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EXCERPT page containing first few paragraphs. 2023-03-21 17:03:42

Voigtlander Nokton 50mm f/1.1

Voigtlander Nokton 50/1.1 — Field Curvature (Towers)

Curvature of field (do not confuse this with distortion) is the effect whereby the subject plane is not imaged as a plane, but rather as a curve. It is usually a parabolic curve, though it can also have a wave-type shape, depending on optical design. Field curvature can also vary with focusing distance, and can be forward (focus closer at the edges/corners) or rearward (focus more distant at the edges/corners).

For more on field curvature, see Making Sharp Images.

A planar subject is one at a uniform distance: the front of a building, any landscape shot at infinity, a resolution test chart, etc.

A lens with field curvature will appear to have poor optical performance on a planar target, and for this reason, a test chart is a misleading way to test many if not most lenses, since few lenses are completely free of field curvature. Such tests lead to erroneous and counterproductive conclusions about the optical performance for real pictures. That said, field curvature for a lens intended for landscape use can be a headache, especially forward field curvature (case in point: Canon 14mm f/2.8L II, see review in DAP).

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f/2 @ 1/4000 sec, ISO 160 0.85-stop pull
Leica M9P + Voigtlander Nokton 50/1.1

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