[rollei_list] Re: Lens Tests
- From: Ardeshir Mehta <ardeshir@xxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sun, 27 Mar 2005 13:10:56 -0500
Thanks (once again!) to Richard K. for this excellent post. Especially
this bit:
> "[...] one reason there is sometimes disagreement about the resolution
> of lenses."
So I was wondering - should not lens resolutions be measured with the
lens ALONE, and film resolution with the film ALONE? For suppose the
lens can resolve 600 lp/mm but the film can't, won't the test end up
skewed?
And if one day digital sensors can be made with elements so small and
so closely spaced that they are capable of resolving 600 lp/mm, won't
that same lens perform "better"?
Or else, could not lens resolution tests be done with extremely fine
grain film like "Gigabit Film"?
Cheers.
+++++
On Sunday, March 27, 2005, at 06:52 AM, Richard Knoppow wrote:
> -----Original Message-----
> From: Fred Fichter <ffichter@xxxxxxx>
> Sent: Mar 27, 2005 12:56 AM
> To: rollei_list@xxxxxxxxxxxxx
> Cc: rolleiusers@xxxxxxxxxxxxxxx
> Subject: [rollei_list] Lens Tests
>
> Rollei vs Hassy vs Mamiya...
>
> http://www.hevanet.com/cperez/test/fourcameras.html
>
> Chris Perez does careful testing and is aware of the pitfalls of
> in-camera testing. I would like to see comparisons of the lenses where
> some of the unknown factors were in better control. I am not convinced
> that film flatness is the only variable. Aerial image testing would
> give a better idea of what actual lens resolution is although it is
> not so good a method for measuring either edge contrast or overall
> contrast.
>
> The compromise made in lens design is between resolution and edge
> contrast by choosing the way in which the energy is concentrated in
> the image. One measure is Strell ratio, which measures the ratio of
> the light energy in the central diffraction peak to the overall
> energy, i.e. how much is in the peak and how much is in the
> surrounding rings. The narrower the central spot the higher the
> resolution, everything else being equal. However, this results in a
> relatively large amount of energy in the rings. By making some changes
> that broaden out the centeral spot the energy in the rings is reduced
> improving the contrast but reducing the resolution. This is sometimes
> known as apodising (removing the feet). A similar choice is made in
> the design of directional radio antennas such as satellite dishes.
> Since lens reso lution and film resolution are likely to be nearly the
> same for practical M F lenses the resolution on film may be
> misleading. Resolution must also be measured at a stated contrast
> between the resolved lines and the background. The lack of such a
> specification is one reason there is sometimes disagreement about the
> resolution of lenses.
>
> [... etc. ...]
>
> --
> Richard Knoppow
> dickburk@xxxxxxxxxxxxx
> Los Angeles, CA, USA
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