Please help me understand... (flash)

Sorry if this question was asked a 1000 times already.

I have a new 400EX. Yesterday we were still without power (5th day straight !) and I decided to play with it. I took a few pics of my son, standing just a couple yards away. Here' s what I don't get:

If I am in any "IQ" mode (P, Av, Tv etc) I couldn't get anything higher than 1/60 s at 2.8 at 800 ISO. Yet when I put it in Manual, set 1/125 at 3.5 and set flash to full power the photo is visibly overexposed. The proper exposure seemed to be 1/3 power at 1/100 at 3.5.

So, it's obvious flash had plenty of power, why wouldn't ETTL let me select shutter speed above 60 and aperture above 2.8 ? I know it meters differently in Av and Tv but it wouldn't let me do it even in P. Shouldn't it know that flash can fire enough light to cover higher speeds and smaller apertures ?

Amamba wrote in post #5716817
Sorry if this question was asked a 1000 times already.

I have a new 400EX. Yesterday we were still without power (5th day straight !) and I decided to play with it. I took a few pics of my son, standing just a couple yards away. Here' s what I don't get:

If I am in any "IQ" mode (P, Av, Tv etc) I couldn't get anything higher than 1/60 s at 2.8 at 800 ISO. Yet when I put it in Manual, set 1/125 at 3.5 and set flash to full power the photo is visibly overexposed. The proper exposure seemed to be 1/3 power at 1/100 at 3.5.

So, it's obvious flash had plenty of power, why wouldn't ETTL let me select shutter speed above 60 and aperture above 2.8 ? I know it meters differently in Av and Tv but it wouldn't let me do it even in P. Shouldn't it know that flash can fire enough light to cover higher speeds and smaller apertures ?

Does the flash show with ETTL on the LCD? If not you are shooting manual.

ETTL will only overexpose if you tell it too with FEC (Flash Exposure Compensation) which you should see via the viewfinder or your LCD screen.

Remember modes like Av mode will not take into account the flash is connected. The shutter speed is designated by the ambient light that the camera meters for.

How are you defining "proper exposure"? You say nothing of getting underexposed images, but you did mention M giving you overexposed images on your command. (justifiable)

When in creative modes and ETTL, the camera's going to choose those numbers based on your ISO. You need to raise that to 1600.

elysium wrote in post #5716830
Does the flash show with ETTL on the LCD? If not you are shooting manual.

ETTL will only overexpose if you tell it too with FEC (Flash Exposure Compensation) which you should see via the viewfinder or your LCD screen.

Remember modes like Av mode will not take into account the flash is connected. The shutter speed is designated by the ambient light that the camera meters for.

Yes, it did show ETTL. I had FEC set to + 1 1/2 but flash didn't fire at full power, so it doesn't explain why it won't let me set higher speeds. I.e. in manual with full power it produces a completely whited out photo, so even if I set FEC high it's not going to use nearly 1/1 power to get proper exposure at speeds higher than 60.

Amamba wrote in post #5716851
Yes, it did show ETTL. I had FEC set to + 1 1/2 but flash didn't fire at full power, so it doesn't explain why it won't let me set higher speeds. I.e. in manual with full power it produces a completely whited out photo, so even if I set FEC high it's not going to use nearly 1/1 power to get proper exposure at speeds higher than 60.

Loser your ISO and set your FEC to 0 to start with. Camera is limited to 1/200 of a sec with flash attached (depending on camera body). You would need to enable high speed sync to go faster.

1/60 is fine to start with. You might want to try ISO400 to start with and FEC to 0. Fire a shot and then adjust from there to be honest.

OK, I think I wasn't clear.

At "auto" settings (1/60, 2.8, 800 in P, Av, Tv) I get OK exposures. But flash fires at some fraction of its power. And it doesn't let me go to say 1/80 and 4 @ 800. So to me it must be "thinking" it can't get enough exposure at these speeds / apertures. Yet when I shoot in M at 1/1 it way too powerful. So, if it has more than enough power to let me go to a higher shutter speed and smaller aperture, why won't it let me to ? I am not talking about going to anything above 200, but in my mind I should be able to chose anything higher than 60 (below 200) and flash should compensate for it with higher power burst.

http://photonotes.org/​articles/eos-flash/

Take the time to read it. It explains what you arent understanding and will make all things eos flash clear. Well worth the time and trouble it takes to read it if you want to understand how your flash is (or isnt) working for you.

amonline wrote in post #5716956
Lower your ISO.

How a lower iso would allow me to go to 1/125 & 5 vs 1/60 & 2.8 at 800 ? Sorry, either I wasn't clear or you didn't understand the question. I do know how ISO work, and I didn't ask why it was overexposed in M.

I know why it was overexposed in M setting. The original M setting was just a wild guess and it wasn't right, neither I expected it to be - I just used this as proof that flash had more than enough power, it would completely overexpose at full flash, so flash was not a limiting factor.

What I don't understand is, why in P mode it wouldn't allow me go to faster speed and narrower aperture. Since there was more than ample flash power available.

The way I thought it worked, I select shutter speed (up to 1/200) and aperture, and FEC if needed, and ETTL adjusts flash power to provide necessary lighting to match these settings, as long as there is enough flash power available.

That it wouldn't go above 1/60 and 2.8 tells me that it "thought" flash wouldn't have enough power, which was not true at all.

I.e. if I can get a very well exposed picture at a given ISO at 1/125 and say 5 and flash at 1/4 power in M mode, I should be able to set camera in P mode to 1/125 and 5 and ETTL should adjust flash power to about 1/4 automatically. That it didn't let me go to anything higher than 60 and kept flashing 2.8 (which I assume meant it didn't think it was wide enough) tells me it missed settings completely. Either that or I misunderstand something about the way it's supposed to work. Perhaps it adjusts flash power based on distance alone, not to provide enough light ?

nadtz wrote in post #5716960
http://photonotes.org/​articles/eos-flash/

Take the time to read it. It explains what you arent understanding and will make all things eos flash clear. Well worth the time and trouble it takes to read it if you want to understand how your flash is (or isnt) working for you.

Thank you. I thought flash in Tv worked differently from Av, apparently not - so now I see how these two modes would not let me change settings. Still don't understand why P didn't - I guess I'll try again !

Amamba wrote in post #5718250
How a lower iso would allow me to go to 1/125 & 5 vs 1/60 & 2.8 at 800 ? Sorry, either I wasn't clear or you didn't understand the question...

You are correct. I misread something up there.

You are shooting in a creative mode. If you continue to choose 800 as your ISO, the camera will adjust the alternative parameters to reflect the settings you are getting. Thus, you will never get higher shutterspeeds.

If you want a higher shutter speed, you actually need to increase your ISO. If you go to 1600, obviously you will be able to gain in your shutter speed at the same aperture setting.

So, set the camera to Av mode. Set the ISO to 1600 and set your Av to 2.8. Now, your SS will double.

Alternatively, you can do this in Tv mode. Set the ISO to 1600 and set your Tv to 200. (or what you desire) Now, your Av will bottom out and your flash will do it's work.

With the flash in ETTL, none of these changes matter. It will do it's job.