Getting the subject sharply focused is such a basic skill that it tends to be overlooked in the examination of other image qualities, such as white balance and highlight prevention. Yet it is arguably the most important quality of all. Many other mistakes are recoverable in post-production, but even a modest loss of sharp focus can make an image worthless. If you are aiming for impressionistic and experimental results, then fine, but for straightforward shooting, pin-sharp focus in the key area of the image is an absolute necessity.
We wrote “basic skill,” but these days few photographers use manual focusing, much less a manual lens, which means that focusing is normally in the realm of automation. This, however, does not eliminate mistakes. In auto-focusing, the most common error is targeting the wrong part of the scene, such as the background in a portrait. Advanced cameras use a variety of methods for finding and keeping sharp focus on a key subject, including scene recognition, but nothing is foolproof. A more subtle error is focusing on, say, the nose rather than the eyes in a close portrait with shallow depth of field. The wider the aperture, as is usually necessary in low-light shooting, the more this is an issue.
The second most likely class of sharpness failure is motion blur, either camera shake because of a slow shutter speed, or subject movement. Ultimately the key precaution is to check, and as soon as possible after the shot. No one expect to do this all the time, but if you know that the shooting conditions are risky, this is the time to pay special attention.
Paradoxically, the cameras LCD screen and its image preview that makes life so much easier and more reliable than shooting film, can also lull you into a false sense of security when it comes to image detail such as sharpness. This is a real danger, and despite experience we’ve fallen prey to it a number of times. You glance at the screen, it looks fine at the size, so on to the next shot. The bad moment arrives in front of the computer when the images being processed. And by then it’s too late.
In fact, it’s impossible to judge sharpness from a full-screen view on the back of the camera. The right thing to do is to zoom in to at least 50%, and ideally 100% magnification. Pan around the image if you have the time, but at least home in on the area that you intended to focus on. In many situation, you have the opportunity to re-shot and get it right.
As for achieving pin-sharp focus, there a variety of in-camera aids specific to each model. High-end cameras have sophisticated procedures such as multi-point focus and focus tracking. There may also be a choice between auto focusing methods; phase-detection uses a special focusing sensor, while contrast-detection analyzes information from the image sensor.