One Stop Under

If you shoot with a Canon EOS camera and use flash (even the in-built one), the Canon EOS Flash FAQ is an invaluable resource. It explains in detail all about how Canon flashes work and how to use them to achieve your desired results.

This part in particular explained some flash exposure problems I was having:

P, Av, Tv and M modes all meter for flash in different ways. See the section on “EOS flash confusion” for details. Here’s the short version, which repeats some of the points made in previous FAQ questions.

Keep in mind that the camera meters for ambient (existing) light conditions and flash illumination independently.

P (program) mode keeps the shutter speed between 1/60 sec and the maximum flash sync speed your camera can handle. It does this so that you shouldn’t need a tripod, even if light levels are low. It then tries to illuminate the foreground using flash.

Av (aperture priority) and Tv (shutter speed priority) modes set the shutter speed or aperture to expose for the existing light conditions correctly. They then fill in the foreground using flash. If light levels are low you will need a tripod to avoid blur.

M (manual exposure) mode lets you set both aperture and shutter speed to be whatever you want. The camera then automatically controls the illumination of the foreground subject using flash.

There’s a good section on the new E-TTL II system, introduced with the EOS 1D Mark II and also supported by the EOS 30V/33V/7S/Elan 7N/Elan 7EN, EOS 20D/20Da, EOS 350D/Rebel X Digital/Kiss N Digital.

It’s definitely worth printing out this page (and the ones linked from it), studying it, and keeping it in your camera bag.

Filed under: Canon,Technique
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