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Quick tutorial (incomplete) on some general concepts regarding AF and aperture.
1. As mentioned earlier, no matter what aperture you set the lens to, the auto-focus process is accomplished at the largest aperture. If you set your lens to f/16, when you press the shutter release it focuses at f/2.8 (on your lenses), then closes down the aperture to f/16, makes the exposure, and allows the aperture to quickly return to f/2.8
2. As you can see, there is no reason to go through the steps you described to AF then switch AF off and stop down manually to shoot. In your case, just set the aperture on the lens that you want to use, AF, and shoot.
3. There are a range of factors that go into deciding on the aperture you will select for a given photograph. Certain apertures might be better for isolating a subject against a complex background, others for achieving the best center resolution that the lens is capable of, or for getting the greatest depth of field. Choosing an aperture that optimizes for any one of these can decrease the "performance" of the lens in the other two areas. For example:
a. If you stop down to, say f/16, while objects across a greater range of distances from the camera will be in good focus, you might sacrifice some maximum potential sharpness to accomplish this. Regarding the overall loss in sharpness as you stop down (due to diffraction blur): in general, f/11 will never create a problem on a full frame camera, you are almost certain to NOT (edit - I originally left out the critical word "NOT" at this point!) be able to see a difference in your prints from shooting at f/16, at f/22 you may well see a bit of softness. Here you trade increased depth of field for slightly decreased resolution. (I generally avoid f/22 unless I really need a lot of DOF and I'm willing to sacrifice a bit of sharpness to get it.) To counter one common misconception, stopping down to f/16 does not make your photograph sharper, though it increases the distance range across which subjects will appear to be in acceptable focus.
b. If you select a so-called optimal aperture for center sharpness you will often get fine image quality, though your depth of field (near-far range) will diminish somewhat. To make a generalization, it is often safe to assume that f/8 is likely to be roughly in the optimal focus range for most good lenses on full frame cameras. If you don't have a good reason to shoot at a different aperture, you could make this your default. For example, with your glacier shots, unless you also want something very close to your position to be in focus, there would be little, if any, advantage in the greater depth of field afforded by shooting at f/16.
c. If you select a very large aperture (such as f/2.8 on your zooms) you can create a much narrower band of optimal focus. Objects much further than your main subject and much closer than your main subject will be blurred/out of focus. In general, there can also be a slight decline in the maximum sharpness of objects at the focus point, though this is normally inconsequential. Counterintuitively, putting a lot of the image out of focus can make your primary subject look sharper. For example, photographing something like a flower at a large aperture (f/2.8 on your lenses) can significantly blur objects in the far background, and the flower can appear to stand out sharply against this soft background - making it appear sharper than if it had been photographed at a very small aperture (such as f/16) that rendered more distracting detail in the background. (I refer to this as "subjective sharpness," as differentiated from "objective sharpness.")
4. If you are selecting aperture because you want to "get the sharpest photograph," you can almost certainly let that idea go. Yes, the optimal aperture for your lenses (likely about f/8 in most cases) will produce the highest resolution - but the other apertures can also provide very sharp images. Unless you are shooting with extreme care and regularly producing very large, high quality prints, you are not going to see these difference in your photographs. In general, photographers usually select apertures for reasons other than sharpness - unless using the f/8 default.
1. Glaciers a good distance away - shoot at f/8 in most cases.
2. A landscape with close and far objects - try f/16. (There is a bit more to it than this...)
3. A close up of a subject like a flower that you want to stand out against the background - try f/2.8
Finally, the good news is that it is really easy to learn to understand this just by taking your camera and making a bunch of test exposures at various apertures. You'll soon see, in very practical terms, how aperture will affect your shots.
Edited on Jul 06, 2013 at 12:24 AM · View previous versions