How to use the depth-of-field preview button on your dSLR
The easiest way to experiment with the button is to be in aperture priority mode. Start at the widest maximum aperture that your lens allows (a small f-stop such as f/3.5) then press and hold down the DOF preview button. Then, close down the aperture …
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dSLR tips for beginners: How to use Aperture–priority mode
As you might expect, you use Aperture-priority mode when you need to control the size of the opening through which light travels to reach the sensor — usually to affect the depth of field — but don't care (much) about shutter speed. You set the …
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