Canon manual 37

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Aperture Scale on the AT Dial

The aperture scale on the AT dial is calibrated in f/stops ranging from f/22 to f/1.2. The f/1.2 setting is indicated by a dot. The dial click-stops at both full and half f/stops. It will not turn past the largest and smallest f/stops. The maximum aperture indicated on the AT dial is large enough for any Canon lens.

Under certain conditions in the aperture priority AE mode, the maximum and minimum apertures on the lens have priority over the aperture set on the AT dial. For instance, if the aperture you have set on the AT dial is larger than the maximum aperture of the lens in use, the camera will match the shutter speed to the lens maximum aperture rather than the aperture you have set on the AT dial for correct exposure. Thus, if you are using an FD 50mm f/1.8 lens with the AT dial set to f/1.4, the camera will use

f/1.8 as the aperture setting. In this case, the maximum aperture of the lens will be displayed in the viewfinder when you take an exposure preview regardless of the setting on the AT dial. On the other hand, if you have set an aperture on the AT dial smaller than the minimum aperture of the lens, the image will be exposed at the lens minimum aperture but the shutter speed will not be adjusted accordingly, resulting in over-ex- posure. In this case, the aperture set on the AT dial will be displayed in the digital read- out.

Since the smallest aperture that can be set on the AT dial is f/22, if you want to set an aperture smaller than f/22, switch to stopped-down AE and manually set the aperture on the lens aperture ring.

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