Steve asked me one of the hardest questions about using small digital cameras: “How can I focus on a flower close-up and have the background soft?” Unfortunately this is one area where pocket cameras are weak, but it’s not an impossible task. There are really two issues involved here. One is that the autofocus system often wants to focus on the background rather than the subject you’re aiming for. The second is that with a small sensor you get a short focal-length lens which equates to great depth of field.
By default, the autofocus in most cameras looks at the center of the frame. So if you’re practicing good composition and placing your subject off-center then you have to “trick” your camera into focusing where you want. One way is to aim the center of the frame at your subject, press the shutter halfway to initiate autofocus, reframe for the composition you want, and then press the shutter the rest of the way to take the picture. This works most of the time, particularly when your subject is fairly large in the frame.
When working close to small subjects, like dainty flowers, you can place one hand in the same plane as the subject, focus, then pull your hand away and take the picture. The disadvantage is that you’re only holding the camera with one hand so it isn’t as steady. Another technique is to use manual focus so you’re in charge of where the image is sharpest. Sometimes for the smallest subjects I’ll set the manual focus as close as I can and then move the camera toward or away from the subject until it is sharp.
When you’re relying on the camera’s autofocus it helps to understand that it is basically a contrast detection system. So you’re more likely to autofocus correctly on subjects that have strong edges, or lines, the system can “see.”
The second part of Steve’s question was how to get shallow depth of field. First, select a subject that is well-separated from the background. The more distance between your subject and background the easier it is to force the sharpness contrast. The second step is to use the longest focal length your lens offers. In 35mm equivalent terms this will probably be around 115mm, but it varies among cameras. Zoom in all the way, then move the camera toward or away from your subject to get the composition you want. Third, use aperture-priority exposure mode (or manual) and choose the widest f/stop. On many pocket cameras that will be f/2.8.
A challenge you’ll encounter is that most pocket cameras focus closest at their widest zoom setting, even in macro mode. That means you may have to adjust your expectations to fit within what is possible. You’ll never achieve the extreme focus contrast you can get with a long lens on 35mm film or a full-frame sensor digital SLR, but with care you can get a pretty good approximation. And like most skills, practice helps. Go out and play.
If you’d like hands-on instruction come to one of my classes this summer.