Thanks for the advice on the center filter for the Metrogon. I actually
had a different problem in mind. As I understand it a piece of flat glass
adds spherical aberration. Someone in this group reported that the
Metrogon was calculated with this in mind by including compensating
aberration of the opposite sign so optimum resolution would be reached when
one of the center filters was in place. (These things are pretty
thick.) Conversely, I have a publication (in fact, two of them) giving
data for the glasses, radii and spacings of the Metrogon and neither
mentions any filter. Then again, just because the published designs do not
include a center filter does not mean B&L did not tweak the design for the
military optics. Thus, I simply have no idea how much difference the
filter makes and, sadly, I have never seen a clear center filter for these
lenses.
What you report on the importance of uniform exposure is very revealing and
makes clear why looking only at resolution data is a mistake and the whole
situation must be considered in the context of making real photos.
Thanks again!
Bob
At 17:42 01.09.02 -0500, you wrote:
>resolution could actually be higher with the center filter, since excess
>exposure (1 stop) can cost you 20% or more of resolution potential, per
>example tests cited at http://medfmt.8k.com/mf/lenstest.html by Roger Hicks
>in Jan 2002 Shutterbug in testing wide angle 21mm on Pan F film. If the
>center filter enabled uniform exposure without a center highlight, you
>would avoid these losses (and more likely 2 or more stops overexposed without
>the filter, so even more of a resolution hit?).
>
>The losses from a decent flat filter to MTF is on the order of 2% or so
>for a good filter, and for a cheapy no-name not so flat filter, maybe 10%
>at high end, per Erwin Puts estimate (he is a noted Leica lens testing
>guru etc. However, this assumes that any focus shifting effects of the
>filter are compensated in setup.
>
>I find this a bit disconcerting (and not often discussed, hence notable), as
>it means relatively minor exposure errors or differences between systems (and
>by implications, developing differences between runs) can have as much of an
>impact on lens test results as the differences between mfgers (e.g. Leitz R
>vs Nikon etc. ) ;-)
>
>But my bet would be that the impact and benefit from avoiding over/under
>exposure would outweigh the minor losses from the filter itself...
>
>* Robert Monaghan POB752182 So. Methodist Univ., Dallas Tx 75275 *
>* Third Party 35mm Lenses: http://medfmt.8k.com/third/index.html *
>* Medium Format Cameras: http://medfmt.8k.com/mf/index.html *
>
>
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