cgardner
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Here's a different take on the topic of exposure....
In practical terms for a beginner the easiest way to learn exposure is to start in P mode and observe what the camera does to select exposure for various scenes. Depending on light level it will pick various combinations of aperture and shutter speeds. It will not always provide the correct exposure, so you need to adjust via the Exposure Compensation control (EC). The easiest way to know when exposure is correct or not is to put the playback into "info" mode where overexposed areas become blacked out.
After using P mode for awhile graduate to Av mode. Av mode allows you to select the aperture while the camera picks whatever shutter speed the meter thinks is necessary for correct exposure. As with P mode its usually necessary to "second guess" the camera results with +/- EC
You will probably use Av mode more than any other automatic one because one of the more useful creative tools in photography is the control of depth of focus, more commonly called "depth of field" or DOF to separate foreground and background. Throwing the background out of focus in a photo mimics the way our eyes focus on only 2% in the very center of our field of view and tune everything else out. So in most shooting situations other than sports where stopping action is important, the first creative decision made is what aperture to use to control foreground / background separation.
For any given lens as you increase the f/stop number the aperture gets smaller and DOF increase, but depth of the zone which is in focus will vary depending on the distance you focus on and the focal length of the lens. You can't actually see the DOF of the selected aperture in the viewfinder because the camera always focuses with the lens wide open to make the viewfinder as bright as possible. But if you press the button to the left of the lens, below the lens release button (i.e. the DOF preview button) it will close the aperture to what you've selected so you can preview the DOF.
Experiment shooting with various apertures using the DOF preview button to get an understanding of DOF control with your lenses at different distances. Many become enamored by extremely shallow DOF, but keep in mind the benchmark for "normal" is the DOF we experience with our eye balls. When DOF gets more shallow than our eyes would perceive in person in an similar situation the extremely shallow DOF can actually become a distraction rather than a means to mimic how the eyes and brain tune out the unimportant background.
In low light situations in Av mode there will be a point where the camera will select a shutter speed too slow for you to hand hold without blur. Your choices there are either to put the camera on a tripod or raise the ISO speed. When you raise ISO the camera doesn't capture more light, it simply amplifies what was captured more. Amplifying the image data also amplifies the noise, which accounts for the rainbow grain of high ISO images.
As in P mode the easiest way to know when exposure is correct or not is to put the playback into "info" mode where overexposed areas become blacked out. To get perfect exposure when shooting portraits have the subject hold a white terry wash cloth next to their face. Keep the towel 1/3 stop (one dial click) below the point where it starts to black out in the overexposure warning and your exposure will be perfect.
Its also quite useful to include a gray card reference for white balance
The one shown above is the 4x5 card from the Kodak Gray card kit R-27 and a tri-tone QPCard
Once you get the hang of controlling exposure via the over-exposure warning (OEW) and those standard targets you'll be able to do it on highlights in scenes like landscapes. Most outdoor scenes will have specular, mirror like reflections off objects. So if shooting something like a white car you'd adjust exposure so the reflection off the chrome was clipping in the OEW but the white paint of the car isn't.
A couple final notes with respect to aperture:
In terms of optical performance most lenses are sharpest about 2-3 stops smaller than wide open. Full f/stops are 1.0 - 1.4 - 2.0 - 2.8 - 4.0 - 5.6 - 8 -11- 16 - 22 - 32 - 45 ... So if you have an f/1.4 lens it will be sharpest around f/4, while a f/2.8 lens will be sharpest around f/8. Beyond f/11 the aperture gets so small physically it begins to distort the light going through it (i.e. diffraction) which degrades image resolution. So unless there is a compelling need for extreme DOF its better to avoid stopping down past f/11 if possible.
Shooting outdoors with flash is a different can of worms. Click the WWW button below and look in the Canon section for tutorials I've written on fill flash exposure control and high-speed FP flash mode for shooting in sunlight at wide apertures.
FWIW - Whenever I get a new camera I do exactly what I suggest here. I first put it in P mode and shoot at random, seeing how the camera handles focus and exposure. Each new camera does it slightly differently so its a very quick way to learn the camera's baseline performance. Then I switch to Av and repeat the same shots. Finally I do it in M mode, seeing if I can outguess the camera.
Chuck
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