Date: Fri, 30 May 1997 14:31:39 +0000
From: Willem-Jan Markerink 
Subject: Re: IR focus on AF Nikkors

> Date: Thu, 29 May 1997 15:04:23 -0600
> From: Mark Walberg 
> Subject: IR focus on AF Nikkors
> 
> Say, all the discussion of the apochromats recently here makes me recall
> something.  I've heard that ALL AF Nikkors are corrected for IR focusing
> and, therefore, do not need to be refocused for IR.
>     Is this true?  If it is, why has Nikon gone to the trouble to make that
> a feature of all their AF lenses?    Would keeping lens designs place any
> significant constraints on performance in the visible spectrum?
>     The only AF Nikkor I have handy to look at right now is a 105/2.8AF
> Micro, and it doesn't seem to have any mark for IR focus correction.
> Mark Walberg        walberg@simmons.swmed.edu

Look in your lens manual/instructions, mostly they refer to a 
particular f-stop for IR-focus correction.
Only the very best high end lenses can do without IR-refocus.
Only Leica and Zeiss use APO in this strict sense, which IMO is the 
only true indicator of full apo correction. 
I have some nice ASCII art from Andy Davidhazy on my homepage, 
explaining the focus differences for normal lenses, achromats, 
apochromats and mirrors:
http://www.a1.nl/phomepag/markerink/mainpage.htm


- -- 
Bye,

Willem-Jan Markerink


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



[note: 'a-one' & 'en-el'!]

Date: Fri, 30 May 1997 14:31:40 +0000
From: Willem-Jan Markerink 
Subject: Re: APO

> Date: Thu, 29 May 1997 21:24:35 -0500
> From: Patrick Singleton 
> Subject: Re: APO
> 
> >A lens that would be totally free of chromatic aberration is
> >just physically not realizable.
> >
> 
> Parabolic _mirror_ systems, common in astronomical telescopes can be totally
> free of chromatic abberation, Right?

They still use refractive glass, although very little.
But it won't allow a lens totally free of chromatic abberation....

- -- 
Bye,

Willem-Jan Markerink


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



[note: 'a-one' & 'en-el'!]

Date: Fri, 30 May 1997 00:12:04 -0700
From: Eric Welch 
Subject: apo redux

>An apochromatic lens is a lens system corrected for chromatic aberration
>at three wavelengths.

Okay, that's the BASIC definition. But when a camera manufacturer says the
word, you have to ask what they mean. In cheapo lenses sloppily designated
APO, that might mean in the center of the lens, or at one particular
focusing distance or whatever.

When Leica uses the term, they refer to all focusing distances, they mean
all aberrations are taken into consideration and they also mean all parts
of the image. They've made microscopes since the 1830s. And microscopes are
instruments that benefit from apochromatic correction, by definition in
Leica's terms. To claim it only applies to process lenses is not correct.
Or only to photography lenses. I photographed the niece of Aldus Huxley who
does research on frog hearts at the Med. School at the University of
Missouri, and she swears by her APO microscope. (Costs five times the
competition).

Nikon, as others have said, uses glass that has low dispersion, or
anomalous disperstion. Don't forget that dispersion is balanced with high
refractive index. Fluorite glass (only Canon uses it in 35mm lenses) has
very good characteristices, but newer glasses are approaching it without
the down side of fluorite crystals. Also aspheric lenses are becoming more
popular as they become easier, and thus cheaper, to make. Does Nikon make
any aspherics besides the old 58mm 1.2 Nocto? Aspheric elements can make a
lens perform better for less money than with exotic glass.

Another APO point is that lenses of less than 100mm don't really need
apochromatic correction (in 35mm). That's because chromatic aberration only
becomes a problem at around that focal length and longer. The longer the
more it needs it, and that is one reason long fast lenses are so expensive.
The glass needed to make the corrections gets very expensive. In fact,
Leica's famed Noctilux (nick name for the first lens it was used in) glass
has a refractive index of something like 97 (or is that 9.7 or .97? It's
been a long time) and it cost well over 400 DM per kg. in the mid-80s.
That's unpolished! Imagine what it costs now! Other manufacturers (except
Sigma?) are probably paying similar prices for similar glass.

What does it mean for the average user? Diddly. :-)

- ---------------------
Eric Welch
Grants Pass, OR

It said,  Insert disk #3,  but only two will fit!

Date: Fri, 30 May 1997 09:15:43 -0400 (EDT)
From: Dr Grover Larkins 
Subject: Re: APO vs ED vs Fluorite etc.

The term APO or Apochromatic is a VERY loose one -- all it really means is
that three colors are focussed on the film plane at the same time. No
specification is made for the amount of deviation between the true focus
points (wavelengths) so you could concievably have Red Blue and Green in
perfect focus and be 200-500 um out of focus in the yellow, orange and
violet for example. 

It is for PRECISELY THIS REASON that Nikon and Canon do not use this term
I have been told by their representatives in the past.

A superachromat is a four (4) color in focus lens NOT SIX! An achromat
focusses two colors on the film plane simultaneously. 

In actuality the really critical part is the amount of deviation from the
focal plane for the off-intercepting wavelengths. Hence an APO lens may,
in all truth, perform worse than a well designed achromatic lens.

Fluorite and *ED* lenses have partially anomolous dispersion
characteristics in the visible band allowing designers to correct for
these deviations more easily (reducing the extrema between the focal plane
vs wavelength intersections) and flatten out the wavelength vs focal
length curve with fewer elements. Fluorite is CaF2 or calcium fluorite --
a crystal which is both sensitive to water and to changing it's index or
refraction with temperature (hence Canon's *white lenses* I've been told)
and is always either encapsulated or in a sealed cell within the lens.
Nikon's ED glass I've heard (correct me if I am wrong) is a Silica glass
which is fluorite loaded (or with rare earth elements) which likewise
have partially anomolous dispersion characteristics. The advantage is
environmental stability (chemical and thermal) of the optical qualities of
the elements (relative to fluorite).

ED Elements tend to be *softer* than flint glass so Nikon in recent years
has included an ultra hard front element (planar) to protect the soft and
expensive ED element from odd objects and lens paper with sand imbedded in
it....

Regards,

Dr. Grover Larkins, Jr.

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl