Russell wrote:

>> www.questarcorporation.com/QuestarPDF/QM_100_30003.pdf
>>
>> Operating range of f/no: 6.0 to 3.5 (less than 14 or 22)

> Those figures are the maximum possible aperture (= minimum aperture
> number) at varous distances. Smaller apertures can usually be used
> in practice at any given distance - just not larger ones.

This is a mirror lens.  I think the aperture is fixed (for a given
focus distance).  I know this is so with my mirror lenses -- 8" x
2000mm Schmidt-Cassegrain telescope is f/10 when used as a lens
(for all reasonable focus distances).

Questar appears to have optimized this mirror lens for close up
focusing.  As such, the effective focal length changes and given
a fixed mirror size, the f/stop ratio changes.  Things get hinky
when you are working at such close focusing distances (related
to the focal length of the lens).

You get f/6 when operating at 33.5cm distance and f/3.5 when at
the closed 15cm distance.

						Lee

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