In the Sept/Oct 2009 magazine "Photo Techniques" (
Photo Techniques Magazine - The Magazine dedicated to professional photographers) there's this article on field curvature's effect on lenses. It makes me wonder which WEB lens reviews are written by folks that really know some of the intricacies of lens performance.
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Some quotes from this magazine article:
1. A lens might exhibit a flat field at close range, yet show strong field curvature near infinity. The Canon EF 14/2.8L II is one such lens, and even f/11 won't overcome the field curvature at infinity focus. Yet another lens might exhibit a mostly flat field at infinity, but show substantial field curvature at close range- the Zeiss ZF 25/2.8 Distagon is one such lens. Such variation usually leads to erroneous conclusions about the merits of any given lens, especially in lab settings and "quick tests"
2. The photos in Fig 2 show modest field curvature using the Nikon 20mm f3.5 AI-S. Optimizing focus for a crisp center yields blurred corners, and optimising focus for the corners yields a blurred center.
3. There is no choice or workaround with field curvature-the optical design of a lens is fixed. All the photographer can do is to understand the behavior and ideally exploit it to advantage while avoiding compositions that are in conflict with the shape of the curvature.
4. Some Leica designs for wide angle have very wavy MTF curves showing both strong astigmatism and field curvature; the Leica Super-Elmarit-R 15mm f2.8 ASPH is one example. The best focus overall for across the frame sharpness is a compromise: adjusting focus for a neutral middle ground, then stopping down, can yield the best results (keep this in mind next time you read simplistic rules about depth of field).
(Phil: I once tried to get the center and edges of a sea wall in focus with a Sigma 10-20 f4. No matter how i stopped down, i could not do it. The above technique might have helped.
5. Detecting Field Curvature: The quickest way to see field curvature without even taking a picture is to use the Live View feature of most cameras (at maximum aperture). A test chart or newspaper taped to a wall can work well. Using Live View, focus at center, then move towards the edges, observing image sharpness. Refocus off-center and determine if the image becomes sharper-if so, you're seeing field curvature (assuming you've aligned the camera squarely to the target)
6. The optical designs of most 50mm lenses are very similar, and nearly all show field curvature that is easily misinterpreted as a lens being "soft". I tested eight different 50mm lenses from Nikon, Canon, Zeiss, Olympus, and Sigma, and found that all had field curvature that became obvious with the appropriate subject matter. I did so because in my testing of the Zeiss ZE 50mm f1.4 Planar, I detected strange variations in image sharpness across the frame, even at f5.6. At first i though this was a bad lens, but subsequent testing of the eight 50mm lenses showed this to be common.
7. The good news is that with most lenses and subject matter you can safely ignore field curvature. But don't be surprised to find odd variations in sharpness at the same desired plane of focus....Especially when comparing different lenses, field curvature is hugely important, because very small changes in focus can shift the sharpness/contrast dramatically.
In fact, comparing the same lens to itself can "prove" that it is both better and worse than itself, depnding on very subtle shifts in focus - one reason that making real images the only realiable way to assess lens performance and a compelling reason to be skeptical of casual lens tests you might find on the WEB.
8. An MTF curve that is flat or slopes off gradually usually indicates little or no field curvature...However, an MTF curve might be measured at a distance at which the lens has a relatively flat field, and neither Nikon nor Canon is clear on their measurements, so MTF is not always a reliable indicator...With Zeiss and Leica, MTF is a very good indicator of field curvature, and they indicate the test distance...
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Last edited by philbaum; 11-10-2009 at 02:13 AM.