Getting good focus isn’t very hard these days, with super smart cameras: Just gently push down on the shutter button and the camera does the rest. With earlier cameras you had to do the focusing manually, twisting the barrel of the lens until the image in the finder became sharp; or else, as with many range-finder cameras, turning the focus ring until a split image in the finder lined up and became one. Thus, focusing used to take at least a little while, but with today’s electronic cameras it happens automatically and instantaneously when you hold the shutter button half way down. This also locks the focus on the subject. Some cameras these days are even smart enough to track a moving subject, like a bird in flight, so that the focus remains sharp all the time. Many of today’s sports photos, capturing crisply the leap of layup, or the lunge of a swimmer doing the butterfly, would have been phenomenal achievements before the advent of automatic focus (1981, in the case of 35 mm cameras). Before then photographers had to focus on a spot where the athlete was expected to arrive, and then trip the shutter at the precise moment the spot was reached. It took anticipation, planning (to set the proper shutter speed and aperture), and then at the precise moment, lightening fast reaction time.
The human eye focuses incredibly fast, so fast that we don’t even notice how quickly it happens. As you are reading the words on this page your eye (and brain) are focused on them. If you keep focusing on these words but pay attention to what you can just make out “in the corners of your eyes” you will notice that the objects to the sides are much less distinct, fuzzier. If you shift your gaze to one side or the other, those peripheral objects come into sharp focus immediately, so quickly that you aren’t even aware that it took your brain and your eyes a wee bit of time to shift the focus.
Pictures taken with point-and-shoot cameras show almost everything in focus, from objects just a few feet away to those at the horizon (at infinity, as photographers say). On a clear day this is more or less the way we perceive the world with our eyes. Everything is in focus, near and far. If you browse a book of famous photographs you will notice, however, that many do not show the world like that. Instead, they show scenes where some objects are in focus while others are not. Such photos do not show the world as we perceive it with our eyes, but perhaps largely because of this they make for interesting interpretations of that world, and photographers therefore take artistic advantage of the limited range of focus which various lenses and settings of those lenses provide.
“Depth of field” is the name for that area in front of the photographer where everything will be in sharp focus when the picture is taken, given certain camera settings. When people view landscape photographs they usually prefer to see objects both near and far in sharp focus. So, for landscapes a photographer wants a large depth of field. Objects just a few feet in front of the lens and those all the way to the horizon should be in sharp focus. To achieve this the photographer will “stop down” his lens so that the iris will open just a small amount, choosing stop numbers like f22, or f16. (Counter-intuitively, the larger f stop numbers are for smaller openings; and smaller numbers, like f 2.8 and f 5.6 are for larger ones. Stopping down means that you are constricting the amount of light coming into the camera. But the numbers of the f stops get larger as you stop down. This terminology can be very confusing to new photographers!
Contrary to photographing landscapes, if you’re taking portraits you want a small depth of field. You want the face of your subject to be in sharp focus, but not much more. Otherwise the viewer will be distracted by the details of the background. To achieve a small depth of field the photographer sets his lens iris at larger openings. F 2.8 is one commonly used. (Although the portrait “scene setting” on many point-and-shoot cameras does smooth the wrinkles of a face nicely by softening focus, the wide-angle and short focal length lenses used in point-and-shoot cameras do not facilitate small depth of field photos. So don’t expect to be able to blur the backgrounds of portrait shots with these cameras. This effect can be achieved only by altering you picture afterwards, with a photo-graphics program like Adobe Photoshop.)
Of course, if you change the aperture of a lens by stopping it down or opening it up you must change your shutter speed proportionally, or else you will under or over expose your photo. For instance, if you stop down by three stops (making your lens aperture quite a bit smaller), you must compensate by setting your shutter to remain open three stops longer. Let’s say that the light meter indicates a proper exposure of f 5.6 at 125th of a second. To increase the depth of field optimally the photographer would need to stop down to f 22. That’s three stops smaller. To compensate for the restriction of light at that small opening the photographer would to reset his shutter speed to f 1/15 second(three stops slower). F 1/15 second is too slow to get sharp focus by hand-holding the camera. (The limit for sharp hand-held photos for most photographers is 1/30 second.) So in this case the photographer will need to use a tripod to insure good focus.
Summing up these basics about getting good focus:
Using an automatic focus camera:
Hold the camera steadily and press the shutter button half way down. This locks the focus and prepares you to take the picture at just the right moment. Press the button gently so as not to move the whole camera as you take the picture.
Using more expensive cameras capable of depth of field adjustments:
For landscapes, use small apertures, like f 16 or f 22. Reset your shutter speed to compensate for the narrow lens opening. Use a tripod if the shutter speed you choose is less than 1/30 second.
For portraits and other photos where you want just one object in focus: Choose wide lens openings, like f 2.8. Reset your shutter speed proportionally to fire faster.