Setting your camera to Aperture Priority Mode will, as the name might imply, give you direct control over the aperture, or opening of the lens. You should choose this setting when you want control over the depth of field (how much of the image is in focus). When you are in Aperture Priority Mode, the camera will choose the other settings for you. If you took your ISO setting out of Auto Mode, that only leaves Shutter Speed. So the thing that makes Aperture Priority Mode so much better than Auto Mode is that you choose the setting that is most appropriate for the depth of field you want, then the camera selects the right Shutter Speed to get a good exposure. You get the best of both worlds, automation but under your control.
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