EXCERPT page containing first few paragraphs. 2023-02-03 06:39:21
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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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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.
- Make better images by learning how to get the best results right away.
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- Real world examples with insights found nowhere else. Make sharper images just by understanding lens behavior you won’t read about elsewhere.
- 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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