Aperture is the opening in a lens, expressed as an f-number. At the same shutter time and ISO, f/2.8 lets in more light than f/4. A smaller f-number means a wider opening; a larger number means a narrower opening.
What depth of field means
Depth of field is the range that looks acceptably sharp in an image. A wider aperture generally gives less depth of field when the other conditions stay the same. Focus distance, focal length, framing and viewing size also affect what appears sharp.
Try it
Place three objects roughly two metres apart along your line of sight. Keep the camera still, choose a wide aperture and focus on each object in turn. Then compare f/2.8, f/5.6 and f/11 if your lens allows them, compensating brightness with another setting.
By Joseph Hubbard, photographer and artist at Hubbard.ART.
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