When you look through your viewfinder and press the shutter button halfway, you can get only a partial indication of the depth of field that your current camera settings will produce. You can see the effect of focal length and the camera-to-subject distance, but because the aperture doesn't actually open to your selected f-stop until you take the picture, the viewfinder doesn't show you how that setting will affect depth of field.
By using the Depth-of-Field Preview button on your camera, however, you can do just that. Almost hidden away on the front of your camera, the button is labeled in Figure 6-15.
To use this feature, just press and hold the shutter button halfway down and then press and hold the Depth-of-Field Preview button with the other hand. Depending on the selected f-stop, the scene in the viewfinder may then get darker. Or in Live View mode, the same thing happens in the monitor preview. Either way, this effect doesn't mean that your picture will be darker; it's just a function of how the preview works.
After the preview engages, you can release the shutter button and then rotate the Main dial to see how changing the f-stop affects the depth of field. (The exception is when the camera is set to A-DEP mode, in which you have no control over aperture.)
Note that the preview doesn't engage in P, Tv, Av, or A-DEP mode if your current aperture and shutter speed aren't adequate to expose the image properly. You have to solve the exposure issue before you can use the preview.
Depth-of-Field Preview button
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