Tamron 28-75 f2.8 is a LIE!

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#tamron #Tamron28-75 #Tamronlie #aperturelie

[@kh1188ish]

Closing down does more than just reduce the amount of light. It also cuts out the edges of the lens design that have the most curve (the most difficult parts to get convergence correct). This is why stopping down reduces if not eliminates CA, ghosting, etc. Imagine if tamron released this lens that had all those issues, even if very slight but then said if you stop down to 2.8, it’s much better? Everyone would say the lens is crap at 2.0 and complain they didn’t do it right. So IMO the design is not a “lie”, it’s a brilliant solution.

[@heezdeadjim]

Well I that case, my Navitar 50mm .95 has near zero blooming and CA at f8!

[@falcofurious]

Agreed. There are always compromises in cheaper lenses. This guy doesn’t understand that.

[@thoraht]

Exactly

[@anonymousl5150]

But everyone half decent at photography already knows if you stop down you achieve superior optical quality between f4 to f10. The difference is hardly noticeable anyways even if they made it f2 to f2.8 variable. The real difference is the marketing. Lens manufacturers lie all the time about the correct aperture and focal ranges, rounding down or up often drastically. An example is the fuji 16 2.8 which is actually closer to fuji 15, but no one would take that seriously.

[@benjhaisch]

many lenses do this to keep a consistent aperture throughout the range and to keep the image looking better optimized as well.

[@officialtiimo]

What other lenses do this?

[@awildtomappeared5925]

if they unlocked it the user could still set the aperature to 2.8 themselves and still have te exact same results, just with the added option of changing it to f2 at the wide end, they could even still market it as constant 2.8 aperature because even if they allowed the aperature to be variable 2-2.8 it would still be able to do a constant 2.8

[@hbp_]

@@officialtiimo One example that comes to my mind is Sigma 18-50mm 1:2.8 EX DC. It doesn't do it electronically but mechanically. I can visually see the aperture changing shape and the blades moving while zooming.

[@sebidoo]

I just checked and the G2 readjusts it's aperture too when zooming in, removing the lens, zooming out turning camera off and putting the lens back on. The aperture does definitely get smaller when turning the camera back on after doing the same experiment. It's mind blowing, now I will never feel that I'm getting this lenses true potential 😭

[@absolutrumo]

Every constant aperture zoom lens does this. I think they should let you use the f2 setting, personally, but it's obvious why they don't. The lens isn't as sharp at f2 as it is at f2.8, and the market prefers constant aperture zooms, they're seen as more upmarket. If it were a 2-2.8, every reviewer would complain about its poor performance at f2 (“why have such a wide aperture when you can't use it?“) and they'd say it's only variable aperture, which is “bad“ for video work. I still think you're right, but they just don't want the bad press, which they definitely would get. They want all the reviewers to say “This Tamron is so sharp and high quality“, which it is.

[@cyrkielnetwork]

@@absolutrumo But they could relese version of firmware that alow to use it. It's like procesors, they have official specs for reviews but you can overclock them, your results may very, system can be unstable and crash, but you have the option. Officialy it still be constat 2.8 but you could use it's full potential.

[@erichramone7812]

@@absolutrumo ohhh your absolutely right. Good insight on that.

[@PowerfulGaz18]

Hey, it's the same with the Sigma 28-70. I suspect it's due to optimisation reasons. It's probably difficult to build tack sharp f2 zoom in such a small body.

[@godsinbox]

yes, polishing the glass for an f2 result would drive up the costs enormously. The vignetting would be an issue that couldnt be resolved with a lens profile correction. The marketing department would hammer the production team for volume output as it would sell like hotcakes if cheap(Tamron 35-150) or not at all if expensive(RF 28-70)

[@networm64]

This video shows how this channel is the most underrated photography channel ever! How many other reviewers have tested this lens sofar? And Dave is the only one who could discover this hidden feature in this lens. I have no idea how many other lenses may have this hidden glory in them. But for now I think we can shout at Tamron to release a new firmware update if they still want to sell this old version! Kudos to Dave for being the nerd he is and we love it!

[@absolutrumo]

EVERY constant apterture zoom lens does this! They have to. Constant aperture number != constant width of the aperture over the zoom range. A 85mm 1.8 has a much wider opening than a 35mm 1.8. To keep the aperture constant for different focal ranges, the aperture blades have to move.

[@networm64]

@@absolutrumo If that's the case in every other lens then why don't they make it 2-2.8 and market it as it is like the Tamron 35-150?

[@FantabMedia]

@@networm64 Because the quality at F2 would be terrible. Every zoom lens does this.

@@FantabMedia not just that, lot of people like constant aperture lenses

[@dougreid1951]

This is done to control vignetting at the wide angle settings.

[@k1k13004]

so you can say exactly the same for any zoom with constant aperture ! Because constant aperture means the diaphragm will actually open wider as you zoom in. Am I correct or there are zoom lenses (constant f2.8) wich physically can't open wider at shorter focal distance?

[@MatheusPratta]

This reminds me of one of my nicest FD lenses, a Vivitar 1 28-105mm f2.8-3.8, which for a modern version would probably have some similar thing to become a f4 constant lens...

[@_rhapsodist]

Interesting find, the reason is probably the market segmentation and Sony's shares in Tamron. The 35-150 on the other hand can get away with starting from F2 thanks to its much higher build quality.

[@ChristosSerranoThoma]

I just did the same experiment with my 24-70 GM and its the same thing! I had a 2/3 of a stop brighter image and it was also very well visible the difference when I detached it when was at 70mm and then brought it back to 24 the iris stayed all the way open! I also have noticed this 3 years ago but I just thought its the way it was designed!

[@zedg390]

It’s really nothing to write home about, a lot of zoom lenses will stop down at its wider angle to maintain the same aperture value. And if you don’t really care about your aperture control and want to keep it at its maximum capability, you can simply remove the aperture or disconnect the aperture control wire at its longer end. I’ve seen a lot of people who’s done this kind of modification to their lenses. But of course, you don’t get to control your aperture anymore.

[@albertacesantosa416]

Great explanation Dave! A lot of constant aperture lenses do this. I have 24-105G lens and I notice that when I zoomed out to 24mm, the iris opening is actually smaller than 105mm's opening.

[@JamesDBuzzard]

I wonder if this lens is actually just losing light at the higher focal ranges and that’s the reason that they need to open the aperture more at 75mm. This would make sense since zoom lenses loose light as you zoom to higher focal ranges so maybe they are compensating for the loss of light at 75mm vs 28mm. I'm thinking f-stop vs t-stop essentially. I'm not an engineer though so what do I know lol

[@LuboBachev]

Wait, I just realized this is exactly why Canon's EF 24-105 f4 sometimes glitches shooting in live view with constant zooming. The glitch is basically the following: you shoot zoomed in with an f4 aperture or more, you zoom out and seemingly nothing changes, but if you go to a higher f-stop and go back to f4, the new f4 is much brighter and you see like a "click" of more light in the photo and waveform monitor.

[@8bitofit359]

Nikkor Z 24-70/4 does it as well

[@juliansmart2326]

@@JamesDBuzzardf stop = focal length / aperture diameter so it does make sense that the lens would adjust to keep it constant. I guess knowing your lens is going to produce the same results no matter what focal length is valuable for a lot of professionals.

[@dragonnyxx]

*All* constant aperture lenses do this. How could you zoom between two different focal lengths, not change aperture, and yet have the same f-stop at both ends? Since f-stop is just the ratio of focal length to aperture, it's mathematically impossible to vary focal length, keep aperture the same, and also keep the f-stop the same.

[@sundarAKintelart]

This technology is old and implemented in minolta rokkor 35-70mm f 3.5. This was termed a compensating diaphragm. So, in that the lens actually was at 35mm f 1.8 and at 70mm f 3.5. But the diaphragm mechanism kept it at a constant f3.5.

[@manoftheroad55]

Yes I believe you are correct....misleading "click bait"

[@sundarAKintelart]

@@manoftheroad55 .... I still have the minolta rokkor 35-70mm f 3.5 zoom lens. There is a prong that actuated the diaphragm. A pin (or one may call a short rod) moves the prong when the aperture ring is turned. The pin is fixed slightly slant . The prong moves within the pin as zooming ring is turned. If you get a chance, try looking at the minolta rokkor 24mm f 2.8 VFC lens. VFC stands for variable field curvature. A similar mechanism is used in Nikon DC Nikkor lenses. Thank you. Be happy 😊.

[@Digi20]

Depending on the optical construction, some constant aperture zoom lenses need to vary their aperture diameter. sometimes its done mechanically, sometimes its done electronically like here. This lens could probably be something like a f2.2 at the wide end, but that would mean high vignetting and softer edges. and of course all reviewers would cry about it, so tamron decided to make it a constant 2.8 and be done.

[@billy9506]

This is incredibly interesting! Great detective work.

[@sclogse1]

Brilliant work. Does Tamron have a Q&A page?

[@jeffrydemeyer5433]

I think that if tamron were to release a firmware update for those who want to turn it in to this they would generate renewed interest in this lens

[@erickruse4679]

Something to assign to the G2's button. Full hog aperture button.

[@daddymoesby_8239]

Maybe its made this way because to keep cost down and calling it an constant apature lens? Its still a great lens and wows me everytime

[@officialtiimo]

Looks like the vignetting might be pretty bad at wide angles and f2. Maybe the lens does not fully cover the image circle at some distances when truly wide open and that's why they did this?

[@AlexanderHaibel]

I think you didn't think this through enough. Any constant aperture zoom is capable of being a variable aperture zoom with a brighter wide end, it's just that they choose not to due to worse corners, vignetting, etc. Looking at the other comments, it seems other people are somehow unaware of this fact too. All lens designs are compromises of some kind, and most companies choose a constant aperture compromise as that's what the market is used to buying as "premium" (Leica being a notable exception), and they're also easier to use with manual ISO.

[@DaveMcKeegan]

Of course they are always a compromise, but usually the compromise is reducing the maximum potential aperture in order to reduce size/weight/cost - rather than restricting the aperture that's made available despite being possible Its perfectly expected that f2 wouldn't be as good as f2.8, but the point is why not allow the f2 and let people decide if they want to stop to f2.8 for better results rather than force people to it instead

[@eamonhickey]

Yes, all constant aperture zoom that I've ever seen operate in this way -- in the old days it was controlled mechanically; with electronically controlled diaphragms, it's controlled electronically. This does surprise many people when they discover it. I remember working it out for the first time 30 years ago with an old AF-Nikkor 35-70 f/2.8 lens.

[@jakubpulka]

Similar situation with Viltrox 85mm 1.8 Mark I which apparently is 1.6 and you CAN install proper firmware to enable that. Mark II doesn't have this possibility, though. Tamron 35-150 2-2.8 also has similar contruction.

[@MrPhilbautista]

Considering you mentioned some flaws when shooting wide open at f/2, maybe this is why Tamron didn't want people shooting at f/2?

[@omarquintana3481]

I have a hard time trying to understand the entrance pupil and the aperture, could you put light on the matter? ( not necessarily pun intended): From the front of the lens with a zoom lens a f/4 at 24mm should measure 6mm and at 105mm is 26mm (entrance pupil not the real physical measure of the aperture…right?… if i use a f/16 at 105mm the aperture is 6.56mm and as all difracción calculators show i have diffraction problems showing up… Why i don’t have diffraction f/5.6 an 24mm with a 4.28mm aperture ? Why there is no consideration of the focal length in any diffraction calculator? Why is no diffraction problems in a wide angle lens much sooner than in a telephoto lens?

[@lucky8057]

All constant aperture zoom lenses do this. I believe it is because as you zoom in, your field of view reduces which reduces the area from where the light is incident on the lens and to keep the exposure same, the aperture widens to let in more light. That's also why super long zooms (200mm +) at constant aperture are a rarety as they would require a huge front element.

[@GoodGuysMedia]

I'm guessing this is what's at work with the 35 to 150mm. The secret to 2.0 on the wide and 2.8 on the tele. Nice find bro! 🎥 😎

[@HussainAlkumaish]

This is normal. This is one way to make the smaller and cheaper lens designs that can still be called constant aperture. Your can find examples like this everywhere. Pros want fixed aperture, then that's what they get and they'll pay premium for it. Losing a little bit of light at the wide-end is a small price to pay. You can actually see similar things in pro cine lens: fixed aperture is actually variable just to keep exposure the same, Or even prime lenses that are actually zoom lenses (micro-zoom, actually) just so they can eliminate focus breathing. Pros know this, they like it, and they'll pay extra for it.

[@awildtomappeared5925]

if it had 2-2.8 aperature they could still market it as constant 2.8 aperature because it would still be able to do a constant 2.8 aperature

[@future62]

For video the constant aperture kind of makes more sense, but a) the enthusiast market is bigger than the pro market and b) I'm certain enthusiasts and pros would rather have an F/2-2.8 zoom than a constant F/2.8 zoom. F/2-2.8 puts this lens in a position where it can start to replace F/2ish primes within its FL range.

[@HussainAlkumaish]

@@future62 I don't disagree with you. My original comment still stands though, as I was only saying this is "Normal" in the pro market segment. Also keep in mind that the lens makers (Tamron in this case) might have other considerations (e.g. sharpness falling considerably at f/2) that would've reduced the projected value of the lens.

[@MrBono303]

@@awildtomappeared5925 did you see the dark corners in the video?

[@svengoj]

this is made video first. in the video world, we need constant exposure. thats why it selled so good. more video, less photo.

[@SirRelith]

Dude great find! Nice video! just subbed. :)

[@benjaminmuller5261]

It's the same with my Sigma 28-70mm. At 28mm the blades are not fully opened as they are when disconnecting the lense at 75mm and zooming out :D

[@jimgraves4197]

I believe Tamron have always operated on a modular design basis. The aperture amd motors in that lens will probably be used in another of Tamron's fine selection of lenses.

[@rogerbradbury9713]

Minolta's 35-70mm f/3.5 manual focus lens had a cam or linkage that adjusted the aperture blades, keeping the f number constant. I took many photos with one. The only reason I can see to do this is to keep the image quality of the lens up. It was certainly known as a very good lens, but as a (let's say) 35-70 f/2.8 to 3.5 instead of a constant f/3.5, it would be just another lens that's soft at maximum aperture. As well as Minolta's Rokkor version, the same optics were sold with a different body as a Leica lens for Leica's SLRs; it had to be good.

[@JerryWoo96]

when i got the lens, i do not even know why the aperture did not even open fully at 28mm when at f2.8.

[@robertgiguere875]

So they didn’t do tho to the 35-150 lense?

[@Honz4Marek]

lol this is exactly how ALL the "constant aperture" zooms work :D

[@DaveMcKeegan]

Except it's not, I've had multiple 70-200's and 120-300's without changing diagrams And a 24-105 & 17-40 which had a shroud in the barrel but never moved the blades themselves

[@Honz4Marek]

@@DaveMcKeegan I don't know how exactly they do it. But if the aperture hole has the same width with different focal lengths, the ratio of focal length and width is different. Therefore the aperture is different. If you want to have constant aperture, on the wide end the hole needs to be smaller by definition.

[@DaveMcKeegan]

Not necessarily because it's a measure of its apparent size viewed through the optics, not it's physical size So as optics in front of it move they magnify the apparent size of the entrance pupil, thus if lenses only use the frontal optics when zooming then the entrance pupil is magnified at the same rate as the focal length changes and so creates a constant aperture If some of the optics behind are also moving then the magnificent rate drops and you end up with a variable aperture

[@hugoingelhammar6163]

Isn't this the case for all constant aperture lenses? For the aperture to be the same at 28mm and 75mm it has to be a smaller diameter at the wide end to maintain the ratio.

[@JimBailey]

Awesome find. Thanks Mr McKeegan. Tell Rusty hi.....

[@JamesDBuzzard]

Super interesting discovery I wonder if this lens is actually just losing light at the higher focal ranges and that’s the reason that they need to open the aperture more at 75mm. This would make sense since zoom lenses loose light as you zoom to higher focal ranges so maybe they are compensating for the loss of light at 75mm vs 28mm. I'm thinking f-stop vs t-stop essentially. What are your thoughts on this?

[@erichramone7812]

That could be

[@renefras2874]

It's because ppl are crazy about constant aperture zooms and frown upon variable ones. Just treat a vari lens as a constant aperture on the lower value🙆

[@chrisogrady28]

This is pretty common in zooms, constant aperture is a lie. But this video did a great job of illustrating it nice work dave!

[@CalzNL]

It's not a lie, it does exactly as it says, have a constant aperture.

[@nogerboher5266]

I forgot the name of the guy but there's a Japanese KZbinr, an engineer who owns a CNC workshop or a factory or whatever that thing is and a few years back he made a quite small 30-110mm lens (which was, if I remember correctly, about the size of your regular 24-70 2.8 or similar) with a fixed f2 aperture, for one of his old fujifilm cameras and it cost him close to nothing to make it. It wasn't the sharpest and it was manual focus, sure but that wasn't the point nor the guys goal, guys point was just to prove that it can be done and not only can it be done, it can be done cheaply as well - and yet here we are, years later and multi billion dollar companies like Nikon, Sony, Canon, Fuji and so on, still dish out garbage, purposefully crippled lenses and cameras - and my guess as to why they do that, is because they just do not want to hinder the sales of their other similar products and their more expensive products... I mean, imagine if e.g Canon made a 35-135mm lens at f2, that lens would cover all 5 of the most used focal lengths in 90% of photography while being only f2, so then nobody would end up buying their f1.8 35mm, 85mm, 105mm or 135mm lenses or the 24-70mm f2.8... But that's just my guess... However... Even though I'm not surprised none of the big name manufacturers made such a lens, I'm still surprised that neither Samyang nor any of the other smaller lens manufacturers who don't care as much about profit, made a similar lens... Well... Tamron did very recently; their 35-150mm f2-f2.8 lens is just pure work of engineering art but it's still no 24-105 or 35-135 at f2... They could have made that lens a 35-135mm instead, at a fixed f1.8 or f2 for the same manufacturing cost and at the same size... But hey... What can we do about it?! Nothing...

[@DACProVid]

Since I'm more of a video guy, I prefer a constant aperture (and I can't afford really great glass yet) What I don't know is why they didn't just put a switch on the lens to turn this behavior off or not..

[@pugdealer82]

pretty interesting! I would guess that tamron chose not to make it a variable 2-2.8 because as is/was, it already took a huge bite out of sony's native lenses, and making it go down to f2, even only at 28mm, would make it even more appealing, and sony, being a tamron share holder, might have said - NO WAY!

[@JeffBourke]

I thought all zooms are like this. Panasonic 12-35 is the same. You can see the aperture maintaining constant f stop.

[@easterlingderek34]

I look at it like this all constant aperture zooms can open more on wide end, because the maximum aperture has to fit with telephoto side first as it is impossible to make a physical hole bigger after it was made too small. So if they made it a f2.8 at 28mm physical with the largest size then when you zoom in it would of been like f4 or more at 75mm. But tamron wants to market as a constant aperture as some people don’t want their exposure changing as they zoom in.

[@kaimelis]

this is neither the first nor the lens that does this. the main reason is image quality purposes, mainly to keep the field curvature under control.

[@CedefZ]

Many constant aperture lens does that actually I've seen it in Nikon lens too, just don't remember which one. It's their to probably look better statistically and on chart such as MTF. Larger aperture will normally degrade things like color fringing and sharpness.

[@portreathbeach]

Just checked my Sigma 18-50 F2.8. This also does the same as the Tamron 28-75 in this video.Would this not happen on all zoom lenses? The F number is the 'focal length/lens diameter (iris)'. So surely if you want to keep a constant 2.8 aperture, when you change the focal length (zoom in or out), the lens diameter (iris), has to change to keep the equation correct.

[@DaveMcKeegan]

Not necessarily, zooming with the optics in front of the aperture magnifies the apparent size of the entrance pupil, so only zooming with this front optics would mean the magnification occurs at the same rate as the zooming so the respective sizes remain the same If some optics behind the aperture are also moving to zoom then the ratio drops and you end up with a variable lens

[@portreathbeach]

@@DaveMcKeegan Just checked my Sigma 18-50 f2.8. Opened it up to f2.8 at 50mm, disconnected it from the camera, zoomed it back to 18 and when connecting it back to the camera, the aperture closes a little bit to get it to f2.8. So this lens also exhibits the behaviour you show with you 28-75.

@@portreathbeach basically every photo lens does

[@MrBono303]

Any zoom lens with a static F value must close the aperture. When you think about it, there is no other way to do it technically differently. Old lenses had something like f2.8 -4.0.

[@DaveMcKeegan]

It's possible by only having the optics in front of the aperture move when zooming The F value is a measure of the apparent size of the entrance pupil rather than its physical size, and it's apparent size increases as optics in front of the aperture move and magnify it

[@SimeonKolev]

I do not want to spoil your discovery but all constant aperture zooms are like this because it is impossible to create a zoom lens with constant aperture or it is not that easy. The way to do it is to close the aperture in the wider end. Even old vintage manual focus constant aperture zooms are like this :)

[@SimeonKolev]

PS the reason those lenses are not sold with variable aperture today is very stupid - marketing :). Just like we see only yellow bananas in the supermarket - there are many other sorts of bananas but people will not buy them because they know bananas are yellow. Constant aperture zooms are kinda known like premium products and if it was 2.0-2.8 it was not going to be so premium for the majority of the users. Even if it is as simple as this - set your aperture in the camera at 2.8 and it will stay 2.8 all the way, if you need it wider at the wide end - you can use it... PS there is light in the end of the tunnel - 35-150 2.0-2.8 came out as it is...

[@DaveMcKeegan]

I believe lenses can be constant aperture as long as the zooming is being done by only moving optics in front of the aperture mechanism as this magnifies the entrance pupil If optics behind the aperture are moved then the magnification rate drops off which then creates a variable aperture ratio

[@SimeonKolev]

@@DaveMcKeegan whatever you do in a zoom lens it always changes the aperture except if your lens design is focused around that parameter making compromises with others. That is why they chose to fix this with the aperture blades :). Check out other zooms ;)

[@DaveMcKeegan]

But even in variable aperture zooms - like an 18-250 f3.5-6.3 Entrance pupil is 5mm at 18mm and 40mm at 250mm, but the aperture blades aren't being used to stop it right down. The entrance pupil gets magnified as you zoom, I've had many zooms over the years but never one automatically pull down the aperture to maintain a slower constant aperture An f2-2.8 that could be stopped down and held at 2.8 would be far more marketable than a constant f2.8

[@SimeonKolev]

@@DaveMcKeegan not a constant aperture zoom :) if it was F5.6 all the way...

[@kriszhli]

But is it not the case on other zoom lenses?

[@SinaFarhat]

Interesting! Have a good week!

[@fs_shots]

Its all true. But it's also like every single "constant" aputure zoom lens does this as well

[@DaveMcKeegan]

Its certainly not 'all' as I've had several 70-200's and a 120-300 f2.8 - all have a really been a true constant with no changing aperture blades

@@DaveMcKeegan which ones? are you sure they don't have mechanical way of blocking light build in, working as "second aperture"

[@DaveMcKeegan]

@ I had the Canon 70-200 f2.8 IS Mk 1 & MK2, Sony 70-200 f4, Sigma 70-200 f2.8 DG OS, 120-300 f2.8 DG OS & Sport versions None appeared to have moving apertures but that may because they were all internal zooms

@@DaveMcKeegan also is it "none of them had moving apertures" or with none of them you noticed? with dslrs it's much harder to notice, only when you're in live view it's obvious

[@marcowuu]

A fool always feels that someone is lying to him.

[@HoomanBhr]

Great review, Lovely dog

[@RhettThompsonFilm]

I’ve seen this in a few different lenses. Panasonic is a very similar thing with its 12-35 2.8 at the very least

[@elnitram4313]

I guess since the MTF charts are usually made wide open and at F8, they thought they rather stop it down than have a lens that isn't that great at F2. Would be nice to have someone test the optical performance at F2.

[@billy9506]

Could you not set the lens to "f2" then tape the connections up then reattach?

[@DaveMcKeegan]

Doesn't look like it, it seems if any of the contacts are taped then the camera treats the lens as a manual focus

[@billy9506]

@@DaveMcKeegan ah... that's a shame. Cameras know too much these days!

[@mhaustria]

Just an idea, put on a tape on the contacts on the lens and screw it back on again. Focus manually and you should get your desired aperture. I have done that many times before with my Canon when I tried to test some stuff for a magazine.

you can't focus manually

[@guyjordan8201]

Fun to see you uncover this evidence as I always assumed this is what occurred in constant aperture lenses. Mechanical linkages would do the same in lenses with mechanical aperture controls as electronic controls do in this Tamron. By definition a 28mm f/2.8 has a 10mm entrance pupil as you stated, and a 75mm f2.8 has one at 26.8mm. Therefore this design "could" have a 28mm f/1.04 setting given that same 26.8mm entrance hole. In the same way a lens with no vignetting actually has a much larger field of view that is artificially cropped to eliminate dark corners.

[@DaveMcKeegan]

I believe lenses with constant apertures are generally done by only moving optics in front of the aperture mechanism as this magnifies the entrance pupil size when the lens zooms Moving some optics behind the aperture will mean the magnification drops off compared to the focal range

[@oasean]

I think that McKeegan is correct: the F stop is determined by accounting for magnification via focal length. As the focal length increases, the "pupil" grows optically, not mechanically.

[@iShineify]

The reason is simple to keep all the optical errors which comes along to a minimum. More like the optical design is optimized for the focal lengths with f2.8. Imagine they released it with the f2 firmware what would all the reviews would say about the IQ, people would probably avoid this lens then.

[@mrbarkan]

This!! The aim a is a constant 2.8 aperture with a high resolution performance whilst wide open

[@photographerjonathan]

I don't get why Tamron would do that, they would have a way better selling lens if it was f2 to f2.8, it would be a big selling point over regular 24/70 f2.8 lenses, just like there new 35/150 f2 to f2.8, it just doesn't make sense for a company to dumb down there gear. that is unless your company is Canon.

[@lucianoag999]

How would you build a zoom lens with constant f stop without this behavior? With the diaphragm outside? Moving diaphragm? As you change the focal length you would need to either move the aperture or change its diameter. In both cases you would have a degree of freedom that would allow a larger f stop at the wider end.

[@DaveMcKeegan]

By placing all of the move optics in front of the aperture - this magnifies the size of the entrance pupil as you zoom

@@DaveMcKeegan that isn't enough

[@danielpopa6243]

I think this is the case or all or most of the "fixed aperture" zoom lenses. In order for the F-stop number to remain constant when the focal lenght changes, so must the diameter of the entrance pupil (effective aperture) also change with it. The only other way this could be done would be to make the barrel of the lens cone shaped, or more exactly a truncated cone instead of cylindrical, which would create a lot more problems for the manufacturers, I am not an expert, but I am not even sure that would be possible. For one thing, the lens elements would have to grow and shrink as you zoom the lens out and in. :D

[@DaveMcKeegan]

Not necessarily because the aperture is a measure of the apparent entrance pupil size rather than the physical size As the lens zooms it magnifies the size of the entrance pupil, so depending on which optics were moving would vary how much the entrance pupil was being magnified

[@danielpopa6243]

@@DaveMcKeegan I can't find any infos right now to verify that, as far as I know it is the physical size. Anyway, the principle stands, in order to have a constant F-stop, the opening (physical or apparent) must change with the focal length. And maybe it's just easier to do it with the aperture than by calculating and installing the necesarry optical elements in order to also acomodate this. It would be an even more complicated formula for zoom lenses and they would maybe need 50+ elements for some zooms and cost a lot more :D ... Changing the subject, did you think about making a video about the effects of the EVF's on the eye? we all know that the smaller the screen and the closer to our eyes, the more damage it does, so I would be curious if you know of any study about this or what are your thoughts on the matter?

[@DaveMcKeegan]

Here is some info on it that I found: reddit.com/r/photography/comments/16oixl/how_does_a_constant_aperture_zoom_lens_work/ Superzoom lenses like the 18-250 f3.5-6.3 - the entrance pupil is 5mm at 18mm to give f3.5, to get f6.3 at 250mm is almost 40mm entrance pupil, but the aperture is nowhere to be seen at 18mm The diameter of the barrel doesn't change, it's just as you move the optics in front it gets magnified - like a magnifying glass in front of a cardboard tube, the apparent diameter of the tube gets larger and you move the glass away from it

[@danielpopa6243]

@@DaveMcKeegan ok. so some lenses do it via the magnification and other lenses do it by closing the aperture. Thanks for the info, I learned something today. I would really also like to get an input from you regarding my question aobut the EVF. It is clear that they do more damage to our eyes than OVFs, but I would really want to know to what extent and if it can cause serious vision problems. I have googled the topic a bit over the past year without finding any worthwhile results and nobody answers when I ask, so... If I was into conspiration theories, I would start to think that the camera manufacturters who push everybody to switch to mirrorless cameras are trying to hyde something LOL

[@nightdonutstudio]

And how come it is still so compact and lightweight.

[@manoftheroad55]

You sample photo ..does not appear as brightness difference being a whole stop..near a 1/2 stop?

[@DaveMcKeegan]

It's closer to a full stop in the centre, if I upped the shutter by 1 stop then the centres were equal - the f2 had more vignetting though which is to be expected

[@93kristof]

This is true of all constant zooms. I've been annoyed by it for a long time, just let us take full advantage of our lenses.

[@KurtisPape]

1 thing I noticed with my Sony G lenses if you shut the apature down to f22 the lens will go wide open when auto focusing, you can see the difference in depth of field change or hear it open. I don't know at what f-stop it will do this but I feel like it does it at f11, will have to do some testing myself. Obviously the autofocus system needs plenty of light still.

[@DaveMcKeegan]

I believe it does this based on available light, if there isn't enough light then it opens up the aperture to autofocus

[@KurtisPape]

@@DaveMcKeegan appreciate the reply, that might explain things why it sometimes does it and sometimes doesn't.

[@351linzdoctor]

This is the same thing Sony did by locking the aperture at f 3.5 on there SLT cameras in AF video with software but if you take a AF lens and lock a Minolta 50mm f.1.7 lens aperture mechanism at f 1.7 like I did it the auto focuses is fine but the camera will still display f 3.5! But turn the camera to still mode and it will show it's at f 1.7!

[@Badhans2007]

Haven't owned any Tamron lenses. This may be my first.

[@kaminobatto]

That is very interesting! You just earned a sub :)

[@Athiril]

I’ve got the Tamron 150-500mm and I find it odd it jumps from f/6.3 at 499mm to f/6.7 at 500mm

[@DaveMcKeegan]

That is actually the basis of the variable aperture video that will be out in a few weeks 😉

[@bene_eins1308]

This is nothing uncommon. actually its normal. If you think about it, all fixed aperture lenses have to do this. They are designed that way so the photographer doesnt have to change settings while zooming. Even the best Gmaster or L-Lens does this.

[@DaveMcKeegan]

Not actually the case, lenses can be made naturally a constant aperture if it zooms by only moving optics in front of the aperture mechanism because it magnifies the entrance pupil at the same rate as the focal length changes These designs can be more complex though, having some of the optics behind moving can simply things but reduces the rate of magnification and thus produces a variable aperture Feels like some lens are being made as variable apertures and then being restricted on the wide end to make them constant ... When in reality the user can make it constant just by setting the aperture to f2.8 to begin with

[@robertconrad6802]

Can we get a developer to hack the lens firmware possibly?

[@SMGJohn]

This is common practise with a lot of lenses that even dates back to the 1930s. Lenses sometimes are for practical reasons limited one stop with the aperture to gain superior image quality compared to if it was a stop brighter. Sigma with their 28-70 f2.8 is is more like a f2.4 despite being the smallest 28-70mm f2.8 lens for a mirrorless camera, ever made. The only practical way to get a 24-70mm~ lens to f2 constant aperture is APS-C or MFT camera with a speedbooster, otherwise you are looking at a very big lens, Canon has one, its not pretty and its not light or practical.

[@die14die]

Is it a misunderstood about a constant aperture lens? In the tele side the physical aperture sure is bigger than the wide side aperture even they have the same F number aperture. There are not any constant apartrue zoom lens the aperture in physically constant size.

[@DaveMcKeegan]

It's not a measure of the physical aperture size but rather the apparent size As the lens zooms, any optics moving in front of the aperture will magnify the apparent entrance pupil size - so if only the optics in front rather than any behind do the moving then the entrance pupil size is magnified in a directly proportional rate to the focal length and thus keeps a constant aperture

[@Issvor]

Good discovery, I wonder if Tamron is willing to answer why this happens. I can't imagine they wouldn't have a good reason, cause who wouldn't want a faster lens?

[@_rhapsodist]

Sony has shares in Tamron and they would never let this happen, that would be too good value for the money and nobody would buy their own lenses

[@ingwarmagn3393]

@@_rhapsodist Let's wait for some custom firmware, like a Magic Lantern!

[@chryseass.5143]

I wonder if agreements with Sony limits Tamron's right to produce an f2 aperture on such a lens. Sony would seek to protect their GMaster share of the "24/28-70/75" market. Could this ability by the Tamron be released with a firm ware update? Fascinating stuff, Dave!

[@sebidoo]

That's exactly what I thought. There's also the fact that sony corp owns 1/8 of tamron and sony wants to sell GM lenses.

[@SmallSpoonBrigade]

I'd wager because this is the latest iteration of a lens that started out as an optimized lens for cropped sensors that could fit into a FF body.

[@hoatd1993]

They made another lens 35-150mm with an f2-2.8 aperture. I guess the only reason here is simple: the corner sharpness when you open at f2 is worst so they don't want to enable it.

[@sebidoo]

@@hoatd1993 that's also a possibility, but still, the overall sharpness of both lenses, especially on the G2 should allow for being able to take reasonably good pictures even at that aperture, as shown in this video, center sharpness at f2 is up there with sharpness at f2.8, I hope tamron eventually gives us the possibility of getting that max aperture, maybe even through their lens utility app

[@ChristosSerranoThoma]

I dont think Sony limits the Tamron cos my 24-70 GM does the same thing! Its a design thing I think!

[@Omenyte]

I need help in choosing Sigma 24-70mm vs Tamron 28-75mm G2. I am new to photography and I wanna get a pro look. Sigma release back in November 2019 while Tamron 28-75mm G2 came out October 2021. Realistically to me I would go with the latest model but there are so many mix reviews!! Hope you are able to help me to decide which first lens should I get. I do not care about the weight but more of the quality and aesthetic of the camera and photos/videos.

[@DaveMcKeegan]

If size/weight is no concern then the Sigma is arguably more flexible as it can go down to 24mm Vs 28 But honestly in terms of looking like a professional, don't waste your energy worrying about it, nobody hires you based on how good your gear looks, they hire you based on how good the results are that you produce with that gear Go and compare their results on comparisons like DPReview or The Digital Picture and see which lens renders results preferable to you and go with that

[@beetwing]

We need a firmware hack here.

[@AbdonPhirathon]

Tamron most likely have designed this lens as an f/2-2.8 zoom, but given the size and dimensions they wanted to achieve, instead of redesigning the lens again they simply decided to stop this lens from opening past f/2.8 at the wide end because the image quality was most likely not good enough, and that would have been mentioned once reviewers started testing the lens. Either way, someone with the right know-how could hack this lens to make it open up the aperture all the way to f/2.

[@EugeneLazin]

Or maybe the lens is just crap at f2 so they have to stop it down in order for it to perform better.

[@anonymousl5150]

@@EugeneLazin Pretty much most zoom lens are suboptimal at both ends of their range, the reason was most likely for marketing consistency (and exposure consistency). It would have to be extremely bad at f2 and I don't see that happening.

[@winterkat8693]

here's the thing... the sigma 24-70 f2.9 also has it... I saw it last week and I kinda guessed why they do it so I never asked.

[@michaeld5888]

A constant aperture 28-75mm starting at f/2.8 being 10mm would be f/7.5 at 75mm. All zooms increase aperture to some degree but not in a fixed ratio to focal length. One problem for any newcomer to photography must surely be the lazy nomenclature calling focal ratio as aperture which is only half of the ratio. The astronomers do not seem to do this. I will of course be called pedantic for saying this as surely everyone knows what is meant. I am not so sure about that looking at comments from beginners in photography forums. I wonder sometimes if it is a tactic to obscure the underlying simplicity of what is actually going on to those outsiders. Probably it is that focal ratio is two words and trying to cram them in to the attention span nowadays is just too much to ask for. The redoubtable Gordon Laing used to say focal ratio but I am not sure that even he hasn't slipped to aperture nowadays.

[@danielshepherd7306]

Firmware update request!!!

[@nshire]

Might be the same with the Nikkor 24-70 F/4 S

[@user-kc9rh8kw7c]

Dude its very smart indeed! Congrats!

[@benuovir]

Sony might limit Tamron as they do with other features

[@fanjan7527]

Tamron probably did it so that it can be a constant aperture lens.

[@TreShoots]

i think it’s funny too tho that the new tamron 35-150 is 2-2.8 🤔

[@ANDRITHORMEDIA]

Nice video 😎

[@horiamihon2253]

Aperture is not only about the blades. If it was that easy, everybody would make 24-70 f0.7 lenses, and obviously that is not the case. It is about optics, image quality, how you make the light pass through the glass, and a looong etc. If it was not like this, you could take off the blades of any f4 lens and you would have a very very fast lens. So I'm afraid the true potential of this tamron is f2.8.

[@davidgatzen1543]

If you cover the lens electrical contacts with thin plastic wrap, and reattach the lens then the camera will not be able to change the aperture. If you cover the lens electrical contacts, then you will not be able to change anything on the lens once to put it on the camera, so you would not be able to focus or change the aperture, so you would be stuck with the settings of the lens when it was off of the camera. If you cover the electrical contacts so that the aperture stays open you run the risk of damaging your sensor, because you could you could get too much light into the sensor if you point the camera at the sun. You should not point your camera at the sun anyway, even if you do not cover the electrical contacts, because you could damage the sensor, unless you have a special filter on your camera for looking at the sun.

[@Steve-Stoytchev]

Ugh this makes me upset. I want a 2-2.8 lens.

[@turusimpala7155]

cmon, any constant 2.8 zooms physically build like that.

[@DaveMcKeegan]

I don't believe that's the case though I've had 70-200's and 120-300 2.8's, none behaved like this I've even since bought Tamron's 70-180 2.8 and even that doesn't behave the same as the 28-75

[@turusimpala7155]

@@DaveMcKeegan Okay, I was wrong, any 2.8 zoom lens that includes a wide angle. 16-35, 24-70. Perhaps this is due to the fact that the wide angle physically transmits more light, due to the optical design of such lenses. I could be wrong, but I think it is.

[@renashbell1203]

Is this what all camera/lens companies doing? Limiting all sh*t to sell more. Like, you can always make a camera able to record 4K but why is there a limiter in an old camera with the same mp and lens?

[@martinruiz398]

Tamron, introduces the 35-150mm f2-2.8 lol they heard you!

[@chryseass.5143]

Has any one now rethought about Tamron's well regarded ( but heavy!) 35-150 f2 /2.8 lens? Tamron's great advertising hook is that this lens starts at F2 but I understand that it quickly does climb to the 2.8 aperture. Apparently, they could have used this claim on their earlier lenses but went down the "constant aperture " route instead to sell lenses ( and very successfully ,too!)

[@pibecks]

I had the same thought after watching this video, my guess is not a lot of people will force out the money to get the 35-150 if they were able to get that f2 on a 28-75, I know I wouldn’t 😆

[@MaciejFersten]

I suppose absolutely every zoom lens has to do that trick to maintain exposure. Opening entrance pupil for 24/2.8 is 8,6mm, for 70/2.8 is 25mm. During zooming opening pupil change its size because of lenses curvatures, but not enough to maintain exposure. Iris has to change to keep same amount of light. Why its firmware blocked? I suppose poor image quality and mid-exposure size final pupil size. 2,2 (or 2,5) to 2,8 wouldnt shake market, and its not full 2,0. For true 2,0 to 2,8 you need bigger iris hence bigger lenses.

[@DaveMcKeegan]

Constant f2 would be bigger but an f2-2.8 variable wouldn't be because it's already there if the aperture wasn't closing down The entrance pupil doesn't really enlarge, it's just it's apparent size as it's being magnified by the optics, if a lens moves the optics in front of the iris then the apparent size of the iris opening changes, if a lens only uses the optics in front of the aperture to zoom then the entrance pupil is magnified in relation to the focal length and so the aperture ratio remains the same

[@danielweber3433]

@@DaveMcKeegan the iris/aperture is one way to define the entrance and exit pupil. It is not the entrance pupil itself. The aperture could be completely gone and you would still have an entrance and exit pupil. The size of them is then determined by other limiting parts inside the lens. To prevent distortion by other parts inside the lens, the aperture needs to control the pupils. I think it stops down to do exactly that in this lens. As you mentioned there is colorfringing etc wide open.

[@DaveMcKeegan]

There is appeared to be a hint, although that could be down to the focus being slightly off The image quality that I saw didn't seem unusable for what would be a 28-75 f2-2.8 for less than $900

[@danielweber3433]

@@DaveMcKeegan that would need further investigation. I can also imagine that coating in the outer areas of the internal lenses is not that homogeneous or not as effective due to light hitting in a steeper angle, thus causing fringing. It would be fun to take a lens and remove the iris and see what happens. Maybe you have a copy of the lens that is better than other copies which might fring more.

[@MatthewMin]

I just checked my tamron 28-105 f2.8 from 1994 and it does the same, but mechanically!!!

[@RuXsel]

We more hackers in the camera community 🤣

[@stutt0711gart]

I have the G2 and it's the same

[@sofronio.]

constant aperture ≠ constant aperture diameter