Re: Apo stuff

["Henning J. Wulff" <henningw@xxxxxxxxxxxxxx>]

Willem-Jan (and now the rest of the IR list),

>> Apo correction means only that three colors are truly corrected for; these
>> usually occur in the visible spectrum. Only the 250 Hasselblad, the
>> 'Superachromat', has two correction points in the IR as well as being at
>> least apo corrected in the visible region. The ONLY lens.
>
>Sorry, you are wrong. The UV-Nikkor is in the same league, and there
>have been two older Asahi Ultra-Achromatic-Takumars as well (the
>85/4.5 corrected from 220nm to 1000nm, the 300/5.6 from 400nm to
>850nm). I believe Ken Sinclair has one of these beasts (Ken?).
>There has also been a Hassy UV-Sonnar 105/4.3, corrected for both UV
>and IR (no data given in my books).

The lenses you mention are very well corrected over a wide spectral range,
but do not have the 5 (6, actually, for the Superachromat) point correction
that I was talking about. There _is_ a difference.

>> Other lenses
>> (including Leica's apo lenses, are very well corrected, but not necessarily
>> in the IR.
>
>But this fact is even confirmed in the BAS reports!....8-))

All I'm saying, WJ, is that the term 'apo' cannot be counted on to ensure
focussing on the same plane as the three visible wavelengths the lens is
corrected for! You keep on saying, 'the Leica lenses need no focus
correction for my infrared needs', which I'm not disputing. It just has
nothing to do with the term 'apo'. It's your use of the word 'apo' that
causes me to write this stuff. You're going to have to read the actual
stuff I'm writing, boring as it may be :-).

>
>> If you put an 87a filter over the lens, and load up with an
>> appropriate emulsion, you will have to refocus virtually every lens made
>> for general photographic use, including the Superachromat, as 1000nm is
>> beyond its correction.
>
>My literature shows correction from 400nm to 1000nm....
>
>> As for 1200nm sensitized emulsions, they are imaged
>> by apo lenses at times, but these are corrected for three IR points, not
>> for visible light. Some lenses are computed for even longer wavelengths,
>> but these are sometimes not even achromats; their correction is for only
>> one wavelength, and they are quite bad off that wavelength. IRTRAN based
>> materials are often used for lens materials, with very sharp cutoff
>> filters, or narrow acceptance sensors.
>>
>>
>> >In my book, and that of Leica (at least primes!), Zeiss, Mamiya and
>> >Angenieux it does.
>>
>> I don't have any Leica 'apo' lenses, but I do have a Mamiya 150 which they
>> claim is apochromatically corrected. It has to be refocussed for infrared,
>> and that includes 800nm stuff. IR settings on lenses are typically computed
>> for 750nm, in any case.
>
>I was talking about the RB/RZ APO's, the 150mm for Mamiya 7 does not
>have APO in its name, though it is indeed mentioned in the text.
>Note that you can only have HIE in 70mm, not in 120/220, and the RB
>is the only one with which you can shoot this format. I am pretty
>sure the designers had the combination of APO lens, HIE and 70mm in
>mind at one time. Konica hardly requires it, although even their Hexar
>allows two AF-settings for IR: 750nm and 850nm; the first for Konica
>750 IR, the latter for HIE.

Maybe they did have it in mind. It just does not have anything per se to do
with Apo correction.

>> >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.
>>
>> Since Apo doesn't have anything per se to do with IR, their lenses don't
>> have to focus any IR reasonably; if they do, it is just a bonus. If you
>> hold Leica (or any other company to the standard that apo lenses must focus
>> IR in the same plane that the three correction points are on, you are
>> holding them to a standard which they have not claimed; in fact, you are
>> holding them to 'superachromat' standards, and only Zeiss has claimed, and
>> achieved that commercially on one lens. Leica, and the other companies, are
>> definitely capable of producing more such lenses, but have chosen not to
>> for economic reasons, most likely.
>
>It is not a bonus, both Leica and Zeiss are quite proud of this
>performance. I am quite sure this was intended from scratch.

Maybe it was intended from scratch. It just does not have anything per se
to do with Apo correction.

>> >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....;-))
>> >
>>
>> It does have a perfectly good definition. It has just been abused by
>> companies from Rodagon to Sigma.
>
>I was recently even more confused, as you might have read on the IR
>list....a Sigma rep claimed perfect focus for IR with their
>APO's....but until I get sufficient proof, I tend not to believe
>it....:-))

Sigma's so-called 'apo' lenses tend to focus a lot of visible wavelengths
rather haphazardly, or rather poorly, so if they use the same standard for
IR, maybe these lenses do focus in the same very general plane according to
their standards. Fuzzy visible + fuzzy IR = fully corrected in Sigma's
terms. Actually, some of their stuff is medium OK optically.

BTW, I talked with a fellow a while ago who had put the Leica 70-180/2.8
lens on an optical bench, and had concluded that this lens was only truly
apochromatic in one focal length region. I don't remember whether he said
which one, and I don't know how good he was, either. In any case, this
makes sense. True apochromatic performance is hard to achieve, and
achieving this consistently over a range of focal lengths would be
incredible. However, for photographic use the deviation of the chromatic
correction curve from the focal plane is what counts, not the intersections
that 'apo' lenses promise. The Leica lens obviously has a very flat
deviation curve, and that would make the actual achievement of apochromatic
correction over all focal lengths and focussing distances meaningless.
Additionaly, correction for longitudinal chromatic aberration is but one
aim for a high quality lens, and 'apo' lenses only promise that correction;
not even a low deviation. The Leica 'apo' lenses are superb photographic
lenses, and, according to you and others, excellent for IR. All the
aberrations have been balanced masterfully, just don't expect more of the
term 'apo' than it can deliver. Also please note: all true 'apo' lenses are
that only for a limited range of focussing distances.

>> Anyway, I thought this was better put on private e-mail, so as not to clog
>> up yet another list with these minutae :-).
>
>Aw, but this is fun stuff!

OK,OK, for lack of being able to sit down with a beer to discuss these
things, this is not bad...


   *           Henning J. Wulff
  /|\     Wulff Photography & Design
 /###\      henningw@xxxxxxxxxxxxxx
 |[ ]|     http://www.archiphoto.com


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End of Infrared-Digest V0 #160
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