Re: Exact fl of lenses... 220 VR

[Bill Glickman <bglick@xxxxxxxx>]

Andrew

     I missed explaining one thing to make my point more clear.... sorry for
the second post.

> I probably got into this discussion a bit late but I can not imagine how
the
> focal length of a lens would be a variable. Focal length by definition is
a
> fixed factor (zooms excluded of course!).

      Even if one knows the exact fl from a bench test done by the lens
manufacture, this is not the total solution for a rotational camera.  For
the sharpest image, with no compression or stretching, the film travel
distance must be perfectly synced with the lens fl.  So even knowing the
true fl of a lens, does not provide the answer one needs.  That fl will only
produce the sharpest possible image if the film travel speed perfectly
matches that fl.  That depends on how many speeds the motors have, the
degree of precision in the gearing, the software instructions sent to the
variable speed controller, etc.  This compression and stretching is not the
exaggerated stuff you would see if you entered the wrong lens, like 80 vs.
150mm...... however, in a rotational camera, the smallest amount of
compression and stretching is the single biggest variable in horizontal
sharpness, most obvious in subject edges.  Assuming constant lens rotation,
not variable such as auto exposure which introduces other variables.

      So for an example, if a lens maker tells you after testing a 150 mm
lens, the exact fl is 146.2 mm, its possible on the 220 VR, 151.0 mm would
make the sharpest image, whereas 146.2 would be slightly compressed and
edges much softer.    I ran all this by Mr. Seitz and he blessed my
understanding of this.  Hope this helps others out there.

Regards
Bill G