Re: UV film

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 14 Jan 97 at 21:35, George Madrid wrote:

>    So any takers on how the focus shift will work out for the UV?  The
>    last time I brought this up I took a wild guess and said it would be the
>    opposite direction as the IR shift, but someone said it's likely to be
>    the same direction.  Any input?
> 
> My input:  
> 
> I was under the impression that the focus shift for IR was due to the
> longer wavelength of light in the IR spectrum.  Since UV would have a
> shorter wavelength, I would therefore expect the focus shift to be in the
> other direction.
> 
> Can anyone comment more definitively on the cause of the IR focus shift.
> That's where we should find our answer.  

The reason is within the different diffraction of IR vs 
visible; longer wavelengths bend less than visible and UV (with 
normal lenses). To compensate that, you need to refocus towards you, 
away from infinity; or increase the lens-film distance (bellows extraction).
Andy Davidhazy and I had a thread about this on PhotoForum many 
months ago, one of his replies contained a master piece of ASCII-art, now 
online on my homepage, where he explains the focus differences:
(use a fixed font like courier to see this properly on your screen!)

wavelength      simple lens     achromat       apochromat       mirror

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

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


As you can see, both ordinary lens and Apochromat require opposite 
focus shifts for IR vs UV, but an Achromat requires a focus shift in 
the same direction. Also note that the special UV lens from 
Hasselblad is called Super-Achromat, not APO.

One of the problems Andy and I discussed is the impossibility to 
refocus for UV with ordinary lenses (wide open): with IR you can 
focus properly on infinity by refocusing closer, towards you, away 
from infinity....for UV you need to refocus beyond infinity....8-))

Such refocus beyond infinity requires high end APO lenses, since
these mostly feature excessive slop beyond infinity, to compensate
for heat expansion of the rare earth glass elements.

Of course, a mirror lens seems a nice solution as well....I am still 
tempted to try the Russian 300/5.6 for this reason one time....
--
Bye,

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      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 17