Before the rush

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by evan-pak

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In the description for the new lens mount for their X-Pro series, Fujifilm says:

diagram from fujifilm

1: Specifically designed to maximize the mirrorless design of the body, the X-Mount has a short flange back distance of just 17.7mm. This means the rear lens elements are as close as possible to the sensor. The wide opening allows the lens to be mounted deeper within the body - up to 7.5mm (approx) from the mount surface - reducing the back focus distance of each lens to the minimum possible, thus achieving high resolution all the way to the edge of the image.

How does this shorter-than-usual distance help "achieve high resolution"? Does it come at a cost? I remember both Olympus and Leica making a big deal about the issues with non-parallel light rays at the edges of digital sensors, and how that caused a sort of vignetting not found with film (where the emulsion is basically non-directional). It seems to me that a shorter flange back distance would force light rays outside of the center of the frame to be even more oblique. Am I misunderstanding how this works?

The Sony E-mount is almost as close. Is this shorter distance all advantage, or does it bring problems as well? (Clearly, it means all-new lenses, but it should also make adapters easy.)

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"Sony E-mount is almost as close" and especially now that they are making a full-frame NEX body. I'm thinking about vignetting here, not the vignetting caused by a lens but by the sensor. – Esa Paulasto Oct 15 '13 at 17:29
up vote 6 down vote accepted

Having the lens sit much closer to the sensor removes the need to have a retrofocal (reverse telephoto) group which results in less extreme image correction, fewer lens elements and often a sharper image.

However the increased angle of incidence of light rays at the edge of the image often results in increased light falloff (vignetting). For this reason a weak retrofocal group is included even when a non-retrofocal design is permitted.

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The short-flange distance gives more latitude to lens designers.

There is no disadvantage at all to a short flange distance because the lens does not have to use it! Case in point, you can adapt almost any lens to a camera with such a short flange distance.

In general, digital sensors prefer light to hit it with a more uniform angle, so lens designers probably wont feel necessary to take advantage of the shorter distance except to create something more compact.

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So, if they do create something more compact, won't that cause problems with the uniform angle thing? – mattdm Apr 19 '12 at 2:43
It may but that is not a given. The point is that, the shorter the flange distances, the more options lens designers have. – Itai Apr 19 '12 at 2:44
What about the resolution claim? (Particularly, they call out edge-of-frame resolution.) How does that tie in? – mattdm Apr 19 '12 at 2:47
Guess they could but an element closer to the sensor which straightens the incidence angle. – Itai Apr 19 '12 at 2:49
Short-flange allow retrofocus lense design ( Having a short-flange is a way to get wide-angle even with the crop factor. – floqui Apr 19 '12 at 7:52

Sounds like marketing BS. Having the back element closer to the sensor lets you get away with smaller sensors and cheaper lenses that can maintain a consistent image quality from center to edge. But there's nothing "inherently" better.

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Doesn't "getting away with cheaper lenses" mean the same thing as "better image quality for the price"? – mattdm Apr 18 '12 at 23:07
Indeed, that's the value proposition for the consumer. – cadmium Apr 19 '12 at 3:21

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