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EXCERPT page containing first few paragraphs. 2023-10-03 09:53:31

Carl Zeiss ZF.2 15mm f/2.8 Distagon T*

Field Curvature and MTF and Focus (Nikon D800E, Mosaic)

The 15/2.8 Distagon has one significant weakness: field curvature. Most lenses have some field curvature with only a rare few having what can be termed a “flat field”, such as the Zeiss 100m f/2 Makro-Planar. And even it might have a faint trace of field curvature on a 36+ megapixel DSLR at some focus distances.

A flat field means that a planar (flat) subject will be focused by the lens onto a plane (e.g., the sensor or film). Field curvature means that a planar subject will end up focused in a warped shape— sharp (perhaps), but sharp on a curved surface (do not confuse this with distortion, which changes the shape of the image).

Since the digital sensor is perfectly planar, the result is an in-and-out-of-focus effect, depending on location within the frame. Often this is quite subtle, but with high resolution digital it can become plain to see, and frustrating.

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Aperture series 2.8, 4, 5.6, 8, 11 available in full article
Zeiss 15/2.8 Distagon @ f/5.6, camera level

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