Curved Field And Flat Field Lenses; A reminder

Illustration of the difference in sharp focus between a curved field lens and a flat field lens

There are many different kinds of lens designs, different designs work best for certain types of jobs.  Two kinds I wanted to remind you of today were curved field lenses and flat field lenses. What's the difference?

As I read a review of a newly introduced lens last weekend, I thought of these two types of lenses.  The lens was sharp in the center, but lost sharpness as you examined the test chart moving from the center to the edges and corners.  Should the lens be equally sharp all the way across?

As in my rudimentary illustration above (which may be somewhat inaccurately constructed so if it is, my fault), the plane of sharp focus with a curved field lens is concave, in other words, it loses the sharpest focus as you move away from the correctly focused center of your subject.

A flat field lens is made to focus in the same straight plane in the center and all the way out to the edges.

Why the difference?  Different purposes.  Most macro lenses are flat field lenses so they are good for keeping flat objects, such as resolution charts, stamps, flat art, and anything else that has a straight plane in focus from edge to center to edge.  

A curved field lens, which is how most general purposes lenses are designed aren't really good for photographing things that are flat, unless you understand the curvature of focus and compensate with additional depth of field.

Both work very well.  I would not hesitate to use a flat field lens, for example a 100mm f/2.8 macro lens for portraiture (however, it may be too sharp for that purpose) or other objects which I would need a moderate telephoto effect, but I wouldn't use a curved field lens for flat macro work.  On the other hand, I would shy away from a flat field lens if I were taking a group photo of several people who normally would be lined up in a slightly concave fashion.

Just a refresher, when I read about reviewers testing lenses showing resolution charts which show the center much sharper than edges, I have to wonder whether or not the reviewer understands that that change may be normal and not a fault.  The center would be sharper than the edges when photographing a flat field with a curved field lens unless you stopped that curved field lens down significantly to overcome the curvature of focus plane.

Just thinkin'....

UPDATE/CLARIFICATION:  Just as a clarification; lenses do tend to be less sharp as you move away from the center.  There are many aberrations that creep in.  My point here is that if a curved field lens is tested and rated by photographing a resolution chart or other flat object, that test for assessing center versus edge sharpness will not be valid. However, if in general photography—travel, landscape, outdoors for instance where there is a three dimensional subject—and the edges and corners lose sharpness and acquire various aberrations, then you can feel that an assessment such as that will be a more accurate evaluation of the overall image forming quality of the lens.

Thanks for looking. Enjoy! 

Dennis Mook 

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