[rollei_list] Re: Whiteface T with Xenar question
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 20 Mar 2009 12:45:51 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, March 19, 2009 7:38 PM
Subject: [rollei_list] Re: Whiteface T with Xenar question
Absolutely. This was mentioned this earlier in the thread.
Unless the
cameras were brought up to factory specs by qualified techs
before the
tests, who knows how out of adjustment they are...
Eric Goldstein
The problem with testing lenses mounted in cameras is
to eliminate the camera. While the overall performance is
obviously important the question is often what the quality
of the lens is.
Since photographic tests introduce the limitations of
the film the performance indicated will always be lower than
the lens actually does. Chris Perez is aware of all this.
We also discussed some odd results and their implications
about the measurement method. For instance, it is rare (but
possible) for a lens to have better resolution away from the
optical center than at the center. Where this is found on
film it generally indicates that either the lens has a
curved field or the film was not flat.
Somewhat more consistent results can be gotten by
examining the aerial image. This is the image produced by
the lens in air and is examined using a high quality low
power microscope. It is the technique used with a optical
bench but its possible to mount an entire camera, if not too
large, on the bench. Care must be used because the optical
method is subject to a number of errors including false high
resolution. This is caused by a phenomonon similar to
ailiasing. One clue is that the number of bars on a bar test
do not agree with the test chart. Many manufacturers use a
Siemens star test to detect defective lenses. This test is
qualitative and not quantitative. If the shape of the star
image is not what is expected there is something wrong with
the lens but it does not tell much about resolution or
aberrations. I am afraid most of the tests run by magazines,
etc., are full of errors and not very trustworthy. Plus, as
others have said, the real criteria is the quality of images
the lens and camera combination produces.
BTW, its possible to get calculated performance
information from any of the lens design programs provided
you have complete prescription information to put into them.
This includes the complete set of glass constants and many
patent or published lens prescriptions do not include this.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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