How do constant aperture lenses REALLY remain constant?

[@DaveMcKeegan]

Apologies for the glitches @4:30 - I had to re-render to amend an early part of the video and didn't notice this rendering glitch - thankfully it doesn't impact any important information

[@oasean]

Suggesting that McKeegan and I are like minds, this video is a surprise, a rare breed: pure information seemingly without practical application.

[@foxorian]

I have several constant aperture zooms from a few different manufacturers, and I wanted to see what their optical qualities were like at the wide end while fully open. I wanted to know if the aperture of these lenses was stopping down ONLY to maintain aperture, or if the optical design of the lens itself was very poor when truly wide-open. Of the constant aperture lenses I own, the Fujifilm XF 16-80mm f4 was easiest to spoof into staying wide-open. Simply zoom all the way to 80mm so the aperture blades open up, then detach the lens while the camera is still on. Now that the lens has lost power, the aperture stays open when zooming back out. (Note powering off the camera with the lens still attached automatically closes the aperture all the way.) Partially reattaching the lens so it's stuck to the mount but not turned all the way so it gets powered by the pins, I can see what the 16mm end looks like when open -- aaand it's not good. Exposure wise, the camera was getting 1 full stop more light (so, f2.8 instead of 4,) but the optics had heavy ghosting around everything. Only OK in sharpness, everything had a heavy soft-focus effect, meaning the glass really was not designed to operate at f2.8 at the wide end. I imagine in order to make the lens a good performing f2.8-4, the optical design would have been more complex and costly. A few more interested cases of this: - I have a Panasonic Lumix S 24-105 f4. This lens has physical mechanical linkage to the aperture blades when zooming, so as they close down at the 24mm end even when detached from the camera. - I also have the Leica 12-60mm f2.8-4 for Micro Four Thirds -- since it's variable the aperture doesn't close down at the wide end. By all normal accounts it looks like it could have been a constant F4, but Leica decided to make the 12mm end VERY usable at f2.8 with extra corrective elements. - I also own an old Canon FD 75-200mm f4.5 lens, which is TRUE constant aperture. It's the style of zoom that you physically pull the entire front barrel out like a telescope. In this lens, only the front element moves contributing to the change in focal lens, making the aperture constant since no elements behind the iris move. The trade-off here however is that the lens is physically VERY LONG. At the 200mm end, you're holding a 10-inch long tube. One of the reasons lens manufacturers started going for these variable aperture zooms is definitely to keep their overall size down, and going constant f2.8 or f4 from there to keep some of the optical design a little simpler and more affordable And one last note: - Olympus, Panasonic and some Leica cameras actually display the aperture of variable zooms smoothly -- 3.5, 3.6, 3.7, 3.8, 3.9... etc. It would seem that Micro Four Thirds and Leica L-Mount lenses report smooth aperture values instead of 1/3 steps.

[@TimvanderLeeuw]

Hey, thanks for this video! I've been wondering about this for a while and I had suspected that one way of doing it would indeed be to just narrow down the aperture blades at wider focal lengths. I did not it could be done optically and you had a very effective demonstration of that! Great video.

[@joaomarques7180]

Great video 🙏 thanks for the information

[@AlexandarHullRichter]

My sigma lens does not stop down the aperture to maintain 2.8 through the zoom range. I play with stuff, and it took me a moment to realize that's why my depth of field preview didn't do anything when I had it set to 2.8. Using the aperture to create the illusion of a constant f: is so stupid. Lots of people would pay more for a 1:1.8-2.8 instead of a constant 2.8, or a 1:3.2-5.6 instead of a 3.5-5.6 if they had the option!

[@JeffBourke]

I have wondered about this actually. Am I sacrifice aperture on the wide end for the illusion of constant aperture?

[@marcbeebe]

That was an excellent and accurate explanation. Now as to whether or not anyone will listen and understand (or accept) it ...

[@kevindiaz3459]

And all this time I thought it was some kind of wizardry. SMH, mind blown! 😁 Thanks for the video Dave.

[@lucianoag999]

Thank you very much! You basically made a video to answer my stupid comment. I appreciate it.

[@DaveMcKeegan]

There is no such thing as a stupid comment when dealing with sort of thing :)

[@faridtahmasebi1]

Thank you so much! finally I understood this concept!😁

[@prestonkelly880]

Would you happen to know if there is a Sony E-mount wide angle zoom lens that exist that has any of these or similar specifications? 1.4 or 1.8 constant aperture, image stabilization, auto focus, etc.

[@DaveMcKeegan]

None that I'm aware of Only full frame wide angle zooms with stabilisation that I can think of are f4, or APS-C is f2.8 and not that wide And no zoom lens I can think of are faster than f2.8

[@prestonkelly880]

@@DaveMcKeegan Ok, Thank you very much!

[@sjm.photos]

interesting stuff

[@arcanics1971]

There's a serious few glitches at around 4:30

[@DaveMcKeegan]

Thanks for pointing this out - seems to be a glitch from when the video was rendering but I didn't spot it