jacques smit photography

Main Menu

contact jaqcuese smit

Correct focusing

Most photographers will not even be aware of the problem of correct focusing. You can choose the correct focusing mode, but your focus can still be out!

Many AF cameras today contain between 11 and 60 AF sensor spread across the frame. We all know that the middle sensor of any camera is the most sensitive. It will give you quicker focusing in low light. Because of that reason many photographers and pro photographers do not select one of the other sensors when the main subject is not in the center of the frame. Instead they lock the focus on the main subject with the central sensor and recompose before taking the shot.

When you look at the picture you will notice that the main subject is slightly out of focus. First you will think that it is the camera's fault, but the actually problem is that you not using the correct technique to get your subject in proper focus. Here's an illustration of the situation:
jacques smit on the correct way to focus
The camera is at the base of the diagram. It's first focused on point A. After focus is acquired, everything on the plane of focus that touches point A is in focus (blue line on the right). Then the photographer recomposes - with locked focus - so that the camera points to point B. The focus distance does not change because AF is locked. After that, everything on the plane of focus touching point B appears sharp (blue line on the left). But now point A is no longer on the same plane of focus.

The plane of focus that would be correct for A is closer to the camera (solid black line). The amount of miss focus is d (red line), and it becomes larger when you recompose farther. It looks like the camera has a back focus issue. It also looks like with wide-angle lenses the problem is bigger than with telephoto lenses. However, this kind of focus error is actually a user error, not a camera problem, and wide-angle lenses just give the user more opportunities to make this mistake than telephoto lenses (because you can recompose more before the subject slips out of the frame).

There also is a secondary problem when recomposing strongly. The cameras mentioned above often also base exposure and flash exposure on the selected AF sensor, assuming it points at the main subject. So the metering cell(s) close to the selected AF sensor are given priority over the other cells. But when you recompose, the selected sensor no longer points at the main subject, and you may end up with incorrect (flash) exposure if you don't have exposure lock also turned on.

The lesson to be learned here is to always select the AF sensor closest to the main subject, even if this sensor has a weaker performance than the central sensor. Recomposing by a significant amount should be avoided as much as possible, for accurate focus and correct exposure.

Comments are now closed for this entry