Re: Using Enlarger lenses beyond their intended format?

The effect of going beyond the normal coverage will depend on the
design of the lens. The same is true in a camera. All lenses have two
limits, the first is called the circle of illumination. This is simply
the size of the circle of light the lens projects. It is limited by
mechanical vignetting. The other is the circle of good definition and
this is somewhat more complicated. There are several lens aberrations
which are proportional to the image angle and some are proportional to
the stop. For instance, coma, which produces a smeared blur spot, it
proportional to both. For coma the coverage of the lens will increase
as its stopped down. The ultimat limit of good definition is probably
from astigmatism. This is rather different than astigmatism of the eye.
That is caused by a slightly cylindrical shape of the lens. In a
photographic lens astigmatism is a difference in the focus of light
entering the lens along a radial line versus light entering along an
annular line. The closer these are to each other the sharper the lens
is. Most lenses are designed to have no astigmatism at the center and
at the point of maximum coverage. This last is called the astigmatic
node. Beyond this point the two fields, called the radial and
tangential, deviate quite rapidly. In some lenses the deviation is
faster than in others. These lenses gain very little coverage by
stopping down. Most modern enlarging lenses are of a generic type known
as a Plasmat. This lens is derived from the well known Dagor type by
air spacing the inner elements, which are cemented in the Dagor. The
lens has similar properties in that it has quite wide coverage, can
have very good correction for astigmatism, and will gain some coverage
by stopping down.
Generally, a Plasmat has coverage of around 70 to 80 degrees so a
135mm lens will cover 4x5 with ease.
Two other factors are important for enlarging lenses. One is that
the coverage of a lens becomes larger as the size of the image
approaches the size of the object. At 1:1 a lens has double the
coverage circle it has at infinity.
The other factor is illumination. The illumination of most lenses
falls off with image angle. When a shorter than normal lens is used for
enlarging it may be necessary to burn in the corners. OTOH, a longer
than normal lens, say a 210mm lens used for 4x5, will have better
uniformity of illumination.
All lenses with fixed elements are optimum at a particular distance.
Camera lenses are usually designed to be optimum at infinity focus. In
practice there is little difference down to a few focal lengths.
Process and copy lenses are optimised for equal image and object
distance, that is for unity magnification. Enlarging lenses are
optimised for the distance corresponding to the magnification the lens
is intended to used at most. This is why there are special lenses for
making photo-murals.
Now, after all this, the real test is to make a print and examine
the quality.

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Ricahard Knoppow
dickburk@xxxxxxxxxxxxx

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