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Re: Wide lenses for 12x20 @ 300mm
Jeffrey Mathias wrote:
>> ...
>> Full coverage begins at the point where
>> you first see a round image through the lens when looking through the
>> corners of the ground glass.
>
>This can work for most lenses. However, I have a 90mm
>Schneider-Kreuznach Angulon which looses focus before illumination even
>when stopped at f-16, f-22, or f-32. It can be difficult to access fine
>focus of a dark image on the ground glass. This has been an unpleasant
>surprise when assuming image deterioration would occur at the same place
>of illumination fall off.
>
>A better determination of the coverage might be to look at a negative
>made at Sandy's condition rather than the ground glass.
>
>--
>Jeffrey D. Mathias
>http://home.att.net/~jeffrey.d.mathias/
What I described was a method to determine full coverage, i.e, circle of
illumination, with no vignetting. This is a fairly objective standard as I
understand it.
Another matter is the circle of good definition, which is rather
subjective. The circle of good definition can vary considerably according
to the printing method (projection or contact), by the viewing distance,
and by other personal criteria.
The condition you describe for the Angulon is to be expected. As was
mentioned in previous posts, the Angulon has a very large circel of
illumination, up to 105 degrees, but the cirle of good definition (even
with loose standards for contact printing) is usually no more than about 90
degrees.
And the 12" Dagor I mentioned earlier, which has full coverage at f/45-64
with the lens focused at 50 feet, probably delivers no more than 5 lpm of
resolution at the extreme corners. Is 5 lpm acceptable definition? You
decide.
Sandy King