on 8/26/03 9:51 PM, Moose at olymoose@xxxxxxxxxxxxx wrote:
> However, this is not a Vivitar or Tamron, etc. The Sigma 600/8 has a
> slide-out filter holder on the side of the lens almost back at the
> mount. Easy to change filters with lens on the camera. With an extra
> holder or 2, switches would only take a couple of seconds.
>
> My suspicion is that only ND filters will work in this posititon (on any
> brand) as the image is close enough to formed that waterhouse stops
> would cause vignetting. I think this partially for vaguely formed
> theoretical reasons, but mostly from the fact that nobody has ever done
> it on a commercial product. I can't imagine that some manufacturer would
> not have tried to get a feature jump on the competition (and an actually
> useful feature, at that) if they could figure out how to do it.
>
> Moose
>
> Jeff Keller wrote:
>
>> Unfortunately the filter on a Vivitar Series 1 resides inside the T-mount so
>> changing the filter involves taking the camera off
>>
Seems like in order to limit the aperture of a mirror lens, you would have
to restrict the diameter of the front aperture -without- blocking so much
that you close off the entire area between the edge and the central mirror.
This probably will reduce the light entering the lens, but it might not have
the same effect as a traditional lens design's aperture with regard depth of
field. It might also simply cause vignetting to reduce the light level in
aggregate, without changing the light level in the non-vignetted areas.
Further, it probably will reduce the resolution of the lens since that is
somehow related to the total aperture opening, at least for astronomical
mirror telescopes. I think the only way to reduce the light on a mirror lens
is with ND (or other) filters, either in front or at the smaller end of the
light path. These won't change the depth-of-field, unfortunately. My
Celestron 300/5.6 uses screw-in 35.5mm (I think) filters in the back of the
lens, and in order to put them in place you have to remove the T-mount. My
Vivitar 600/8.0 Solid Cat uses a similar setup. To use a polarizer
practically it would have to go in front, so it would be big and
expensive...
--
Jim Brokaw
OM-'s of all sorts, and no OM-oney...
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