Re: Design

[Chris Albertson <chris@topdog.pas1.logicon.com>]

I can answer the question of why professional photographers
don't use mirror lenses.  Two reasons:

1) Mirror lenses tend to be slow.  Mostly f/8 but a few are f/5.6

2) More importantly, in general photography, when doing high
magnification work (with telephotos or close up photos)
most of any photograph is out of focus.  Yep blurry.  Say you do 
a head shot of a person or animal.  You focus on the eyes (not the 
nose or ear) and let the rest go soft.  The lens after all can only 
focus one plane. So the photographer, working in a three dimensional 
world must choose which plane.  When working with a 500mm f/4 lens 
depth of field will not help you as it is only a few inches.

So if a picture has much of it's area out of focus, how that out of
focus area looks is important.  All lenses render the in-focus
area about the same (sharply and realistically) but those mirror
lenses make out of focus highlights look like little round doughnuts
(bright circles)  These are distracting and in many cases make 
the image un-publishable.  It can be an interesting special effect
like when the sun reflects off the aperture blades and you see
a line of hexagons making a sun ray  or crosses on stars in astronomical
images.  But images with artifacts in general don't sell.

In astronomy you need not care how bad the out of focus image looks.
There are no foreground or background objects.  With camera lenses
designers try to make the spot diagram fall apart in artistically
pleasing ways.  A few lenses fail at this badly, all mirror lenses
and many zooms fail. 

I think you are right about Glass being best.  For a wide field,
fast, short FL system and your 45mm required image circle, I'll
bet some kind of symmetric (but with smaller rear elements), camera 
lens like design is what you will get.

Tom Droege wrote:
> 
>  My logic goes something like this, if the mirror lenses worked
> great, then the paperatzi would not be lugging around all that glass.
> Also I see rows of *big* glass lenses at the super bowl and other
> sporting events.  The few people doing similar things are also buying
> glass.  

> 
> Tom Droege

-- 

--Chris Albertson

  chris@topdog.logicon.com                Voice:  818-351-0089  X127
  Logicon RDA, Pasadena California          Fax:  818-351-0699