Guide Number Misconceptions / Understanding Flash Power on Strobes & Speedlights

[@AParticularPointofView]

This video blew my frontal cortex completely out. 👍

[@geraldundone]

Haha. Hard to tell if that's a good thing or bad thing, but coming from you, I know that means you liked it. 😃👌

[@marcomark8]

Just rambling on and on and on. Slow down and make a point that can be understood by all. That's called teaching not explaining.

[@duncandee2514]

@@marcomark8 flash is so confusing 🙃

[@Belle5942]

Yah same sentiments. I thought I mistakenly searched for "physics tutorials".

[@sentand-sg3yr]

@@geraldundone 7：07 Hello can I ask you if you use studio flash and I use the accessories to calculate the same way you can?

[@annedouc]

I had to watch this video about 3 times and do mathematical exercises to fully understand it all. But I must say this video is the most comprehensive and accurate video on guide numbers thus far. Thank you, and very well done!!!

[@n1ckotene]

I've used flash for years but never realised the guide number equated to f/1.0 - the old Canon 50mm f/1.0 is the only lens I can think of that requires no conversion! Excellent and most informative as per usual Gerald, thank you.

[@geraldundone]

I know, right?! It's a surprising fact that no one talks about. Thanks for the comment and kind words. 😃👍

[@azrinaz8993]

Woah the best technical explanation of Flash controls I’ve seen on KZbin. Thanks Gerald.

[@geraldundone]

Thanks so much! That's very kind. Cheers!

[@jeno5821]

Thank You! I have been sifting through countless resources in an attempt to understand the logic behind flash power settings and this has been the best explanation so far. Very well stated.

[@correavelez]

and incredibly fast :/

[@luisarevalo6112]

I've watched a number of other videos with explanations including diviiding the GN by the Distance that still did not complete the picture. Your video has made a difference in my understanding all the components and that you've included the speedlight power and corresponding fstops is a big plus. Still usefull 3 years later! Well done

[@_Jess]

Been waiting for this video to come out! You covered it better than what I've been able to find, and provided actual practical information on using it. I'm glad I subbed!

[@geraldundone]

Perfect! Happy to have helped. Cheers! 😃👍

[@carlosdias1940]

Hi Gerald! My sincere congrats! I've learned this concept in 10 minute video than a whole class, that I was actually there!!! Honestly I ket back and forth to understand and took the examples given to my needs (camera\flash) and been busy with the last couple of hours. Paper and pencil of course!! Brilliant!!! Cheers

[@pebmets]

The power of math, I love it. Thank you. Another useful video added to the collection.

[@geraldundone]

Thanks, Phil! Appreciate your comments as always. 👍😃

[@marvoski547]

Awesome! I was wondering about shutter speed the whole time and then you mentioned it. Thanks!

[@MiaogisTeas]

As a dyscalculic, I thank you for your stop-to-stop work around. Definitely need to play around with flashes more!

[@geraldundone]

I'm happy to hear that. Whenever I can I try to provide a solution for people that don't consider math/numbers a friend. Appreciate the comment. Cheers! 😃👍

[@ramongutierrez8249]

THANKS! After almost 9 years taking pictures I never took the time to find out what were those guide numbers on my flashes! :D

[@geraldundone]

Interesting! Well, I'm glad you enjoyed the video. 😃👍

[@bigfatfitnessguy6479]

2=4X9XISO+FSTOP- SPACE TIME CONTINUUM = black hole/ gravity= Get to know your flash better. Got it!

[@geraldundone]

😂👍

[@uctynguzb]

thank you. this answered my questions that others' videos didn't.

[@martingranger40]

Wow - best video I've seen explaining GN on flash, aperture, and distance. WELL DONE...or should I say UNDONE!

[@andreslinn69]

Just when i want to buy flash for the first time! Thank you for the explanation. A bit confusing, but great start for beginners. Cheers

[@geraldundone]

Thanks! I hope it proves helpful. 😃

[@BrasicOne]

Had to go chapter by chapter and write this out to follow…thank you for this great video with examples. Very very helpful.

[@dogstuff8337]

Brilliantly explained! So useful. Thank you. I’m just in the process of getting to understand if a Godox 350 will provide enough power for my Lumix S5 in the shooting scenarios I’m likely to find myself in. And I can make an informed comparison with my Fuji 60 flash on my Fuji X-T4, and hence get to understand everything so much better. Fantastic!

[@eguitarstar]

Genuinely loved the maths on this video, very detailed and described.

[@patriciashetler4736]

Best intro ever! This was an awesome video and made so much sense. I am better at memorizing than math. But your explanation made it easy. Thank you.

[@tusslemania]

pertinent information for flash user. explained precisely and thoroughly, much appreciated Gerald. I especially like when you smesh misconceptions. would be curious to see a high speed sync video at some point for similar reasons

[@geraldundone]

Thanks! And thanks for the suggestion as well. Cheers! 😃👍

[@hazelvee2527]

I really like the alternating intros. keep them, please. 🙂

[@mehditaba4657]

ThAnX for this video and pronouncing the ISO correctly!

[@zahreelashehzaadi1174]

Thanks Gerald, what an amazing explanation... I am a super-rookie and just started to use the "kit flash" that comes with those Christmas sale bundles (now I know that it is a manual slave flash so I am still few hundred test shots away from getting to a point of right exposure that takes the ambient light into account) and will revisit this video sometime soon. The introduction clip is dope... wondering why just 15K views...

[@geraldundone]

Thanks so much!

[@Photographicelements]

Math: ..and now I'm going to angle the flash, so only a little hits the subject, for more flattering light, or, there's a low ceiling that is, conveniently white, I'll now bounce the flash up to the ceiling to illuminate the subject, but then angle it as well, now I'll add a Lumiquest, Gary Fong Light-o-Sphere or Rogue Flashbender to create more flattering light, and also zoom out to the widest setting (24mm, or__) .. and carry the one.. and.. Oh screw it, I'll just do some test shots and go from there.. (or just give up & use TTL-- aka: "mheh, good enough."). ..(OK, I should have watched the whole video first :[]) This is actually really helpful. Using full stops is easier in a lot of ways. Where was this video when I was first learning flash photography?! Thanks!

[@geraldundone]

You raise a good point though. The guide numbers are only really useful if you don't modify or diffuse the light. Silly guide numbers! 😃

[@cooloox]

Or take a quick reading off a flash-meter (shooting with bounce or modifiers, etc.) and get it right first time.

[@cooloox]

@@geraldundone Many modifiers rob you of 2 stops which halves the effective distance at the same flash power/aperture settings. It's not rocket science LOL. I've made 2 posts not quite agreeing with you, but please note I love your channel and agree with you 99% of the time. I gave this video a thumbs up too because you taught beginners what guide number means, which most probably wouldn't know. Keep up the great work. 🙂

[@geraldundone]

@@cooloox I think I'll save light meters for another video. I try to balance explaining one topic thoroughly while keeping the length manageable. There's usually so many rabbit holes I want to go down, but I restrain myself. Hell, this whole video was one of those holes from a previous video. It's a never ending dig. And thanks, I'm glad you like the content. Appreciate your comments. 👍😃

[@Photographicelements]

@@cooloox A flash/light meter? I thought they stopped using those when ppl stopped using film cameras. Haha, yes, that would be the EASY way to do things :D

[@kwadamson]

Perfect timing! I needed to learn about this this week for a job. :D

[@geraldundone]

Awesome! I'm glad to hear that. Thanks! 😃👍

[@tedk2814]

Wow, great presentation, clear and articulate. This is the best I've seen on you tube and I subscribed to see some more. Thanks !!

[@geraldundone]

Thanks, Ted! I think you'd really enjoy my playlist "Nerdy Explanations". Appreciate the kind words!

[@BinniamEskender]

Well done! Thank you for a very illuminating video

[@geraldundone]

Thanks! Cheers. 😃

[@jellevanthuyne1984]

You made things soo clear for us!

[@thegreatujo]

Loved the presentation. Very clear and logical.

[@tearoha.hohaia]

Very helpful. Very entertaining.

[@geraldundone]

🙏😃

[@khp333]

Fantastic video!! Thank you so much!

[@DaveMcKeegan]

If the GN is based at full zoom on the flash, how does reducing the zoom alter the calculation? Relative to change or inverse square? (I hit information overload so maybe you mentioned it and I missed it)

[@geraldundone]

Good question! I didn't cover that one because it's finicky. It's subject to design by manufacturer and the angles they've chosen. The bigger companies put it in their manuals sometimes, but here's one for the Canon 430EX III to give you an idea: 105mm = 100%, 50mm = 76.8%, 35mm = 65.1%, 28mm = 60.6%. So it has a diminishing effect similar to inverse-square, but each flash is different.

[@cooloox]

Fortunately, manufacturers often show GN at different lens coverages in their spec sheets which come with the speedlight.

[@dukenukem5768]

If the optics of the flash were as good as the optics of a camera lens, the GN would come down linearly with the "focal length" of the flash optics. But the flash optics are far from perfect, and the light spread is much wider than strictly necessary (to avoid vignetting), so in practice it is no-where near linear. The only way to tell is to look at the manufacturer's data table.

[@poundingthenail6189]

im way too smooth brain to fully utilize the specifics but the generalities struck me well and salute to you good sir

[@transitionphotography1392]

This video is so good ! Just wish you had a spread sheet to down load the data to get my head around it :)

[@leonarddavis8449]

If you do a lot of flash work with all the variables of diffusion and modifiers it’s Well worth investing in a flash meter.

[@mirasga]

Glad to see another flash/light meter guy.

[@vistek]

Oh my goodness, I'm glad you understand all this math. My brain hurts - but damn, once again Gerald, you know your stuff. Best researched man in show business.

[@geraldundone]

Haha, thanks! I like the idea that I'm in show business. I'm going to steal that. 😜👍

[@007xyzzy]

Excellent! Loved the tech!

[@kickfila]

Woooooo that was a awesome amount of info lol thank god for pause and rewind. Question?! Does this apply to film cameras as well. Especially when you talk iso, if I’m shooting on a 400 iso roll, the my guide number would double?

[@showdaKOs]

I want the What's happening intro AND the changing ones ! Don't care it-f it sounds greedy ! Well, maybe you can alternate, or something ??? You work is just AMAZING (I wish english was my native language so I could understand it better, but still, I am amazed by the amount of sense you help me make out of so technical subjects) !

[@geraldundone]

Thank you so much. This comment really means a lot. Cheers! 😃🙏

[@photographywiththesun]

This video is short and useful,I had many questions about guide number and distance.I searched allot but i couldn't find my answers.I have three Canon Ex600 speedlites and i wanted to know should i keep them or buy strobes for studio,I though Speedlite is weak in F-stop 8 or 11 but this video helped me. Thank you

[@geraldundone]

I'm glad to hear that. Thanks for saying so. Cheers!

[@JoshSandlin]

This is the best explanation of this I have ever heard. Very well done!

[@paulm8157]

👍Good academic dissertation. (Were you ever a CPA, or a bookmaker? Kidding) Different outcomes when using light mods, like gels and diffusers. Best use of guide number, in my opinion, is as a buying yardstick for comparing light output capabilities of different flash units. For easiest flash exposures/settings invest in a mid-priced flash meter, or begin with TTL and then adjust. Flashers can be entertaining, but that’s another context 😁.

[@geraldundone]

Because I like counting the beans? 😜

[@pranamsk]

Soo sooo useful, always was searching for something like this and got this video from my favorite :-) keep going sir, best info always.

[@geraldundone]

Thanks so much! I'm really glad to hear it was useful. 😃🙏

[@pranamsk]

@@geraldundone always informative videos sir, I learn something out of every single video and I'm thankful for that:-)

[@sahzookey]

You’ve convinced me you’re a rocket surgeon.

[@geraldundone]

Board certified! 😜👍

[@TonyMacina]

Knocked it out the park again, mate.

[@geraldundone]

Thank you! Cheers. 😃🙏

[@TheDark12]

Lmao this video is more incredible than most people can understand. So useful to me as a nerd, thank you.

[@josecolon8143]

You nailed it!!!!!

[@1b1uster]

When god watched this video, he said, “Did I really create a being who could talk this fast?” Lot’s of valuable info here. Glad KZbin allows playback at 1/2 speed! :-)

[@maurice_morales]

Fantastic video

[@organicskincare]

Nice work! Need advice; hybrid style shooting; should I buy GH5, G9, EOS R, or Z6?

[@DayakDreams]

Wow! Your'e like Stephen Hawking without a wheel chair. I was spellbound throughout your video. I can't help but wonder how much editing it took to keep the pace. Really well done.

[@omerserifkuru3390]

Amazing ❤

[@karlozbg1]

Great intro, rather original!!!

[@ForSquirel]

Holy Shit. TIL. I've always been wondering about this.. Just gonna mark this to refer back to it when I need to.. Thanks for the knowledge!

[@geraldundone]

Awesome! You're welcome. I'm glad it's helpful. Cheers! 😃🙏

[@PrabhjotSinghTech]

Well researched, well presented 👏

[@geraldundone]

Cheers! Thanks. 😃🙏

[@valentinogarrycinematograp2297]

just realized...u are a genius! lol

[@eXaviar]

Best explanation i have found 👍🏾

[@NerioAguirre]

Very helpful bro!!! Thank you very much! Honestly 🙏🏼🙏🏼🙏🏼!!! Do the math it will be hard, but I think that will gonna be a good exercise!!! By other hand, I watch the video about lux, etc... Very interesting, I founded because I want to know how choose the right light or kit light for different situations. That I'm trying to say is, how I know that a light it gonna to have enough power or intensity to illuminate anny situación, interview for example or a scene with 3 actors. How I do this calculation using watts, lux, etc... I hear some DP saying: - I want a 1000(watts) fresnel and two 600 ( for a scene with 2 actors for example)- But how they know? just for experience? There is a way to Lear how much light you need?

[@kasiedike1264]

thank you SO much 😭😭😭ive been looking for a break down like this E V E R Y W H E R E. Quick question--I'm interested in doing beauty photography (up close to the model's face pretty much to show detail to the makeup) I have my canon camera aand 100 mm macro lens and I was wondering if I have my aperture at f/11 would it still be okay to put the power at 1/8 on my strobe or would I have to adjust? sorry im very new to photography so I barely know anything

[@georgealexender]

Can a speed light like the Godox V1 be used as a video light for a moving subject who is more than 40ft away?

[@frang6]

What's haaaaaaappening!!!! needs to come back!!!

[@rickvestuto]

Great Content for sure in my older age, I have been using LED as it's much easier especially on indoor shoots with mirrorless cameras (WYSIWYG) however, I still use my GODOX AD600 and HSS to overpower the sun for outdoor work but again with mirrorless, I just take a few shots at different exposures and adjust from there best regards,Rick

[@geraldundone]

I'm with ya, Rick. I've been finding I use constant light more and more these days.

[@Dave_Stafford]

You legend. Thanks..

[@chandraprakashmogha1745]

Hello, i have a osaka tt 990 pro speedlight which has a guide number 72 feet and i cannot find the at which zoom level it give the full illlumination

[@jimbelcalicdan7442]

Hi, thanks for this very informative video. I will just also ask "GN 60m @200mm" does it mean that i always need to set my focal length to 200mm to achieve the perfect exposure and it's given GN?... What if my focal length is only set to 50mm, does it mean that i cannot use that given GN 60m?...

[@brandonw81]

very talented.

[@dukenukem5768]

Pity the zooming of the flash wasn't covered. You need to consult the maker's data sheet to find the GN at other flash zoom settings. Makers used to standardise on the GN when set for a standard lens (50mm full frame), but then they started cheating by quoting at max flash zoom, to sound better than their rivals. 200mm is really stretching it, maybe relevant to paperazzi kept at a distance by celebs' minders.

[@povertytaretv]

Thank you

[@woodworkerroyer8497]

I love this video! Well done! But one of your subs has now died, and stopped taking photos because his mind melted from the maths...

[@sentand-sg3yr]

7：07 Hello can I ask you if you use studio flash and I use the accessories to calculate the same way you can?

[@peepers4763]

I may need to watch this a few more time. I understand the concept but can’t hang onto all the variables at once 🥴. What changes using TTL? Is the camera doing the math? High speed sync is a tool I’m interested in working with. Any suggestions or videos you suggest?

[@geraldundone]

It does do the math, yeah, but it does it by measuring the light coming "through the lens" (TTL). In many cases this involves multiple smaller flashes to do a quick test meter. I'll get back to you on the HSS thing. 😃

[@huynhminhtruc1718]

I have a question: if the GN decreases, should we decrease it with zoom or flash so that the GN remains the same if the zoom is changed?

[@aprilthunder]

When measuring a flash unit's power,, I usually make a reading indoors at a 1 meter distance ( or 2 or 3 meters, depending on the flash's power). And if the flash unit has a zoomable flash head, I take the reading when it is fully retracted. This way, I get more accurate results. At greater distances (or even 60 meters,) there will be too many variables at play, and there variables will even make more of a difference as the distance between your flash unit and subject (or light meter) increases. What are these factors? This could be the reflectivity of the floor, ceiling, or walls, or their color (or the size of the room or enclosure that you are in), or the way your flash unit project the light, or the light-gathering ability of your camera lens.. If you are outdoors, you will get a guide number much lower than what has been written into the flash unit's specs. (Try making a reading at night, when your are on a black asphalt road (no walls!) and try figuring out the guide number! I surmise that Guide Numbers have been figured out using a test conducted indoors, and given a certain average reflectivity of the surroundings. Guide Numbers are not absolute numbers. The operative word here is the word "Guide". They're there to give you general idea of your flash unit's power, and it's up to you to take some variables into consideration, and make the necessary adjustments.

[@mirasga]

For studio work, or semi-controlled outdoor setups, I use a light meter to save my brain cells from dying. On the plus side, you will look more professional compared to the guy who does trial and error with his multi-flash setup. For fast paced event photography, TTL FTW! :D

[@heffelumpphotoco]

So, to get the most power from my flash, would I want to set the fresnel to 200mm when using it in a soft box? I know that doing so would concentrate the light into a very tight beam, but with the two layers of diffusion, the reflector ring, and the silver interior, I’d think that would be negated. Would there even be a benefit to doing so? Or maybe the opposite is true and I should set the beam to as wide an angle as possible?

[@geraldundone]

Correct, the tighter the beam, the more powerful. Typically though, for the nicest quality of light, you'd want to the beam to fill the diffusion to create a larger light source.

[@pedzsan]

I didn’t follow the bit about ISO. If I halve or double the GN, I am going down or up by a stop... right? More or less that’s what 6:51 is saying. So if I double the ISO, I’m going up by a stop so I need to halve the GN (by my logic) but that’s not what you said. You implies that going from 100 to 400 is the same as moving by a stop ... at least that’s how I interpret what you said.

[@geraldundone]

I know what you mean. ISO has always been a bit of a different animal though. It's specifically tuned to each camera and doesn't reflect as much of a standard as the name would suggest or that film used to. But it's kind of the same idea as when you find full frame equivalents. (I hope this doesn't make this more confusing) If you were to convert a M43 setting to Full frame, the crop factor is x2, right? So if you were at 35mm, you'd say that's a 70mm full frame, and if you were at f/2.8, you'd say that has the properties of f/5.6. But if you were at ISO 1600, you would not say ISO 800 full frame, because it's actually crop factor squared so 2^2, so ISO 1600 M43 is actually ISO 400 on full frame. It's just kind of a rule. Does that help at all? So if on the flash you go from ISO 100 to ISO 1600, that's a 16x multiple, take the square root of that, you get 4x, so multiply your guide number by 4.

[@johnhaynes9910]

I am now lost in a purple haze of numbers ! However and fortunately, Matron doesn't allow me out at night so I won't have to master this :)

[@geraldundone]

Haha. 😂

[@herurishardana5618]

Are you using a different LUT for this video? Seems like the skin tone is slightly different than your usual video. Thanks for the content tho, as always, it is great and enlightening.

[@geraldundone]

Nope. It's probably just that I'm wearing a light shirt instead of a dark one. I just drag and drop the preset I made, so everything is the same. 😃 Thanks for the compliment on the content. Appreciate it. 🙏

[@lewismunyua9814]

you have spoken about the distance and flash power but when it comes to the DOF is there no relation?

[@disco4178]

This is amazing!!!!!! But I'm not doing that.

[@catnamestaken]

Cool

[@leevons_home_vids]

Do you ever inhale? Haha, kidding. But this gave me a lot of good info, so thank you.

[@Hdfromabove]

Thank god for TTL... :-)

[@dukenukem5768]

.... and Auto mode. But the GN is still relevant for knowing the _maximum_ distance you can shoot at.

[@jacobnathanielzpayag3885]

This is with TTL metering right?

[@geraldundone]

No, TTL is different. You don't generally set the flash power with TTL, the flash sets it for you.

[@jacobnathanielzpayag3885]

@@geraldundone Thanks

[@kevinwang7894]

When unspecified is the GN always in meters? Apparently not…I guess the ones that look “too good to be true” are in feet >.>

[@mediamannaman]

So with all the discussion about your intro, looks like you decided to "tread lightly" this time. See what I did there? Groan. Heavy sigh. I have to go to work now. But yeah. Good topic that I need to understand better. So thanks!

[@geraldundone]

Haha. I love that you sighed at your own pun. 💯 Thanks for the kind words! 😃

[@curtishustace]

I always wonder how the power of a speed light compares to watt seconds.

[@KreaCity]

Man it's the first time you really lost me... And as it doesn't happen often (me getting lost in an explanation), you get two thumbs up from me...

[@geraldundone]

Haha. Thanks! I know you meant it as a compliment, but if there's anything you want me to elaborate on, let me know! 😃

[@KreaCity]

@@geraldundone To be perfectly honnest, i looked at the video because of you, because i like how you explain fast and to the point. But in reality, I don't give a ... about flashes at this time in my photography/videography learning process, so maybe it's why i was lost. Lumens, watts etc was very interesting and easy to understand. Keep up the good work, and see you on your next "same bad time" live ;)

[@harleyspork9293]

What happened to the 7.5 meter example? I thought it was going to lead to a solution for power setting for 7.5 meters and then we end up being told to move the subject to 10.6 meters. Haz disappoint.

[@jody1622]

OK so I kind of got it, but I'd love to put the video on slo-mo!

[@xnautro]

till the very end of video i was hoping you say something like 'to calculate this easy you can download my app' ......

[@jody1622]

Can I please.play this in slo mo!

[@markpix]

Nice video, but flash in hhs doesn’t really work this way

[@dennispeterson3691]

I wish flash manufacturers would bring back auto flashes.

[@tw9535]

You are very, very good at explaining this. Great video! Thanks so much for creating this. Hope you don't mind I played at 0.75x speed so I could keep up easier. Hey, a great video that is worth watching is also worth spending fractionally more time with.

[@geraldundone]

Thanks so much! I don't mind at all--as long as the video was helpful. 😃👍

[@Dangoldmedia]

My brain melted

[@pranavgaonkar8660]

Hey Siri, order a lightmeter for me

[@bobceffo]

I owe you a beer

[@Reyfox1]

Thanks! But now I need aspirin.... :D

[@geraldundone]

😜