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Dorian Gray
Veteran Member
 
Join Date: May 2005
Location: Paris, France
 
2012-02-03, 11:12

Quote:
Originally Posted by Matsu View Post
The transluscent mirror doesn't swing, so I highly doubt that all 44.5mm of flange are needed for clearance.
Yeah, you don't need 44.5 mm for the mirror. The figure to keep in mind is 39 mm, which is the approximate back focus for all full-frame SLRs on the market. The mirror on all full-frame SLRs swings within this distance from the focal plane. The distance from flange to focal plane is less important, since that doesn't in itself restrict the position of the last element.

You and PB PM mentioned there are problems with putting the exit pupil too close to the focal plane, even if you make that possible by eliminating the mechanical restraints (mirror, etc.). In fact, two big problems arise, both caused by the very acute chief ray angle (i.e. incoming rays are very tilted from perpendicular at the corners of the sensor):

1. Individual photosites suffer from shading by the metal on the sensor and/or the microlens array and/or the colour filter (Bayer) array; if the shading is reduced by using less metal, then optical crosstalk occurs. This causes heavy artificial vignetting and colour shifts at the periphery of the image.

2. Astigmatism is induced by the filter pack between the lens and the sensor. The thicker the infrared and anti-aliasing filters are, and the more acute the chief ray angle is, the more astigmatism is induced. This is why Leica dumped the anti-aliasing filter and used a thin (0.5 mm) infrared filter in their rangefinders.

Astigmatism causes the tangential and sagittal foci to separate fore and aft with variation in image height, i.e. the field curvature of the tangential and sagittal foci is different. It is this astigmatism, caused by the filter pack (which has a different refractive index than the air that was assumed to be there during the lens design process), that results in NEX cameras producing soft corners with most rangefinder lenses.

If you're designing a lens that will only ever be used with one sensor (a fixed-lens camera), then you know the characteristics of the sensor and the filter pack, so you can design your lens around those properties. This lets you put lens elements really close to the image plane. For example, a large convex element near the focal plane converges the diverging bundle of light rays, effectively magnifying the exit pupil and pushing it away from the focal plane: a good thing for corner performance.

On another topic, I like Marc Newson's work on the new Pentax:



The camera looks simple and functional, and the materials are top class: aluminium alloy, natural rubber, etc. Nice to see a camera with some thought put into it. It's pretty big for a mirrorless camera of course, since it uses the K-mount, but that's not all bad as PB PM notes. Good to have some choice on the shop shelves.
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