T3D Re: Iris placement

[john bercovitz <bercov@xxxxxxxxxxx>]

Bill Carter writes:
> The duliable John B. writes:
>> If you don't want vignetting when you stop down,
>> you put the iris where the chief ray crosses the optic axis.

Duliable?  Boy, you've got me there, Bill!  I don't think I've
ever seen that one so cough it up - what does it mean?  Do I  
assume the Greek root meaning slave?  That makes me a service-
oriented guy?

> When photographing through the eyepiece of an existing optical 
> system, for instance a microscope, would one be creating an aperture 
> stop where the exit pupil of the ocular coincides with the entrance 
> pupil of the camera lens???

Unless the exit pupil of the ocular is a long ways from the ocular (a 
long eye relief ocular may well do this), it will be hard to make its 
exit pupil reach to the position of the entrance pupil of the camera lens, 
though this would be a desirable thing, because of the resultant minimal 
light loss and minimal vignetting.

In your quotation from me I was referring to where the iris is placed 
relative to the camera lens.  To make an ad absurdum argument, if you 
placed the iris at the film plane, you'd get an exposed patch of light 
the size and shape of the iris.  You'd have to have an iris bigger 
than the image gate to have the whole image equally exposed!  As you 
move the iris further from the film plane and more towards the lens, 
more of the film is exposed but the central part of the image is still 
exposed more than the outer reaches until finally you put the iris where 
the chief ray crosses the iris.  Now no matter how far you stop down, 
the chief ray still makes it through to the film plane.  This goes 
for all chief rays so it's best to pick a highly oblique chief ray and 
use its crossing of the optic axis as the location of the iris.

You've got an analogous situation with a microscope and a camera lens
only the stop of the system is that of the ocular.  So you want to 
have the camera's iris wide open in hopes of not getting any vignetting.  
Now you can stop down the exit pupil of the _microscope_ to cut down the 
exposure and to get less vignetting.  Another thing that will help cut 
down vignetting will be to focus the camera's lens out and refocus with 
the microscope.  When you focus the lens out, you get its stop out farther 
to a better postion to cover the whole image gate.  Best, however, is to
get the exit pupil of the microscope co-incident with the entrance pupil
of the lens as your intuition told you.  Then there are no losses and
you don't have to take these half measures of focussing out the camera 
lens and stopping down the microscope's exit pupil.

Aha!  Here's a good way of thinking about it: You don't want the entrance 
pupil of the lens to obscure the exit pupil's view of any part of the 
image plane.  Make sense?  So you want to open up the camera lens.  You 
know, you could do this with a camera and your eye's entrance pupil 
standing in for the exit pupil of the microscope.  Probably to see any 
part of the film gate's perimeter at all, you're going to have to paste 
your eyeball to the lens and open the iris all the way up.  You just 
wish you could get your eyeball even closer but it's physically impossible.  
However, if you are using the type of ocular on your microscope which has 
extended eye relief, its exit pupil will be out in space and so it's 
possible to get it at least much closer to the entrance pupil of the camera 
lens. 

In a followup post you write:
> 1.  If I'm using an ocular as a field lens, what's the best configuration
> and placement for the relay lens? 
> 2. How do I create an aperture stop?
> 
> Should the exit pupil of the field lens coincide with the entrance pupil 
> of the relay lens, and is an aperture stop created there? Or am I totally 
> lost here?

A field lens images the exit pupil onto the entrance pupil.  That is, it 
redirects the rays exiting one system into the entrance of the other.  
Field lenses typically must be quite large in diameter to capture all the 
exiting rays.  Relay lenses do about the same sort of thing except they're 
transferring images so they also have to be large.  In your case I would
just get a long eye relief ocular and dispense with these extra optical 
components.

Sorry to be so slow responding.  I've been on a business trip and I'm 
just catching up now.

John B, writing for the steresoscopic and etymological discussion society


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End of TECH-3D Digest 236
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