On 2 Aug 1998 18:01:41 GMT, rwh56@aol.com (Rwh56) wrote:

>I cannot say that I have had a bad experience with my inexpensive ($100) used
>Spiratone 500mm mirror lens.
>
>The concern with mirror lens design is that they have a very small depth of
>field, smaller than a refracting lens at the same aperture ( f8). The depth
>of field is more a function of lens diameter than light transmission. A
>mirror has a larger diameter than a refracting lens for a given focal length
>and f-stop. One of mirrors is attached to the front element thereby blocking
>light that would otherwise enter if it was a plain refracting lens. So, the
>frustration I experience is that I would like to stop down the lens further to
>increase the depth of field.

Hmmm, if the diameter is increased to keep the transmission constant,
the difference in DOF would be equivalent to that of only a part of
a stop change - not very significant... In practice, mirrors tend to
be slower than rating, though. And I find that my Nikkor (older-style)
500mm f8, possibly due to its "bad bokeh" (contrasty, hard rendering
of out-of-focus areas), actually has more than the expected DOF
(often universal, even with subjects that have depth - for an example,
see the 7th photo under "Phun Fotoz - photographs - places" on my web
page [of a building face shot at a strong angle to the camera...]).