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EXCERPT page containing first few paragraphs. 2023-03-21 16:51:55

Zeiss ZM 35mm f/1.4 Distagon

Vignetting vs Effective Aperture at Periphery

On axis (at center), stopping down one stop reduces brightness by one EV, by definition.

But at the periphery, stopping down one stop hardly changes the corner brightness: the center is a full exposure value darker going from f/1.4 to f/2, but the corners effectively do not change in brightness. At f/2, the overall illumination is now more even than at f/1.4, but the corners have not changed in brightness. In other words, the lens diaphragm (iris) is excluding very few oblique rays at f/2 for the periphery and essentially none at the corners (versus f/1.4).

The foregoing is why in outer zones there is no gain in depth of field and no change in image quality when stopping down from f/1.4 to f/2. Summarily speaking, there is negligible change in image quality from the mid zones to the corners in going from f/1.4 to f/2.

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Zeiss ZM 35mm f/1.4 Distagon relative illuminance (vignetting)

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