Dealing With Lame Autofocus
Do you ever wind up with pictures like the one shown here? Digital camera owners will probably say "yes" a little too often. In the neverending quest for technological advancement, most camera makers build automatic focusing (AF) mechanisms into every camera they make, and frequently do not include a good manual override. When the camera gets confused and won't focus right, you're hosed.
This article first examines the factors at play behind blurry pictures and lame autofocus, and then presents some advice for photographers at the mercy of their "smart" cameras. It's intended for digital and film camera owners whose camera does not have a good manual focus system, but allows them some control of aperture or shutter speed. Many new digital cameras have "aperture priority" and "shutter priority" modes which will be essential to the discussion below.
Understanding depth of field
When you focus a lens system, you actually get a range of distance which is in focus. You've surely seen photos where a foreground subject is sharp but the background is totally blurred, or a landscape where everything from the foreground to the vast distance beyond is razor-sharp. The term for this range where elements are in focus is depth of field (DOF). The former case where the background is blurred has a short DOF, and the latter case has a long DOF.
I'll cover this quickly since any beginning photography book will discuss depth of field. DOF is a factor of the focal length of the lens (how 'zoomed in' you are) and the aperture (the diameter of the opening in your lens system, which controls how much light gets through). Typically you vary the focal length to frame the picture as needed, and vary the aperture to control DOF and exposure. Because the lens aperture controls how much light gets to the film or digital CCD, it also affects shutter speed; if you adjust one you must adjust the other proportionally to keep the same exposure. A large aperture means more light, which means faster speeds.
Aperture is measured in f stops, often written like "f/4" or just "f4." Smaller numbers mean larger apertures, so f/4 is a larger opening than f/5.6, has shorter depth of field, and a faster shutter speed is used.
So what does this have to do with focusing? Because digital cameras have slow CCDs -- they need lots of light -- the camera tends to use a large aperture, and therefore a short depth of field. As such, a slight error by the autofocus mechanism will have catastrophic results. Short DOF leaves little room for error.
The focusing system for most digital cameras uses contrast in the image to determine if it's in focus or not. If the shooting target has little natural contrast or the environment has little light, the camera's AF may not figure out what to do. This scenario is far more common than I'd like with my Nikon Coolpix 950; even times when I think there's plenty of light and contrast, the camera just won't have a clue. It's extremely frustrating that my 1975 manual-everything film camera will outperform my 1999 mega-high-tech digicam -- because I focus it manually.
Many cameras have an "AF illuminator" or "AF assist," a light which they'll activate to help them focus in a low-light environment. (My camera does not.) Also, some external flash units may have an AF illuminator which the camera can use. (My flash has one, but the camera can't use it.) Some form of AF assistance can be a huge help when shooting in low light.
Using depth of field to your advantage
Finally, we get to the beefy part. It should be obvious that a long depth of field can help compensate for not-quite-right autofocus. Hopefully your camera has some indication of whether the camera has focus lock or not, and will let you take a picture without AF lock. (On my Coolpix, it will take the picture if the LCD is turned on.) If you can't get AF lock or the lock is questionable, you can use a small aperture to get a long depth of field, and therefore a larger margin of error.
For my airshow article I took several hundred photos of airplanes in flight. The first shots were all hopelessly blurry, like the first photo on this page. Even setting my camera to infinity focus didn't yield acceptable results. Finally, I switched into aperture priority mode, set a small aperture, and the long depth of field was enough to compensate for the camera's inability to focus on the planes. The day was saved, at least for most shots.
There's a downside to this technique, however: small aperture means slow shutter speed. The airplanes were in bright daylight, so I could still take hand-held shots and freeze their motion. As already mentioned, however, autofocus systems tend to get stupid most often in low-light environments, exactly where you need more light, and therefore larger apertures. It's like having a dentist who listens to the Spice Girls as he gives you a root canal -- torture never comes in small doses.
The balancing act
So, then, what do you do in low light when the autofocus won't lock? You need to choose between fast shutter speed and sharp focus. If your subject can stay still and you have a tripod -- or a very steady hand -- you may get by despite the slow shutter speed. (Remember, if you reduce the speed enough, your image may get blurred due to camera shake. Watch both numbers!) If you have a film camera, use faster film so you can use a smaller aperture. Some digital cameras allow you to increase the CCD gain to allow faster speeds, e.g. my Coolpix 950's CCD is ISO 80 but can be increased to ISO 400. Image noise increases considerably when you do this, but it may be worth it if you don't have any other choice.
Sometimes you can use your flash to provide enough light to overcome the small aperture. For example, if you are close to the subject you might be able to set f/5.6 or f/8 and use a flash. I do this at times, with my powerful SB-28 flash compensating for the small aperture. If your camera has an AF assist light, you may not need to mess with this, as the camera should get AF lock.
Picking up the pieces in Photoshop
Photoshop can't save a hopeless image like my MiG-17 shot that opens this page. (Yes, that's actually a MiG-17; can't you tell?) It can, however, do wonders for images which get close. Not only can you sharpen images to some degree, you can also simulate a shorter depth of field if you were forced to use a long one while shooting.
First, read the manual on Photoshop's "unsharp mask" filter. This filter can do wonders for your images, especially ones with soft focus. I usually start with settings of 150%, 1.0 pixel radius, and 10 level threshold. These are just rough numbers, though, so play with it until you find something good. Note that some digital cameras do sharpening internally, making this step unnecessary. If possible, however, I recommend disabling this feature and doing the sharpening yourself with Photoshop's unsharp mask, as you will have more control and Photoshop will almost surely deliver better quality.
Going the opposite direction, if you took a picture with too much depth of field because of autofocus problems, you can artificially shorten the DOF in Photoshop. This takes more time and skill, but digicam photographers are usually the type with too much time on their hands anyway. For example, the blues guitar guy from my article on Boulder, Colorado:
The background was noisy and I wanted to make it fade back a little, so I applied subtle after-the-fact DOF. To do this, make a copy of the image on its own layer and apply a gaussian blur. Then add a mask to the blurred layer and airbrush the blur off foreground subjects. The gradient tool is also very useful for making a smooth sharp-to-blurred mask for any ground spanning the entire depth of the photo. Note that this effect is best used sparingly; if you try to add too much, it'll quickly start looking fake.
Try to get the best images you can to begin with, because it's hard or impossible to fix things later. Too much depth of field is usually better than too little, since sharpening afterwards is harder than blurring. Consider carrying a film camera for backup, preferably one with good manual focus.
I wish I didn't need to make the kinds of compromises explained in this article. I wish my digicam had good manual focus control, but that wasn't in the cards. We all need to adapt to the limitations of our equipment, and sometimes that means sacrificing technical or artistic quality for getting something "workable."