Understanding Lighting - Strobist

[KevinStecyk]

Hi Folks,

I have another lighting question from the Strobist.

Strobist: Lighting 102: 3.3 - Balancing Flash/Ambient Indoors

In article, the author states the following:

This is simply because we are working so far from the ambient exposure of the room. If the flash is much more powerful than a given, combined ambient setting, I call that "working above the ambient," as in, "I was shooting flash at f/4, working 4 stops above the ambient."

That tells you that at f/4, my chosen shutter speed was four stops too high for the ambient exposure.

How does he know it is four stops? Just slightly earlier, he made reference to the room being five stops underexposed. How does he know precisely how many stops?

Second question, as he progresses through his series of shots, the background becomes brighter--which I understand--but the lighting on the camera almost doesn't change. Why doesn't the camera become markedly brighter as well?

I understand that aperture controls the subject brightness and that time controls the ambient brightness, generally. I am puzzled because the time duration is sufficient to completely light the room (ambient area) yet the difference on the lighting of the camera is not large. So to my thinking as the light has increased greatly--from 1/40 second to 0.6 seconds--there ought to be large difference on the lighting of the camera itself.

Put differently, if the flash was removed, the 0.6 seconds ought to have lit the camera by itself. The added flash with the 0.6 seconds of exposure should really brighten the camera compared to his earlier shot with a flash and only 1/40 of a second of exposure.

I am probably missing something simple.

Thank you for your indulgence.

Best regards,
Kevin

[michaelnotar]

dont know hows he trying to get at it, but i do it this way, and its simple.

TV=controlls ambient exp, AV=both flash and amb, flash distance to subject/power= controls flash exp.

expose for ambient as much or little as you want it, then add flash. if you want less ambient, choose a faster TV, for more ambient, slow down the TV. you can make equivalent exposures with different TV/Av and get different ratios betweeen flash and ambient.

when ambient is 3 + stops under flash exposure if wont influence exposure.

[ChrisPerry]

I've worked on this in my head for trying to balance ambient with flash when working outside near sunset or in a church for the formals where I don't want the BG to be a black hole.

I'm not going to explain this very well, but here goes how I do it.

I set up the flash at some arbitrary power setting (I'm using a studio strobe on battery power, but a flash is a flash as far as the procedure goes). I use my meter to read the flash at the subject's position. (sekonic L358). It tells me an exposure.

I'm guessing on ISO at this point (400 usually) and see what it says - 5.6, fine. 2.8...add flash. F7 is fine too for groups in a church. F8 or more I'll dial the flash down a bit, as I don't need that much DOF and want faster recycle.
I then pick 1/30 on the camera at 5.6 (if that's what the meter read the flash as. the shutter speed is arbitrary at this point) and take a test shot - how does the BG look? Too dark? Then slow the shutter down. Too bright? Faster shutter.

If the BG is way too dark then up the ISO to 800 - but now you've also doubled the flash contribution so you need to 1/2 the aperture or turn down the flash to balance it out again.

I don't know how to figure the stop difference between the flash and ambient on shutter speed alone - up the shutter to 1/200 and the subject's exposure won't change, but the BG will go black - 1/30 is about 3 stops under 1/200. Does that mean it the ambient is 3 stops under the flash? I suppose. But I don't really care unless I want the BG to be truly black.

[KevinStecyk]

Chris and Michael,

I am being fairly specific in my questions because it is the learning that I am after, not the technique to accomplish a specific shot.

Michael, I understood your post perfectly--thank you. I am still wanting to increase my understanding of the material covered in Strobist.

Chris, I understand your methodology as well. You are chimping to assist you in getting the correct exposure.

I want to know if I missed something in the Strobist article.

Thank you.

Best regards,
Kevin

[michaelnotar]

not to brag, but i was lucky enough to attend brooks institute of photography. what i outlined was info from my time there. it simply works. i dont know what the strobist is trying to say in his technique and perhaps he is going to fast to type and didnt include some details in the article.

but learn and practice what has been said as it gives you a methodical system to get through any challenging situation effeciently. i wouldnt waste your time trying to find another way to do it like the strobist.

[KevinStecyk]

Hi Michael,

Yes, I realize that to

1. Alter flash and ambient, adjust aperture;
2. Alter ambient, adjust time;
3. Alter flash and not ambient, adjust both aperture and time.

I've got that.

It isn't the formulaic process that I am after. Rather, I want to understand how the Strobist knew 4 ƒ stops and why when the Strobist increased time dramatically, the flash (camera) barely changed. On the last topic, intuitively I would think that ambient lighting plus flash would increase the exposure noticeably.

I just discovered the answer to the 4 stops. He increased the time in small increments over one stop and there was no change in the background light, except for his last change. Thus, he was 4 (=3 + 1) ƒ stops over ambient. As others have mentioned, 3 stops and no difference. So in this case where the very last adjustment made a tiny difference, he knew he was 3 ƒ stops away at that point. He starting point was an additional ƒ stop. One question solved.

I just copied and pasted an early photo and the last photo in the series to compare each side by side. Although not dramatically different, the last photo of the camera does appear brighter and lighter. So I think flash plus plenty of ambient light does affect the flash lit subject.

Going back to point 2 above, my guess is that, if you make dramatic changes in time of a couple ƒ stops, it will have an effect on the flash exposure. This is what I want to understand better.

If I have a solid understanding of the theory, then I will be better able to adapt to different situations.

And I agree, you are fortunate to have gone to Brooks.

Best regards,
Kevin

[michaelnotar]

#3 is wrong perhaps a typo. to adjust flash, change power of flash (1/2-1/4 etc) and/or change the distance of the flash to the subject.

"I just discovered the answer to the 4 stops. He increased the time in small increments over one stop and there was no change in the background light, except for his last change. Thus, he was 4 (=3 + 1) ƒ stops over ambient. As others have mentioned, 3 stops and no difference. So in this case where the very last adjustment made a tiny difference, he knew he was 3 ƒ stops away at that point. He starting point was an additional ƒ stop. One question solved."

i think your 3 stop reference is differant the one i meant. i was talking about additive exposure compensation. are you familiar with this.?

" So I think flash plus plenty of ambient light does affect the flash lit subject."

if both sources illuminated the subject, then the subject will be brighter. perhaps the small change in brightness is from one of the light sources that contributed only fractionally to the exposure.

dramatic changes in TV will NOT change flash exposure. all cameras sync less than 1/1000th sec or less, most around 1/250th. the time a flash produces light or the flash duration is around 1/1000-1/2000th sec. so the camera shutterspeed can not go truncate the flash exposure since the flash is on and off in a 1/4-1/8 of the time the shutter is open. so flash power, distance to subject and aperature are left to control its exposure.

[Pro Photo Store]

Visit the B&H Pro Photo Store for the best Gear Prices and regular specials!

Brilliant Color for any Digital SLR
White Balance Cap
The Pros Agree on ColorRight
What People Are Saying