Aperture Priority and Flash Adjustments

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Apr 7, 2008
Hi, I am a Canon 450D user and I have some doubts that I hope you guys can enlighten me.
When I use aperture priority mode and adjusted the aperture to f/2.8, I get a corresponding shutter speed of 1/10 at a constant ISO of 800. When I activated the built in flash, and set the aperture size to the same f/2.8, I still get the same shutter speed of 1/10 at a constant ISO of 800. The picture appears overexposed. Why is this the case? I thought when I use flash, the shutter speed should become faster? :dunno:

(Apparently, under P-mode, when the flash is activated, the shutter speed goes to 1/60 and when it is not activated, the shutter speed becomes slower than 1/60)


Staff member
Jan 3, 2008
Pasir Ris
I have checked the 450D manual before recommending you a "RTFM" :) - but this manual is terrible. Having a look at the 350D manual I have found a single page explaining the difference in using flash in P mode and Av mode.
The 350D manual can be downloaded from http://eosdoc.com/manuals/ - Page 96 describes it.
P - For fully automated flash photography.
That means it's all done as in Auto mode, just that you can decide whether you want to use the flash or not. The cam will adjust accordingly.
Av - Aperture remains priority, shutter will be set according ISO and metering without flash.
In night shots the flash is used to light up foreground objects. The shutter speed remains to enable correct exposure of the darker background. This is called "Slow Sync" and will require a tripod. You can adjust the exposure manually if the foreground is too overexposed (page 89 in 350D manual, page 78 in 450D manual).
If you don't want this at all you can switch to "Fast Sync" (in the Custom Function) - then the shutter is set to 1/200sec if you activate the flash. Whether this is useful for your picture is up to your judgment.
Best is to play around with both functions. Both make sense depending on purpose, intention and conditions around.


Senior Member
Oct 4, 2004
This is a little weird actually - under low ambient light the flash and ambient should be about 1:1 (well, this depends on how it meters, perhaps a little more flash) in Av mode.
I just tried with my 400D - same framing (blank wall actually), F/5.6, ISO 1600, it reads 1/20s when flash is off, 1/45s when flash is on.

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