Hi--

>Hi there. I find the information on your page very interesting and quite
>valuable.

Thanks for the comments.

>I have a question related to your lens evaluations:
>
>I have just bought a Nikkor 500mm f8 mirror lens, and can't easily tell
>which type it is, with respect to your evaluation. The lens focuses down
>to about 15 ft or so. I would assume that it is then in the category of
>the 'later', and thus less capable.

No, the former (both types are good, the earlier a bit better) - if the
lens focuses normally, it is the earlier - if it rotates twice around
for close focus, it is the later.

>I had noticed, at least at near closest focus, that the image was a bit
>soft and wondered how soft it should be (yes yes I know it's much less
>$$ than real glass!)? I could still not get a 500mm lens of any type for
>anything close, by a factor of 5 or more, to what I paid for this.

Getting sharp images with a low-mass, short, diaphragm-less mirror is
DIFFICULT. Try setting it up on a tripod to make accurate focus easier,
shoot a rounded detailed subject, and make the exposure with flash to
see what it can do (or shoot 400 ASA Royal Gold [or Tri-X, in D76 1:1]
hand-held on a very bright clear day, and shoot five identical frames
of each thing at at least 1/500th second [hold the camera-lens lightly,
and squeeze the shutter release VERY gently - ignore the slow shift of
the image in the finder, don't "clamp" the gear to stop it, since this
raised the frequency of the shift) Down-rate the film speed 1/3-1/2
stop for correct density.

>Also, another weird question...some of the fixed aperture lenses like
>this 500mm have the ability to add neutral density filters to supply
>even LESS light... what possible purpose would one have to REDUCE the
>light coming into such a lens, compared to say a f1.4 lens?? I couldn't
>figure out a good reason? What's bright enough that f8 wouldn't make it
>easy enough to just speed up the shutter?
>
>Thanks in advance for any help you might give me!
>
>David Smith dws@austintx.net

Most newer cameras have very high shutter speeds, but way-back-when,
a 1/500th top speed was not uncommon, and Tri-X in Acufine even then
was a true 800 ASA - though I agree that the ND filter is pretty
useless unless shooting the sun (I do that...;-).