Reader Jeffrey recently read my post about focus points and commented to me. Because this is such an important issue – sharpness is essential – I thought it would be a great idea to reproduce his point here and to give my response below.
The distance to the plane of focus is the shortest in the center, longer
everywhere else. If you compose focus then recompose, you’re guaranteed to put the target in front of the plane of focus.
To test an exaggerated example, use your 16-35 2.8L at 16mm f/2.8, and
focus on something relatively close to the minimum-focus distance. Then
recompose with that something to the far edge of the frame, and shoot.
Inspect in Lightroom.
Unless unintended in/out movement of the camera during recomposure just happens to offset the change in distance to the focus target, you should find quite a difference… (and even if you don’t, do you really want to rely on a two-wrongs-make-it-lucky close-your-eyes-and-hope philosophy?).
Modern bodies have a bazillion focus points for a reason… so you can have one in the part of the frame you actually want to focus on, specifically so you can avoid the pitfalls of “Focus, Recompose, Shoot”.
Jeffrey is right that there is an issue. I understand the geometry (I am an engineer): when you recompose, the plane of focus as you move about your camera in a circular fashion around you is not a plane: it is a sphere. The sphere of focus means that seen from the subject you focused on, your focus points gets closer to you as you swing the camera.
But there’s a few moderating factors, and reasons focus-recompose-shoot often still makes sense:
- There are only so many focus points. Specifically, few cameras have any around the “rule of thirds” points.
- The centre one is more sensitive and is both-way sensitive.
- Moving points takes a lot of time on many entry-level cameras.
- It is only when you are close that this matters. Unless you’re close and at a large aperture, you will not see the difference.
That is why “focus-recompose-shoot” makes sense, or is often even necessary. But Jeffrey is right that it should be used with caution when close.
In fact when I use my cameras wide open at f/1.4, I hardly recompose, and when I do do it, I compensate by moving my camera ever so slightly before pressing the shutter. Works for me, but be careful and check your focus!
Logic says a focal plane should be curved so that the focal plane is equi-distance from the lens in the middle and corners. If that were true, then recomposing wouldn’t be an issue.
I’ve never thought about it before. So either the lenses are designed to counteract that effect, or the different distance from the rear of the lens to the middle and corners of the sensor results in the flat focal plane.