# Do lenses have focal planes or focal spheres/ellipsoids?

The title says it pretty much all. I'm asking this because I find myself always using the focus point in the center, and after I focused, I rotate my camera to make the framing right. However, if lenses do create a plane in which everything is sharp, then the focus will be slightly off after rotating the body.

Doing some maths: if I focus at distance x meters, and rotate my camera for alpha radians, then the focus plane would be off for:

``````∆f = x*tan(alpha/2)*tan(alpha)
``````

So, let's plug in: x = 2 meters, alpha = 20 degrees = 0.349 radians, we get: ∆f = 0.128 meters. So this means, my optimal focus point is now almost 13 centimeters away from the original focus point.

If they are not spherical, I will experience my focus being off. So is it safe to use other focus points except for the one at the center and immediately put my framing right? This will slow down the process of taking pictures?

• It depends on the lens. Some have practically planar focal, well, plane, others have a more curved surface. Jun 5, 2014 at 14:56
• Related, regarding focus-and-recompose vs. selecting a non-central AF point: photo.stackexchange.com/a/12152/2138 Jun 5, 2014 at 18:21