Archived posting to the Leica Users Group, 1997/07/27

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Subject: Re: 70-180 vs. The World
From: "Willem-Jan Markerink" <w.j.markerink@a1.nl>
Date: Sun, 27 Jul 1997 16:17:03 +0000

On 26 Jul 97 at 19:07, Henning J. Wulff wrote:

> Willem-Jan Markerink wrote:
> 
> >I might have asked this before, but don't think I saw a confirmation:
> >Does the 70-180 still need focus correction for infrared film, or is
> >it a true APO in this respect too, just like all other Leica APO primes?
> >Does it show a red dot on the DOF scale, or is this mentioned in the
> >manual?
> 
> Apo correction does not imply that refocussing for infrared is superfluous.

In my book, and that of Leica (at least primes!), Zeiss, Mamiya and 
Angenieux it does.

> It is unlikely that even if the 70-180 were a true apochromat for all focal
> lengths, one of the three correction points would lie in the infrared; and
> besides, which wavelength in the infrared is it you want the best
> correction for - it extends quite a way. 

This does apply to all Leica APO primes....I don't think they allow 
two types of APO....either it complies with Leica standards, or it 
doesn't, but I doubt they would call it APO in that case.

I am pretty sure the above mentioned brands go up to 950nm at least, 
otherwise they wouldn't claim perfect focus for infrared film. I even 
don't rule out the possibility that it goes higher; there are some 
special infrared films that go up to 1100nm, but those require 
special handling (very cold storage, immediate processing).

> It might need very little
> refocussing for most infrared photography, but the Apo designation goes not
> guarantee it.

For the better brands it does....someone has to put the benchmark if
APO doesn't have a decent definition....;-)) 


- -- 
Bye,

Willem-Jan Markerink


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


<w.j.markerink@a1.nl>
[note: 'a-one' & 'en-el'!]