[rollei_list] Re: Whiteface T with Xenar question
- From: Eric Goldstein <egoldste@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Fri, 20 Mar 2009 16:17:41 -0400
Hi Richard -
Since we are shooting the camera/lens system, we do not want to
eliminate the camera. We want to know about focus, film flatness,
surface reflections/flare, and any other issues involving the entire
system.
This is why what Carlos is saying makes so much sense...
Eric Goldstein
--
On Fri, Mar 20, 2009 at 3:45 PM, Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:
>
> 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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