[rollei_list] Re: OT--for Richard: Telephoto chromatic aberration
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 18 Feb 2010 00:15:12 -0800
----- Original Message -----
From: "John Wilton" <john@xxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, February 16, 2010 8:25 AM
Subject: [rollei_list] OT--for Richard: Telephoto chromatic
aberration
I recently acquired a 300/4.5 Nikkor-H, a 1969 design with
6 elements in 5 groups. I was surprised to find that it has
noticeable CA. I compared it with a 280/4.8 Leitz Telyt
(1961, 4 elements in 4 groups) and found a similar degree
of CA. I then tried my 400/5.6 Novoflex Noflexar (old,
coated, 2 elements in one group): slightly less noticeable
CA. The first two, true telephotos, had red fringing; the
long-focus achromat had blue fringing.
The CA here is not terrible, only visible at hard
light-to-dark edges. But much more than one sees with fast
lenses covering a wider angle.
I wonder if there is a layman's explanation for CA in
relatively modern lenses with many elements covering a
very narrow angle.
---
I don't have a specific answer for this. Some
aberrations scale with focal length and that may be the case
here. Also, there are two forms of chromatic aberration,
lateral and longitudinal. Longitudinal is the variety one
usually thinks of. This is a variation in focal length, and
hense focus, with color. It is the aberration that is
usually balanced out of lenses by choosing the correct
combination of average index of refraction and dispersion
for the positive and negative elements. Lateral chromatic
aberration is something else, it is a variation of
magnification with color. all the colors focus in the same
plane but the size of the images is different. This
aberration is automatically balanced out when the lens is
symmetrical, actually the cancellation is complete only
where the image and object distances are equal, but it is
substantial even when a symmterical lens is focused for
infinity. Lenses like the Dagor and Apochromatic Artar are
examples. Lateral chromatic can be eliminated from
unsymmetrical lenses but it is more difficult.
These lenses could have either or both. Longitudinal
chromatic can be detected by using colored filters over the
lens and measuring any shift in focus between, say blue and
red. Lateral chromatic can be detected the same way except
that there will be no change in focus, only a difference in
magnification, i.e., the size of the images.
Good quality telephoto lenses should not have either
aberration and it is disturbing that lenses made by rather
respectible manuacturers display it.
I will ask if you are seeing this in a reflex finder or
if you have observed it in actual photographs. It _could_ be
the aberration is in the finder. If in the lenses it should
be visible in test photographs.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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