Re: Medium format (was: plastic sheets

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

On  6 Jul 97 at 19:39, Jaap Los wrote:

> I own a Nikkor 2.8/80-220 mm ED zoomlens, a beauty. I understand from
> your mail messages, that I don't need to refocus this lens for IR
> photography?? Mind you, there is a special IR focus indicator on this
> lens, meaning, that a refocus is needed. Am I right? 

You are right, and it proves my theory that ED doesn't tell anything 
about performance in this field.
I am not even sure about the Leica 70-200/2.8, which would otherwise 
be a very nice solution for a range of focal lengths.

> Or will the
> ED-element sufficiently correct also for IR? Is there any difference for
> using HIE, Konica 750 or SFX200/Agfa200 as regards to IR focus
> correction?

The further into the IR-spectrum, the harder a true APO correction 
becomes. I vaguely recall a upper limit of 1000nm for either Zeiss or 
Leica....

And to make things more complex: it also depends on which filter you 
are using....a #25 requires less shift than a #87A, simply because 
you cover a broader range of the spectrum, and the broader the 
spectrum, the harder it is to compensate for the entire range.

Best proof is with Ekachrome IR, as for normal use it covers the
range from 450nm to 900nm, much more than HIE (which is always used
with at least a #25, ie from 600nm to 900nm).
Ektachrome IR does not require a focus shift, simply because you 
*can't* correct for the entire range....what you gain with infrared 
is lost in the yellow spectrum. Hence Ektachrome requires either 
stopping down (perhaps combined with a focus shift halfway), or....a 
true APO. Guenter Spitzing reports that APO's significantly improve 
image quality compared to full aperture shots....I can't prove that 
fact as long as I don't own a true APO....8-))




- -- 
Bye,

Willem-Jan Markerink


      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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