Date sent:        Wed, 05 Jun 1996 17:17:52 -0400 (EDT)
From:             ANDPPH@ritvax.isc.rit.edu
Subject:          Follow-up to IR discussion
To:               List for Photo/Imaging Educators - Professionals - 
		  Students [photoforum@listserver.isc.rit.edu]
Send reply to:    photoforum@listserver.isc.rit.edu
Originally to:    photoforum: ;

Willem,

... regarding our discussion of focusing IR manually vs. by autofocus.

>> If they were in the same direction, then it is possible the lens 
>> was achromatized in such a way that both IR and UV came to a focus 
>> closer to the lens than green. Normally IR _and_ UV are focused 
>> farther from the lens in achromatic lens designs ... which is the 
>> color correction in most lenses. 
>
>Ho there! Perhaps I am getting into a mine field of specific lens 
>designs, but I not only believe an achromat is a quite common lens 
>design, but I also know pretty sure that UV correction is mostly, if 
>not always in the opposite direction of the IR correction. The 
>general rule for visible light is that red focuses behind the film 
>plane (ie you need to focus closer), and blue in front of the film 
>plane (ie you need to focus further away). 

This applies to simple lenses but not mirror lenses and the chromatic
"focal length" for compound lenses (such as those used in cameras) is 
different and can be quite complex to the best of my info.

>So there respective  extremes (IR for red, and UV for blue) should 
>require some more correction in same respective direction. In all my 
>German IR & UV literature it is also mentioned that UV needs the 
>opposite correction of IR. Should those books be rewritten??....:-))

;-)   YES!   Just kidding!!!  but kidding aside the "classical" 
description of chromatic field "plane" is as follows:


wavelength      simple lens     achromat       apochromat       mirror

800                |     /         |  /            |/             ||
-                  |    /          | /             |              ||
700                |   /           |/             /|              ||
-                  |  /            |             ( |              ||
600                | /            /|              \|              ||
-                  |/            / |               |              ||
500                |            (  |               |\             ||
-                 /|             \ |               | )            ||
400              / |              \|               |/             ||
-               /  |               |               |              ||
300            /   |               |\             /|              ||
                   ^               ^               ^               ^

---> light         |
                   ^ indicates optimum location of film plane 


Note that if you use a simple lens you can bring ONE color to a sharp 
focus. Generally, in the olden days this would be set to a blue 
wavelength since films were not sensitive to longer ones. In this case 
the green wavelengths are chosen to be sharply focused on the film. 
Note than an achromat bends the chromatic focal plane so that TWO 
wavelength come to a common focal point - 350 and 600 in this 
illustration. More importantly the uv and ir come to a focus further 
from the lens! 
The classic layout for an apochromat is that the filed is bent twice 
(as well as the departures from a single plane being smaller) and IR 
and UV are focused at opposite sides of the "light" image plane. 
However, sometimes a very well corrected achromat is also called an
apochromat, to add to the confusion.

A true mirror lens is for all practical purposes free of changes in 
focal length due to changes in wavelength because angle of 
reflection = angle of incidence regardless of light wavelength.

>> In simple lenses the IR focuses farther from the lens than "light" 
>> and UV closer. I wonder if one could design a lens in which it is 
>> not the UV rays that are made to focus farther but rather the IR 
>> rays nearer. I do not know.
>
>It would surely collide with several optical laws, I am sure....;-))

I claim ignorance on this topic but leave the door open for further
discussion...  ;-)

Oh, BTW, how do I know that most camera lenses are achromats? I don't 
but I suppose any manufacturer who thought they had the best possible
correction would probably claim they had an APOchromat ... simple 
marketing advantage. 
However, I am not sure about this. OTOH, it would be impossible to 
bring UV rays to a focus if they came from infinity if the UV rays 
focused closer to the lens than light rays ... something to think 
about too as you mention below. I do not trust in "extra play" ...

>Yes, the normal 'beyond infinity' focussing required for UV
>photography struck me too. Some lenses might have enough play to go
>a little beyond, but certainly not all. 

I will have to devote just a bit more reflection on the rest of our
discussion. And, in fact, I will ponder a bit more about the points
you've raised above as well.
Maybe a little at a time we will both come to a common focus!  ;-)

regards,

Andy

 o o  0 0 o   o  Andrew Davidhazy, Imaging and Photo Tech - High Speed Photo
  \/\/\/\/\/\/   andpph@rit.edu,  http://www.rit.edu/~andpph,   716-475-2592  
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