11 Jan Roger Cicala: Understanding field curvature for fun and profit (part 2)

Source: DP Review

Hi, Roger here – I’m going to talk about field curvature again. And first, some admin: If you only use primes with flat fields, shoot only at F11 or F16, or like to pretend your zoom lenses have flat fields, then this article will be of no interest to you.

In my last article I discussed how to evaluate your lens for field curvature and why that’s worthwhile information to have sometimes. Today I’m going to show an overview of what to expect from different types of lenses. There are some general tendencies of field curvature, but remember, there are also a lot of exceptions to those tendencies.

I’m going to use just one type of graph, hopefully minimizing the chaos I so often create. Sagittal field curve on the left, tangential on the right, and the graph curving up towards the top means the field is curving back towards the camera.

By F4 or F5.6 you won’t notice a mild field curvature very much. At F2.8 or wider you often will

We all know the depth of field increases when the lens is stopped down and the field curvature graphs show that. When the depth of field increases enough, sagittal and tangential fields overlap, reducing astigmatism and the edges come into focus at the same plane as the center. The field curvature doesn’t change shape, it just get broader.

Here’s an example, stopping down a lens from F1.4 to F2.8 and then F5.6. The field

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