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 This article will be expanded to include Canon EOS and other Nikon lenses, as time allows. For now, the following pages available: Autofocus Accuracy — Nikon D2X
with 17-55/f2.8DX Autofocus Accuracy — Canon EOS 1Ds Mark II / 85mm/f1.2L II USM Plane of focus — The plane (2-dimensional geometric surface) upon which the lens is focused, assuming a lens has a perfectly flat field (no curvature of field). Curvature of field — The degree to which a lens focuses in a shape other than a plane (a 2-dimensional geometric surface). Virtually all lenses have curvature of field. Even macro lenses, though usually well-corrected for planar subject matter, can exhibit trace amounts of curvature of field obvservable with suitably high resolution images. A lens with curvature of field does not focus on a a plane but rather has uneven focus from center to corners. Subject matter in front of or behind the desired plane of focus might be rendered sharply in such a lens even as subject matter on the plane of focus (but elsewhere in the frame) might be rendered sharply. The Nikon NOCT-Nikkor 58mm/f1.2 is an example of a lens with strong curvature of field. Back-focus — Focusing of the lens behind the plane of critical focus. Critical-focus — Focusing of the lens with sufficient accuracy such that there is no detectable difference in image sharpness and/or contrast for the particular recording medium used. Greater accuracy is required for a higher resolution camera than for a lower resolution camera (assuming equal sensor sizes). Front-focus — Focusing of the lens in front of the desired plane of critical focus. Today’s high resolution digital cameras, even “prosumer” models with “only” 6 megapixels have resolution high enough to reveal even very small focus inaccuracies. But focus accuracy becomes critical with cameras like the Nikon D2X and Canon EOS 1Ds Mark II when used with “fast” lenses at large apertures like f1.2, f1.4, f2, f2.8. The D2X, with its 5.5 micron pixels, and the Canon EOS 1Ds Mark II, with its 7.2 micron pixels, quickly become 2-megapixel cameras at f1.2/f1.4 if focus is not extremely spot-on. [See The Sharpest Image and D2X vs EOS]. Plug in 5.5 microns to a depth of field table, and the number might as well be zero for an f1.4 lens. I might update this article for Canon EOS in the future, and I might add a Nikon 50mm/f1.4D for comparison. Its always a good idea to watch my blog for those sorts of things. Conclusions Please see the conclusions in each individual article: [1], [2]. Any test which evaluates sharpness must determine whether the intended plane of focus has been achieved. This can be achieved by shooting multiple frames and/or fine-grained focus bracketing. Failure to do so invalidates the results of any such test. Before you conclude that a particular lens is “soft” or a “dog”, check whether you have focused it accurately! Real-world subjects often do not require such high precision, because the lens is stopped down and/or the subject is suitable for focus within a range of areas. However, certain subjects, such as close-ups and/or images using wide apertures such as f1.2/f1.4/f2 (or even f5.6 on a 300mm) also require precision focus. Mis-focusing on such subjects can have a huge impact on resolution in the resulting image. The Nikon D200 and D2X (or likely any Nikon digital body) cannot be relied upon to achieve critical focus with either autofocus or manual focus. Whether this is true for Canon EOS will be the subject of a future addition to this page. Contact: email diglloyd a comment on this article.
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