A question about ETTL and underexposed picture

bobbyz wrote in post #12037396
Bold part is incorrect - changing aperture affects ambient just like changing ISO. I don't know when this shutter controls ambient and aperture controls flash nonsense will stop. Hope Dave Hobby and folks give proper information on their blogs.

But there is a great amount of truth to what is in green, but it is an incomplete concept statement, as it is typically stated. If we restate it, this is 100% true:

• Shutter affects ambient without affecting flash (ignoring HSS for this discussion) and
  
• Aperture controls brightness of flash exposure when flash is in Manual, and if shutter speed offsets the change in aperture the ambient exposure is unaffected by the aperture change

smorter wrote in post #12037145
You have a point in that I am being extreme. It is what I do, but not suitable for all situations

I do have to disagree about ETTL vs manual though. ETTL is excellent but you have to know when it is (designed to) fail. It handles white tablecloths horribly, but it is consistently bad, and reliably bad...if that makes sense. It is totally consistent and reliable if you know it's weaknesses.

I would never dare use manual flash for anything other than setups or off camera flash. ETTL takes just a fraction of the time and headache of manual flash trial and error

haha I completely understand
It's like when you know their limitations, you do things to compensate for it.
After getting more into OCF and understanding more about my camera AND flash, I've become more comfortable with manual. Just a personal thing of mine I guess.

bobbyz wrote in post #12037396
Bold part is incorrect - changing aperture affects ambient just like changing ISO. I don't know when this shutter controls ambient and aperture controls flash nonsense will stop. Hope Dave Hobby and folks give proper information on their blogs.

yes true, but the real world impact of aperture on ambient vs flash power, I'd still stick to my original statement.

Don't understand what you mean by real world impact. Leave flash at say 1/4 power. Open up aperture by 1 stop. Both flash exposure as well as ambient exposure went up by 1 stop. Now stop down aperture by 1 stop. Both ambient as well as flash exposure went down by 1 stop. The ratio between flash and ambient remained same in both cases.

Do same with ISO, same effect, nothing different.

bobbyz wrote in post #12038738
Don't understand what you mean by real world impact. Leave flash at say 1/4 power. Open up aperture by 1 stop. Both flash exposure as well as ambient exposure went up by 1 stop. Now stop down aperture by 1 stop. Both ambient as well as flash exposure went down by 1 stop. The ratio between flash and ambient remained same in both cases.

Do same with ISO, same effect, nothing different.

OK,, take a flash meter reading and at that given distance the correct exposure for the subject reads f/5.6 (for argument sake), irrespective of the shutterspeed selected.. Open up to f/4 and the subject will be overexposed by +1 EV.. Close down to f/8 and the subject will be underexposed by -1 EV.. The background exposure will also be effected..

Now,, take a meter reading and keep the aperture to the exposure the flash meter reads,, say f/4.. Decrease or increase the shutterspeed and you'll find the flash exposure for the subject won't change but the background brightness will.. The subject brightness won't change until your shutterspeed is slow enough to register any ambient light falling on the subject..

If you change the ISO and keep the aperture and shutterspeed constant, the brightness of the entire image will change by the amount the ISO is changed..

Wilt wrote in post #12037566
Just curious, Smorter...do you shoot with ETTL in Evaluative CFn, or in Average CFn?

I use Evaluative, but I have heard suggestions that average gives more consistent exposures. Maybe you're able to shed some light on it, but I suspect it may be because average is less "fooled" by things like mirrors and tablecloths in the frame, whereas evaluative may overcompensate for them

yogestee wrote in post #12040645
OK,, take a flash meter reading and at that given distance the correct exposure for the subject reads f/5.6 (for argument sake), irrespective of the shutterspeed selected.. Open up to f/4 and the subject will be overexposed by +1 EV.. Close down to f/8 and the subject will be underexposed by -1 EV.. The background exposure will also be effected..

Now,, take a meter reading and keep the aperture to the exposure the flash meter reads,, say f/4.. Decrease or increase the shutterspeed and you'll find the flash exposure for the subject won't change but the background brightness will.. The subject brightness won't change until your shutterspeed is slow enough to register any ambient light falling on the subject..

If you change the ISO and keep the aperture and shutterspeed constant, the brightness of the entire image will change by the amount the ISO is changed..

You said exatcly what I am saying except that bold part should end with "background exposure will also change by 1 stop." Try it yourself.

ISO and Aperture effect both flash as well as ambient by same amount. Folks here are saying only ISO will change ambient and flash by same amount.

smorter wrote in post #12042869
I use Evaluative, but I have heard suggestions that average gives more consistent exposures. Maybe you're able to shed some light on it, but I suspect it may be because average is less "fooled" by things like mirrors and tablecloths in the frame, whereas evaluative may overcompensate for them

Outside evaluative, indoors average, that is how I do it.

smorter wrote in post #12042869
I use Evaluative, but I have heard suggestions that average gives more consistent exposures. Maybe you're able to shed some light on it, but I suspect it may be because average is less "fooled" by things like mirrors and tablecloths in the frame, whereas evaluative may overcompensate for them

That's why I asked, after your prior comment, " it is consistently bad, and reliably bad...if that makes sense". I was going to suggest trying Average, not Evaluative flash metering, as bobbyz already has done.