Focus + recompose = back focus

Some photographers use the ‘focus, then recompose’ technique. They place the central focus point on their subject, let the camera auto focus, and then change the composition by rotating the camera. Some photographers even think this is the most accurate method, because this way they use the central focus point rather than a periferal focus point and the central point is usually a cross sensor. Unfortunately, these photographers are wrong. This technique is a recipe for out of focus problems. It’s the main cause of so-called ‘back focus’ problems. The reason why this technique is flawed, is because the focal plane of a camera is flat, not curved. If you rotate the camera however, your subject moves on a curved path relative to the camera. The images below hopefully make it clear why you should never use the ‘focus, then recompose’ method. The top image shows how focus is obtained by pointing the central AF-sensor on the woman’s face. The bottom image shows what happens after the composition was changed by rotating the camera. The red line is the actual focal plane, the green line is where the focal plane should have been for correct focus. You may get saved by the depth of field and so you may not notice it, but it’s wrong nevertheless.