flash - exposure question

ok.
sorry but i am trying to wrap my head around this

i had a friend with more experience claim that if I were to use manual settings, I would get a more fill light instead of ?ambient? light than if I used the AV setting. Being that he had more experience than I , I just nodded and vowed to check it out. Can't figure this one out.

if i had a composition on M
with
f2.8, ISO 100, 1/125 and flash activation on ettlII of X
and the exact same on AV, I would expect the exact same exposure, right? (i assume that the variable would be the flash activation of X in this example)

but then if I had the exact same composition
but instead one of the parameters changed by one stop

e.g.
f4.0 ISO 100 1/125 and flash activation of X + 1 stop (assuming that X is under the 100% ability of the flash unit)
or f2.8 iso 100 1/250 and flash activation of X +1 stop (assuming that X is under the 100% ability of the flash light)

would one expect that there would be a difference in the quality of the fill light (assuming same position and direction of the flash, full recycling times, and otherwise controlled environment)

rereading my longwinded question - I guess it can be boiled down to - is there a difference in ettlII behaviour in different modes- specifically M vs AV.....

I hope I have made the question clear and not too messy (and no technical errors), but if that is not the case, please focus on the idea and not my mistake in numbers. many thanks.

Let me know if i make the wrong assumption. But in FILL LIGHT do you mean the primary light from your flash, and ambient light is light in the room? I'm confused by your use. Usually FILL means a secondary source.

With Flash on ETTL, the aperture and flash worry about making the correct exposure on your subject for any given ISO. The Shutter will expose for the ambient light, whether you are on manual or AV. The only difference is on manual you are setting the shutter and aperture, and on AV the camera is setting the shutter.

So in your example, with all other parameters being the same, changing the aperture from 2.8 to 4, would make the flash produce more like, I would think, the shutter would stay open longer in AV to maintain the same ambient light. If you were in manual, changing from 2.8 to 4, with the same shutter would make the room darker by one stop.

thanks kjonnnn for your answer. I am trying to understand something that someone else said, so again, please excuse the inaccuracies.
as far as I understood from the guy - that given the same parameters on AV vs M, that the composition taken while in M mode would benefit from the ambient light more and make the on camera flash (we didn't specify about on camera or OCF) produce a fill light.

you last paragraph of course makes sense....and maybe that is the basis of my not understanding. Given these basic statements, I don't understand how a flash would act differently given the same parameters (assuming that all the parts work! hehehe) unless it is a part of the ettl II protocol to act differently in M mode vs AV mode.

the problem here is :if the existing light is low , setting iso100 is wrong, the background will be very dark. the flash will not capable to iluminate the entire scene. Increase iso , put the camera on manual mode , large aperture and you ll see the difference!

ceriltheblade wrote in post #11992814
thanks kjonnnn for your answer. I am trying to understand something that someone else said, so again, please excuse the inaccuracies.
as far as I understood from the guy - that given the same parameters on AV vs M, that the composition taken while in M mode would benefit from the ambient light more and make the on camera flash (we didn't specify about on camera or OCF) produce a fill light.

you last paragraph of course makes sense....and maybe that is the basis of my not understanding. Given these basic statements, I don't understand how a flash would act differently given the same parameters (assuming that all the parts work! hehehe) unless it is a part of the ettl II protocol to act differently in M mode vs AV mode.

I think I know what you're staying. If the the camera had the same settings, would there be a difference in the exposure, JUST because you were in Manual or in AV. Is that what you are saying?

If so, the only difference in exposure using Manual or AV, would be if the light on the scene changed. If you were in manual, and the scene lighting changed it would be over or under exposed. If you were in AV, the camera would make the adjustment. Im I on the right track?

Question. What do you mean by Flash Activation of X?

kjonnnn wrote in post #11992854
I think I know what you're staying. If the the camera had the same settings, would there be a difference in the exposure, JUST because you were in Manual or in AV. Is that what you are saying?

If so, the only difference in exposure using Manual or AV, would be if the light on the scene changed. If you were in manual, and the scene lighting changed it would be over or under exposed. If you were in AV, the camera would make the adjustment. Im I on the right track?

exactly correct - where the variable would be how the flash behaves.

there is a differnt thread which was going on this past week about differences in the AV vs M without flash.....and then this guy/friend went on and confused me on something that I THOUGHT I understood at least on a basic level.

thanks a lot!

kjonnnn wrote in post #11992877
Question. What do you mean by Flash Activation of X?

i didn't know how to notate some value of the strength of a flash burst - so I just called it "X"
if it was in manual control I could call it some fraction of the fullpower of the flash (e.g. 1/1 1/2 1/4 etc), but ettl-ii system contols that (and I've not seen any specific notation of power used when in ettl-ii mode in the exif) when not in manual flash mode - and I am specifically speaking about the ettl-ii anyway....

so I assumed that ettl-ii had the flash fire with amount "x" assuming that "X" is not 100% of full power

hope that clears that up
thanks again

canonistul wrote in post #11992840
the problem here is :if the existing light is low , setting iso100 is wrong, the background will be very dark. the flash will not capable to iluminate the entire scene. Increase iso , put the camera on manual mode , large aperture and you ll see the difference!

thanks for your input. the question is a bit theoretical in that I have not actually taken the photograph described above.
if it helps, please understand the same example with ISO 400
and or aperture value of f1.4 vs aperture f2
I am looking a bit for the principle involved.

again thanks.

Theoretically ETTL flash will always give the same correct exposure. It doesn't matter what the camera settings are. (P, Manual, Av, or Tv, etc.)

If you do something to change ISO, aperture, and shutter speed values, the ambient light will change in the areas that are not illuminated by the flash. But theoretically, the areas illuminated by the flash will not change. To understand this you must realize that every photo using flash is actually TWO exposures. One exposure for ambient, and one exposure for flash. With ETTL, the flash portion of the exposure is fully automatic and theoretically will not change.

thanks.
knowing the person who told me this - and he is not a dummy by any means
i can only assume that i misunderstood what he was trying to tell me.
maybe he was trying to convince me to move from AV to M mode because of the greater flexibility in taking advantage of ambient light and trying to use the flash only/mostly for fill.

thanks for the help in trying to understand all. I hope I can make use of the advice he gave.

Actually, AV mode actually causes the camera to use flash in a different manner than M or P, a simple test will confirm this. If you put the camera in AV mode and use f4, the camera will use a shutter speed that gives a proper exposure as if flash was not going to be applied to the scene. Then when the shutter is tripped and ETTL does it's magic instant output calculation, there isn't really much light that needs to be added.

With M mode, it will give you enough light (up to limit of max power) to make your settings work..

P/Auto mode drops to some default settings (usually 1/60, ISO 400, f5.6) and uses flash as M mode above

SO while the way ETTL calculates output is the same on every mode, the photos you get in AV mode are not always as you'd expect coming from P or M mode. If you are using AV indoors and set ISO 200 and F5.6 and expect the camera to pick a normal shutter speed, you will be surprised.

ceriltheblade wrote in post #11992718
i had a friend with more experience claim that if I were to use manual settings, I would get a more fill light instead of ?ambient? light than if I used the AV setting. Being that he had more experience than I , I just nodded and vowed to check it out. Can't figure this one out..

That statement is NOT NECESSARILY true.

With Av, the camera determines how much background light to gather via control of shutter speed, while the ETTL flash metering determines how much flash to output for correct exposure of the subject at the focus point.

With M, the photographer determines how much background light to gather via control of shutter speed, while the ETTL flash metering determines how much flash to output for correct exposure of the subject at the focus point. The photographer might want the background to be underexposed (compared to flash exposure) by -2EV or -1EV or 0EV (same as ambient). The main benefit of M on camera with flash is that the camera will never try to use a stupidly slow shutter speed (which the photographer cannot hand hold without camera motion blur!)

tkbslc wrote in post #11995289
Actually, AV mode actually causes the camera to use flash in a different manner than M or P, a simple test will confirm this. If you put the camera in AV mode and use f4, the camera will use a shutter speed that gives a proper exposure as if flash was not going to be applied to the scene. Then when the shutter is tripped and ETTL does it's magic instant output calculation, there isn't really much light that needs to be added.

With M mode, it will give you enough light (up to limit of max power) to make your settings work..

P/Auto mode drops to some default settings (usually 1/60, ISO 400, f5.6) and uses flash as M mode above

SO while the way ETTL calculates output is the same on every mode, the photos you get in AV mode are not always as you'd expect coming from P or M mode. If you are using AV indoors and set ISO 200 and F5.6 and expect the camera to pick a normal shutter speed, you will be surprised.

Red text is a very important point which is widely misunderstood and miscommunicated. It often gets stated "The flash behaves as fill". No, the flash always behaves exactly the same, it is merely the balance of flash vs. ambient which determines if you call it 'Main light' (i.e., more powerful than ambient light exposure) or 'Fill light' (i.e., less powerful than ambient light exposure)

wilt and tkbslc - thanks for the explanations.

So there **IS** a different protocol on flash use between settings.

I was missing that piece of information.

Now with those facts, I guess I need to figure out better how to use the flash. (thinking to himself outloud - I think i saw a video about the 430 and 580 exii - maybe I will check into it)


for those poeple who understand - WHY does the flash system act this way? thanks a lot!