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