During my last vacation I made a conscious effort to shoot my Canon EOS T3i almost exclusively on Aperture Priority (AV) mode. For the most part, this worked out really well. It gave me some creative control, but didn't have the I'm paying more attention to my settings than my subject feeling that Manual mode gives me. I noticed that there were a number of times when I was falling back to the basic automatic mode and it wasn't immediately obvious why that was.
Then it hit me as to what the scenario was: I was often indoors and needed to shoot with a flash. When I turned on the flash in AV mode, the shutter speed wouldn't really change. If I was turning on the flash because there wasn't enough light to let me get the shot while holding the camera steady, adding the flash didn't appear to help. What the heck was going on? Thanks to Google, I now know that my camera wasn't behaving randomly. Found these three articles which explain the problem and solution well: Canon Cameras – Using Flash in Aperture Priority (Av) Mode , Flash Photography with Canon EOS Cameras and Fill-in flash with program & aperture priority modes.
First off, this article articulates the problem quite well:
Canon engineers decided long ago that the best way to use flash is to have it blend in with the scene’s ambient light rather than over power it. Because of this a Canon camera in Aperture Priority (Av) Mode will assess the available light at the photographer selected aperture, set the proper shutter speed for exposure accordingly and then balance in some flash fill light.
This method of adding flash produces lovely results, unfortunately when Av mode is used with flash indoors the shutter speeds can be far too slow for hand holding the camera.
In other words, turning on the flash in AV mode on my EOS may not appear to do much (the shutter speed and aperture don't change), but the camera is working. Specifically, it's working to add just enough light to the scene so the flash works with, not overpowers, the ambient light. This is wonderful to know, and means that I'll be more inclined to turn on the flash while in AV mode. In theory, it shouldn't overpower the scene, but fill it in.
But what about my original problem just needing to get the shot minus any ambient light? Turns out, Program (P) mode is designed to handle this case. I just tried it: I pointed my camera to a dark scene and switched to P mode. The result is a shutter speed at 1/60th of a second and an aperture to match the flash. This article explains it even better:
The overriding principle of Program (P) mode in flash photography is that the camera tries to set a high shutter speed so that you can hold your camera by hand and not rely on a tripod. If that means the background is dark, so be it.
The P mode is actually a little smarter than that. Apparently, it uses two rules:
1) If ambient light levels are fairly bright (above 13 EV) then P mode assumes you want to fill-flash your foreground subject. It meters for ambient light and uses flash, usually at a low-power setting, to fill in the foreground.
2) If ambient light levels are not bright (below 10 EV) then P mode assumes that you want to illuminate the foreground subject with the flash. It sets a shutter speed between 1/60 sec and the fastest X-sync speed (see above) your camera can attain. The aperture is determined by the camera’s built-in program.
One last bit of useful information. The Manual (M) mode also plays nicely with the flash:
In manual exposure mode you specify both the aperture and shutter speed, and your exposure settings will determine how the background (ambient lighting) is exposed. The subject, however, can still be illuminated by the automatic flash metering system since the flash can automatically calculate flash output levels for you. This is a marked contrast to the olden days, when photographers would carry around little flash exposure tables with them in order to work out manual flash settings.
Ever since high school, I've had a keen understanding of how P, Av, Tv and M modes work. Sure, I've slacked off and relied on fully automatic at times, but the concept of exposure was always there in the back of my head. What I hadn't realized till recently was how little understanding I've had about how the flash works with all of this. I assumed the flash was pretty dumb, just dumping out gobs of light. As I've learned, this couldn't be farther from the case. So now I've got a whole bunch of new tools in the toolbox.