# Are points off-center from the lens axis out of focus? [duplicate]

If I take a picture of a flat surface perpendicular to the direction of the lens (i.e. the surface is facing directly at the camera), and I focus on the center, are the points off center in perfect focus, since they are further from the lens than the center point?

Alternatively, do camera lenses have a way to account for this?

If in fact the points off center are slightly out of focus, couldn't this have a significant effect on achieving focus across the entire surface, especially if the camera were close to the object?

Thanks to anyone who can shed some light on this.

Theoretically, the focusing plane should be flat, so the focusing distance should be the same and all points should be in focus. This is not always true due to aberration called field curvature. This causes that the field is not always perfectly flat, but curved, some lenses with more complex corrections can even have wave shaped curvature.

• Sometimes the field curvature is a desired quality and the lens is designed specifically to not correct for it. The Canon EF 50mm f/1.2 L and the EF 85mm f/1.2 L are two such lenses. Sep 22, 2016 at 1:19

The other answers have pretty well covered the way lenses are designed to deal with (or not deal with) a flat focal plane. This answer addresses the concept of "in focus."
First of all, there's no such thing as "perfect focus" in the real world. The best you can do, diffraction aside, is to minimize the geometric blur spot size for a specific location in x-y-z object space. Not only will there be some degradation for off-axis objects in the x-y plane, but there's a degradation in the z-axis as well. This latter is usually referred to "depth of focus," which defines the range of object distances (for a given x-y coordinate) for which the image blur spot size on the sensor is acceptably small.
As you probably know, depth of focus and off-axis blur get better with smaller lens aperture. Theoretically, a pinhole camera has perfect focus and zero distortion across the entire field. Since theoretically (and in reality :-) ) a pinhole lens has rather poor light collection, camera lenses open up the aperture to achieve useful illumination levels.

It depends on the lens. Some, such as most macro/micro lenses and perspective control lenses, are designed to have a flat field. Others, such as many lenses intended specifically for portraits or fisheye lenses, are designed to have a spherical field of focus.

• Do you have any reference for this? I could not find anything. I also doubt anyone would want to make a lens like that, because it would look horrible in the MTF chart. I know that should not be the biggest consideration, but unfortunately in the real world, it makes a big difference. Jan 5, 2019 at 22:36
• All uncorrected single element lenses of real thickness demonstrate field curvature. Field curvature is one of the classic seven optical aberrations. Only a pinhole camera has a theoretically flat focus plane. Jan 5, 2019 at 22:55
• Pinhole cameras don't have a focus plane, they are in focus everywhere. I have searched quite a bit some months ago, but I could not find anything about lenses that intentionally have a curved field of focus. Jan 5, 2019 at 23:08
• Jan 5, 2019 at 23:24
• @Orbit It doesn't have to be intentional. All lenses with real thickness already demonstrate field curvature until they are intentionally corrected to minimize it. But as with all things involving optics, there are compromises that have to be made. Lenses that are highly corrected for field curvature (e.g. so-called "flat field" lenses) render out of focus highlights harshly. Lenses with more field curvature render out of focus highlights much more smoothly. Jan 5, 2019 at 23:25