Aperture Priority AE mode, denoted as Av on the Mode dial, is the semiautomatic mode of preference for everyday shooting because it offers quick control over the aperture and, thus, control over one factor that determines depth of field. In this mode, you control the aperture by turning the Main dial, and the camera automatically calculates the appropriate shutter speed based on the light meter reading, the Metering mode, and the ISO. The 5D Mark II displays the selected aperture in the viewfinder and on the LCD panel.
Pressing the Shutter button halfway down initiates the camera's exposure calculation based on the aperture you select. If the exposure is outside the camera's exposure range, the shutter speed value blinks in the viewfinder and on the LCD panel. If 8000 blinks, the image will be overexposed. If 30 blinks, the image will be underexposed. In either case, adjust to a smaller or larger aperture, respectively, until the blinking stops or you can set a lower or higher ISO setting.
You can preview the depth of field by pressing the Depth of Field Preview button on the ^QjJ front of the camera. When you press this button, the lens diaphragm stops down to the selected aperture so you can preview the range of acceptable focus. This button also turns on a Speedlite's modeling light if you have a powered-up flash attached.
3.4 Av mode is great for controlling the rendering of the foliage for this indoor portrait. Because it was raining the day of the shoot, my senior subject posed by a window with foliage in the outdoor background that created great bokeh. ISO 100, f/2.8, 1/200 sec., with an EF 100mm f/2.8 USM lens.
For both Tv and Av modes, you can set C.Fn I-6 to enable a safety shift in exposure. The shift comes into play if the subject lighting changes enough to make the current shutter speed or aperture inappropriate. Enabling this function causes the camera to automatically shift to a suitable exposure.
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