Re: ATM beginners basic mirror question

[Stephen Tonkin <sft@aegis1.demon.co.uk>]

Michael Perata @ p-solutions <michaelp@p-solutions.com> wrote:
>From a beginner.
>
>When I stop down a camera lense, the focal length doesn't change, the depth
>of field does.

Ah, but the focal *ratio* does. (focal ratio = aperture/focal length)

>
>Why would this be a different in a mirror.

It isn't. However, one way of looking at coma in mirrors is that it will
exist for abaxial objects in any non-spherical mirror, and the degree of
coma will correspond to the degree of non-sphericalness (non-
sphericity?). When a mirror is stopped down, what is left approximates
more closely to a sphere; coma is correspondingly reduced.

Are you asking why the depth of field changes? If so, respond by email,
and I will reply likewise -- it's a tad OT for this list.

>
>----- Original Message -----
>From: Stephen Tonkin <sft@aegis1.demon.co.uk>
>>Ermm -- isn't this the wrong way round? A classic method of coma
>>reduction is to stop down the aperture, i.e. *increasing* the focal
>>ratio.
  ^^^^^
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