[rollei_list] Re: OT: Leica vs. Zeiss
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 20 Nov 2009 02:28:28 -0800
----- Original Message -----
From: "Frank Dernie" <Frank.Dernie@xxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, November 19, 2009 11:52 PM
Subject: [rollei_list] Re: OT: Leica vs. Zeiss
Quite so.
Several manufacturers have MTF curves on their sites, some
calculated, some measured, presumably from a minimum spec
lens???
Canon quote the calculated MTF for their lenses, which I
suppose rules out sample variation.
One example I have studied, simply because I have both
lenses and opinion is somewhat divided on discussion fora,
is the difference between the 400mm f5.6 and the 100-400
zoom at 400mm f5.6.
The MTF of the zoom is markedly better than the prime wide
open, the radial over the whole field and saggital over the
centre, but the prime is considered to be sharper by many
users. My copy of the zoom is quite close to my copy of the
prime.
The zoom has 17 elements and a image stabiliser, the prime
has 7.
FD
On 19 Nov, 2009, at 21:14, Richard Knoppow wrote:
the performance of an actual lens can often very
considerably from the ideal
Modulation transfer function can be arranged in several
ways. First of all, as you know it varies with stop, angle
of the image, and for sagital or radial fields. The MTF can
be presented as for a constant f/stop in which case its
ploted against image angle for both radial and tangential
(or sagital if you prefer, both terms are used) fields. It
can be plotted for a constant image angle where the variable
is the f/stop. If pure diffraction is plotted the lines have
a constant slope with image angle but for practical lenses
they often have waves in them. These are from higher order
aberrations.
One difference in lenses is the shape of the MTF curve
where resolution is plotted against contrast for a specific
image angle and stop. One can find a lens which has
resolution that stays up to some point and then drops
sharply. When compared to another lens where the resolution
slopes off at a slow rate, that is the slopes for the two
cross each other at some point after which one drops off
quickly and the other does not, the one with the lower
resolution limit but higher mid value resolution often makes
sharper looking images. This is related to the same property
of human vision that results in the apparent shapening of
images due to acutance effect on film. Simply, high contrast
transitions between light and dark are interpreted as
sharpness even when the actual resolution is lower.
It is possible to design a lens for either effect. I
can't tell you in detail what the designer does that it
different but it has to do with the balance of higher order
aberrations and the illumination of the acceptance window of
the lens. For instance the shape of the MTF curve will be
different for an obscuring stop than for a normal centered
stop where the two have the same effective stop and not
considering the difference in aberrations (spherical will be
higher for the obscuring stop). For the same reason the
shape of MTF curves are not the same for lenses of different
geometrical mapping properties, i.e, a fish-eye lens or for
lenses with different illumination, for instance a lens with
a "tilting entrance pupil".
There are other lens properties that have a stong effect
on the acceptability of the image for pictorial (and other)
purposes. For example coma or oblique spherical both produce
similar sorts of tear-drop shaped blurring which increases
as you move away from the optical axis. This effect is much
more unpleasant than plain spherical aberration which
produces a sort of haze all over the image.
This is all meandering: some lenses are poor because the
design is poor, some because they are made badly. Most
expensive consumer lenses seem to get about the minimum
individual inspection needed to eliminate real dogs. More
expensive lenses and those who will be sold to really
critical customers (like professional motion picture
photographers) probably get more. But you never know,
remember that the defective mirror for the space telescope
was tested using quite sophisticated methods. Some tests
made using simple "first principal" methods indicated it was
faulty but they were ignored because the people at
Perkin-Elmer trusted the more complex stuff. BTW, a back up
mirror was made by Kodak. Its perfect, its in storage
somewhere.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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