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Ray Angle and Off-center Color Shift
The Leica M9 suffers from off-center vignetting and color shading, caused by extreme ray angles. The M9 takes this into account for “coded” Leica M lenses, and one can also choose any such setting for a non-coded lens of any brand. In this way, most of the vignetting and off-center color shading is eliminated provided that the lens you’re using is a good match for a Leica M lens offered as a choice in the camera.
Some lenses are not really usable because of this color shading. Shown below is an image from the otherwise wonderful Zeiss ZM 21mm f/4.5 C-Biogon. It has a severe ray angle, the most extreme of any ZM lens, and this shows up with color shift at the edges: magenta at left and cyan at right.
The chief ray angle follows from the exit pupil position and the frame size.
In the ray angle column of the table, the first number refers to the object side, the second to the image side. So the
first number is nothing else than half the diagonal field angle; and of course it has nothing to do with the sensor response,
the colour shading etc.
Two lenses with identical chief ray angle on the image side may still show some difference in shading, if they have different speed. The chief ray angle is just a simple indicator parameter; the real function of a lens+sensor combination is based on a range of ray angles around the chief ray. The chief ray is that light ray which passes through the centre of the iris; in the wide open lens there are rays to the same image point, which are less inclined and some which are more inclined. The total energy is transferred by many rays which might have different efficiency.
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