on 1/28/02 10:09 AM, William Sommerwerck at williams@xxxxxxxxxx wrote:
> A word about lens compatibility. The cells in a CCD sensor sit slightly below
> the conductive paths that surround them. This means that, the more-acute the
> angle of light striking the sensor, the greater the shadowing effect of the
> paths, further exaggerating the normal edge falloff that occurs with
> wide-angle
> lenses.
>
> Olympus claims their new lens design gets around this problem by moving the
> exit node farther from the film plane, thus making the angle of attack less
> acute. This doesn't sound right -- the angle at which the light rays strike
> the
> film is, by simple geometry, unavoidably linked to the lens's angle of view.
> How can you make the angle less-acute without simultaneously reducing the
> angle
> of coverage?
>
> So I did some research. In every reference I checked, a lens's focal length is
> _defined_ as the distance from the exit node to the film plane. In other
> words,
> you can't change the position of the node without simultaneously changing the
> lens's focal length. QED -- Olympus's claim is not kosher.
>
> Regardless, the problem of edge falloff -- whatever its cause -- is easily
> solved in software. It's a trivial matter to "map" the falloff for each lens,
> then process the image to remove the falloff. This could be done in the camera
> or with an image-editing program.
But in a 'retrofocus' design, the apparent exit node is out in the middle of
the mirror chamber, whereas the actual exit (last element) of the lens is
further out front, to allow for lens/mirror clearance.
Think of it as a projected image... the light falling on the film is
'projected' by the lens... The image is collected from the area of the lens'
'field of view', then this angle of view is concentrated and aimed onto the
film as a projected image. In theory it would be possible to design a
fisheye lens that collected light from a 180 degree field of view and then
projected it out the back several inches and onto the film. This would be
necessary if the apparent focal length of the lens (say 8mm) doesn't allow
enough room for a mirror, shutter, etc. In practice the projection is done
over ~45mm or so (the depth of the mirror box) but it could be longer which
would have the 'side effect' of having those light rays more parallel with
each other and more perpendicular to the film (or CCD sensor) when they get
to it... They would still project the apparent field of view, but the rays
would strike more perpendicularly and not have as much shading from the
electrical traces.
The downside is that to get the benefit of this the lens-to-CCD depth is
greater, so you could wind up with a camera having a mirror box 3 or 4
inches deep. But the lenses could be designed to accommodate that... if the
designers wanted to. That would necessarily mean that older SLR (OM series)
lenses wouldn't work even if you had an adapter machined so they could be
physically mounted; it would be like having a bellows or extension tube
permanently in the way.
--
Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney...
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