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MTF: Zeiss ZM 28mm f/2.8 C-Biogon
Contrast wide open is already very high at center, but exhibits a steep drop-off to the corners. This is a pronouncd wave-type field curvature which persists to f/5.6.
Realistically, the 28/2.8 C-Biogon will be a marginal choice for a planar (flat) target unless stopped down to at least f/8 (a building, distant horizon, etc). In this regard it is similar to the Leica 35mm f/1.4 Summilux ASPH FLE, which has a a similarly strong wave-type field curvature. The zone of sharp focus most likely arcs forward (closer to the camera), then reverses back starting around the edges.
Stop down to f/8 - f/11 for subjects where sharpness across the field is needed. Moreover, a focus bias to the distance is likely to better balance total sharpness across the field.
Actual micro contrast is likely close to 80% across the frame—just not in a geometric plane.
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Diglloyd Guide to LEICA contains in-depth coverage of Leica M system cameras and lenses, with additional coverage of Leica M Monochrom, Leica Q.
Special emphasis is placed on Leica M lenses and certain Zeiss ZM lenses.
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- Aperture series from wide open through stopped down, showing the full range of lens performance and bokeh.
- Optical quality analysis of field curvature, focus shift, sharpness, flare, distortion, and performance in the field.
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