The difference between the exposure values in both highlights and shadows can be as much as two to three stops (or more). In cameras with multi-spot option, like the discontinued Canon T90 or the EOS-3 and the pro EOS-1V models, the metering function has been programmed to recognize this and will usually choose an intermediate after taking into account the differences in the exposure values of both highlight and shadow areas. If the film used has exposure latitude within these ranges, you will get good results. Aperture-priority AE In the camera brochures, the types of pictures usually associated with the Av mode are portraits with blurred backgrounds, close-up shots of plants or landscape scenes with plenty of depth-of-fields. A typical shot with plenty of depth-of-fields But it can be used for more than these ranges of subjects, like available light shooting. Yes, Program AE is much more versatile when shooting in such conditions but the Av mode allows you to shoot at a preferred aperture so that the camera can set the required shutter speed. In P mode, the camera will always choose a handhold-able shutter speed in conjunction with the focal length of the lens used and the chosen aperture may not be the one that you want. Shot at 1/30 sec. f/4 Shot at f/2.0     Fill in TTL illumination in Av mode In these three photos shown above, the first was shot with the EF 135mm f/2.0L USM lens at 1/30 sec. at f/4.0 - in P mode, where the camera would have chosen 1/125 sec. at f/2.0 automatically. In the second shot, the maximum aperture of the same lens was used to great effect by throwing the background out of focus while the EF 50mm f/1.4 USM was utilized to get a blurred background with fill-in flash for the third photo. Using fill-in flash at f/1.4? This was one of the situations when the lights were dimmed to accommodate the mood portrayed and with an ISO 100 on the EOS-1N RS (effectively ISO 64 equivalent), TTL flash illumination was needed to add light to the subject, which would otherwise turn out dark, in the shadows. Metering for the face without fill-in flash would have overexposed the background. << Back to Main . 1 . 2 . 3 . 4 .