Lens aperture also affects how much in front and behind the lens focus distance will appear to be in focus in the image. This is called Depth of Field. The most common reason for wanting a narrow aperture (high f-number) is to have a very deep depth of field in which everything (or very nearly so) appears in focus. A very deep depth of field is almost always desired for landscape photographs. A wide aperture (low f-number) will create a shallower depth of field. The wider the aperture, the shallower the depth of field. One of the common uses for wide apertures is in portraiture, to isolate the subject from a distracting (possibly cluttered) background.
The last thing you want when taking portraits is cluttered, distracting backgrounds, that leads the eye away from the subject. The background appears out of focus, but not in a way you would blur in photoshop or some other photo editing software, but looks like circular or hexigan shaped blotches, reflecting the shape of your aperture. This is how you know whether the photography has been manipulated after a shoot or has been done in-camera.
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