I'm using an EOS-3 and a 420EX flash. In Av (aperature priority) mode, in dim light, let's say I meter a scene with the flash turned off at f/2.8 and 1/10sec. Now I turn the flash on. Once the flash is powered up, the indicated exposure changes to f/2.8 and 1/20sec. If I repeat the same experiment except in brighter light, turning the flash on does not change the indicated exposure (for example, I might have meter readings of f/5.6 and 1/15sec with flash off or on).

Anyone know what's going on here?

My assumption had been that, in Av, Tv, or M, the indicated exposure readings from the meter are for the scene, completely independent of the flash; and that the flash has it's own metering (E-TTL system) via preflash. But clearly there is some interaction here, and I would like to understand it.

-- Seth Pollack (sethpollack@hotmail.com), January 07, 2002

Answers

Check my previous thread on this subject:

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=006SBz

I am convinced the behavior you described is normal for the EOS-3 and EOS-1v.

-- Kenneth Katz (socks@bestweb.net), January 07, 2002.

Does anyone know why Canon did this? Unfortunately it makes Av and Tv modes a bit less predictable, and so a bit less useful. Any clever workarounds? I guess M mode is the way to go, which is fine when you have time, but Av is so convenient for quick shots.

-- Seth Pollack (sethpollack@hotmail.com), January 07, 2002.

Pretty Bizarre system activity!!

I checked out changing a few of the custom functions and the camera and flash still behaved the same way. I turned off auto fill flash reduction(EOS 3 CF 14) and switched e-TTL to TTL (550EX CF 3). The camera behaved the same way. SO, I guess it has nothing to do with either of those.

-- Roger S. (rashrader@hotmail.com), January 08, 2002.

maybe the reason this happened in the low light was that the camera also metered the relatively bright auto assist lamp of the 550EX?

-- jeff nakayama (moonduck22@hotmail.com), January 08, 2002.

maybe the reason this happened in the low light was that the camera also metered the relatively bright auto assist lamp of the 420EX?

-- jeff nakayama (moonduck22@hotmail.com), January 08, 2002.

i meant to say 420 EX.

-- jeff nakayama (moonduck22@hotmail.com), January 08, 2002.

try disabling the auto focus assist light and see if it still happens.

-- jeff nakayama (moonduck22@hotmail.com), January 08, 2002.

It's not the focus-assist lamp. The behavior still happens with autofocus disabled, and the behavior does not happen in M mode with autofocus and the focus-assist light.

-- Seth Pollack (sethpollack@home.com), January 08, 2002.

I can think of two possibilities:

1) The metering pattern changed when the flash was turned on. See the old EOS Flash FAQ for details. A new Flash FAQ is being drafted by NKG et al.

2) The "Flash ready" indicator in the viewfinder might be spilling light onto the metering sensor. If this is so, the effect should be even more prominent with the lens cap on and complete darkness.

-- Julian Loke (elan7e-owner@yahoogroups.com), January 08, 2002.

“The metering pattern changed when the flash was turned on”

When I did my tests, (a) camera was on evaluative and (b) I metered an evenly lit blank wall .

“The "Flash ready" indicator in the viewfinder might be spilling light onto the metering sensor. If this is so, the effect should be even more prominent with the lens cap on and complete darkness.”

And this adds 1 full stop of light! Besides, lens cap was off and EV 7-8 is not that dark, but is equivalent to a dimly lit interior. The flash ready light also goes on in “M” mode (which did not show this + 1 stop ambient behavior).

I agree with Seth’s response with respect to the AF assist light. In all due respects, perhaps only people who use EOS 3’s and/or EOS 1v’s should respond to this post.

-- Kenneth Katz (socks@bestweb.net), January 09, 2002.

clearly there is some interaction here, and I would like to understand it.
... In all due respects, perhaps only people who use EOS 3’s and/or EOS 1v’s should respond to this post.

Since Jorrit de Jager responded in November that the EOS 50E and I can confirm that my Elan 7E and 550EX also behave in the same way (Evaluative metering), it would indicate that the +1EV background exposure compensation is a feature of E-TTL and may not be limited to the EOS 3 and/or EOS 1v.

I therefore consider that your suggestion for only EOS 3 and/or EOS 1v responders runs contrary to Seth Pollack's original request for greater understanding on this EOS FAQ forum.

-- Julian Loke (elan7e-owner@yahoogroups.com), January 10, 2002.

I stand corrected.

This is the first time I heard that this behavior exists in cameras other than a 3 or a 1v (so sorry). I checked my Elan II again and it does not underexpose ambient light when either the 550EX or the pop- up flash is powered up.

-- kenneth katz (socks@bestweb.net), January 11, 2002.

reckon u fellas should check out this site...pretty good place to understand how the eos flash system works... hav good links 2..

http://www.teladesign.com/photo/eos-flash/

-- rendra (seraphim_75@yahoo.com), January 29, 2002.