Guide Number Misconceptions / Understanding Flash Power on Strobes & Speedlights

Explaining the math behind a flash's guide number, how it relates to f-stop, and more practical formulas for nailing exposure on your strobes & speedlights.
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Table of Contents:
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0:00 - Intro
0:48 - Guide Number Formula
1:02 - Canon 600EX-RT Guide Number Explained
1:57 - Misconception: Forgetting about f/1
2:57 - Using Guide Number to Find Subject Distance
3:51 - What If the Subject Was Closer? (Inverse-Square Law)
4:48 - Practical Uses: How to Figure out Correct Flash Power
5:23 - Why Flash Calculators Don't Solve for Flash Power
5:50 - How to Calculate Flash Power in Your Head
6:12 - Find the Correct f-stop at Full Power
6:36 - One Stop of Flash Power Equals One f-stop
6:58 - Powers of 2 Make This Even Easier
7:25 - How to Memorize All the f-stops Easily
7:44 - A Simpler Way without Memorizing Numbers - Turn the Dials!
8:08 - ISO Impact on Guide Number
8:33 - What about Shutter Speed? Does It Affect Flash Power?
8:57 - Final Thoughts: Know Your Lights!

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#flash #photography #guide

[AParticularPointofView]

This video blew my frontal cortex completely out. 👍

[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!!!

[nckotene]

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.

[azrinaz]

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

[jeno]

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.

[luisarevalo]

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!

[MiaogisTeas]

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

[poundingthenail]

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

[ramongutierrez]

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

[carlosdias]

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.

[zahreelashehzaadi]

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...

[andreslinn]

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

[BrasicOne]

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

[bigfatfitnessguy]

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

[marvoski]

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

[hazelvee]

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

[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!

[uctynguzb]

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