I see I stirred up a hornet's nest here. Let me try to clarify a
few things.
- I *originally* commented on making an optical adapter similar to a
teleconverter (only wideconverter). It could be used to "tighten" the
image circle of 35mm-sized lenses down to APS-sized sensor.
- The second bunch of comments was a "what if." If one could bring the
sensor forward in the mirror box (and compensate with a spacer flange out
front), it would allow more flexibility. Older lenses could be used
without the spacer flange and thuns have a smaller image circle (albeit
with a focus shift... focusing *beyond* infinity). New lenses could be
designed to take advange of the shorter registration distance if
necessary... or not. Other brand lenses could be used if there was now
enough room for a non-optical adapter.
On Sat, 23 Jul 2005, Godfrey DiGiorgi wrote:
- How do you drive the older lenses mechanical linkages?
just as you do in extension tubes
Which means that there would be no change from what we have now.
... except you would have the option of using a *shorter*
registration distance. You cannot do that now because the mirror will hit
the lens.
- More optics in the path means performance degradation.
what "more optics"?
See Bill Robb's post.
Wrong. No optics needed for standard K lenses. Just a simple
spacer to bring the flange out the same distance the sensor moved forward.
- Reduced room for an adequate mirror, etc with the "new K" standard
lenses.
sure, with smaller sensor size. is that a problem?
They've already reduced the size of the mirror in the D/DS. The size
reduction doesn't change the basic layout by much, certainly not by 10mm in
mirror box dimensions.
Maybe not 10mm, but a few probably.
Again, what's the point of it?
See above.
it also means that any "new K" lenses would be completely unusable on
the older Pentax bodies,
and they are not right now anyway (smaller image circle)
Not entirely true. You can use DA lenses on K mount bodies, assuming that a
24x24mm image circle is enough to satisfy your needs. In some cases, the DA
lens might cover the full frame perfectly (DA40/2.8 is like that), and
Pentax' D-FA lenses are fully optimized for the digital sensor while also
being 100% compatible with 24x36mm format.
Basically the "through" adapter is like a K-mount teleconverter,
but without any optics.
Then it would do nothing other than add complexity to the lens line.
actually, no.
Once again, what would this do that hasn't been done already with the current
lens mount? It would create a new, incompatible line of lenses, it would
allow older lenses to be used but only with an adapter. It would NOT change
the field of view characteristics of the older lenses on the digital sensor
bodies at all, not without optics to do that, and that would degrade the
performance of the lenses that you were trying to use as well as cost some
speed.
Unless you can articulate exactly what benefit this might have, I see no
advantage to it whatever.
Godfrey
Basically, it is making a body that has "built-in" a
"negative-length" macro tube. It comes with a macro tube on it of the
same length. If you choose to *remove* that macro tube, you get wider FOV
(with a focus shift, of course). If you remove the macro tube and replace
it with one that is the same length, but has another brand flange on the
end, you can now use those as well.
If the geometry of the mirror box can tolerate this with the
smaller sensor, I don't see why it couldn't be done.
-Cory
--
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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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