Re: OPTICS FOR TASS: REFLECTOR OR REFRACTOR?

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

aah@nofs.navy.mil wrote:

>   Chris -- I've never tried to use a projection lens for viewing. What is
> the difference between a 'projection lens' and a normal refractor (that is,
> can they be used the same way)?

I'm not an optics expert either but I've talked with a few.  Projection 
lenses are about the same as camera lenses except that they lack all the 
mechanical complexity of aperture blades and focus helix.  It is normal 
in most projectors to move the entire lens,  barrel and all,  to focus. 
This means no moving parts in the lens.  You just pay for the glass.  
Cost is pretty low for example a Kodak 105mm f/2.8 lens made for 
projecting 36mm x 24mm slides sells for $50.00 new.  The same by
Schneider for $240.  There are also lenses for projecting 6x6cm slides
and 6x7cm slides made by Hasselblad and Mamiya. that could better
cover a big CCD.

As I remember, our video projection lens looked a lot like a Kodak slide
projector lens.  Both are just a black cylinder filled with glass with no
moving parts.  The video lenses were bigger, just over a foot long and 
about 5" diameter.  I think we paid about $5K each for them and shipped
two with each projector.  The lens has more coverage then required for
the 2K chip so that is why the high cost.  

The difference between a camera/projection lens and a telescope is with the
image circle.  In most cameras the ratio of image circle to focal length
is large.  A 24mm FL lens has to cover a 43mm diagonal frame.

Before computers and zooms.  A traditional camera lens used a symmetric
design.  Kind of like two lenses back to back, one creates the image the
second projects it onto the film.  The idea was that whatever defect
was introduced by the first set of lenses would by removed by going
backward through the second set.  That's lens design ca 1900  Telescopes
use much simpler optical designs because thier narrow field and high
f-numbers let them get a way with it. 

Process lenses like those in Photographic enlargers and in graphic arts
are not corrected for infinity focus and are flat field when used as
intended in the 4x to 24x magnification range.  They are normally slower to
as they get long.  About f/5.6 at best.  I dont think we could use
any of these type lenses

Most camera lenses are not flat field as you focus closer than infinity.
They don't need to be as people don't take take pictures of flat walls.
Enlarger and process lenses are flat.  One exception is with some slide
projector lenses (they are marked "CF") that are purposely designed to 
focus a 35mm slide which as typically mounted is not plane but curved.
Kodak sells both CF and FF lenses are the same price.

It may be worth looking at projection lenses as optically they are like
camera lenses but mechanically simpler.  However I think the cheapest
400mm f/4 lens would be one you could build with off the self optical
elements and a custom machined barrel  The barrel could even be plastic.
I think the trick is to get an optic catalog and see stock lens elements
sell for.  I can envision a tube design where you just drop elements
and spacer tubes into a lage tube then screw on one retainer ring.

Is the math correct below?

With only 2k pixels across 40mm we only require 25 lines/mm resolution.
as we are sampling at 50pixels/mm.  If the lens projected an image with
highr spacial frequency it would be a waste. This is in the range of what 
a single use disposable camera can do.

--Chris Albertson

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