Originally posted by seachunk2 ... if I opt to shoot at f11, the aperture of the lens can't physically get any smaller because it's limited to the 5.6 diameter opening ...
Apertures are confusing at first. See, f/5.6 looks like a smaller NUMBER than f/11, but it's a larger / wider APERTURE, a larger fraction. Aperture is a fraction, the ratio of the focal length to the widest opening. So an f/2 means the opening is 1/2 the focal length (FL), and f/11 means it's only 1/11th of the FL. The SMALLER the f-number, the WIDER the opening -- and more light gets in. So f/4.5 is much wider than f/11, just as 1/2 is much larger than 1/7.
So when a zoom lens is rated as f/4.5-5.6, that means that at the shortest zoom, the widest possible aperture is f/4.5, and at the longest zoom, the widest possible aperture narrows down to f/5.6. If you have set the aperture to f/11 or f/16, or any other setting narrower than f/5.6, the camera stays at that narrower setting (LARGER F-NUMBER).
The aperture number is also called an f-stop. Setting a narrower aperture is called stopping-down. Whatever the widest possible f-stop, you can always stop-down as far as the lens allows (maybe f/22 or f/32, not alway desirable).
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What effect does this have on shooting? A larger, faster f-stop lets in more light, so you can use a faster shutter. But it also reduces depth-of-field (DOF), the distance from the lens where stuff is in focus. When you shoot wide-open with any lens, at its widest f-stop, you have to be more careful about focusing. You might aim at a face, and the face will be sharp, while everything nearer or further is blurry. This is great when you want to emphasize a certain part of the picture.
A smaller, slower f-stop lets in less light, so your shutter speed is slower. But your DOF is increased -- more of what's in front of you is in focus. If you're shooting a deep landscape, and you want everything in focus, crisp and sharp, from the nearest flower to the farthest mountain, you use a smaller f-stop.
This is especially so on wide-angle lenses. I can put a slow cheap 21mm lens on my camera, set the f-stop to f/8, set the manual focus to 6 feet -- and my DOF is 3 feet to infinity! I don't have to refocus on anything further than a yard away! This is great for shooting in crowds, and in crowded places. I don't even have to raise the camera to my eye! Yeah, I like sneaky street-shooting.
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All this stuff is related. Depth-of-field (DOF) is a result of aperture (f-stop) and focal distance (camera to subject). A wider lens or a slower f-stop mean thicker DOF, more is in focus, but you need good light or a slower shutter speed. A longer lens or a faster f-stop means thinner DOF, be more careful of your focus, but you can shoot in less light or with a faster shutter. Using the camera is all about making compromises, the trade-offs between focus and f-stop and shutter speed, and learning how to exploit all the possible combinations. Have fun.