The two case that come up most frequently for me are:
Originally Posted by ic-racer
1) Negative curvature. The peak allows one to get the focus spread on the baseboard, from center to edge. I always try to use a non-glass carrier if I can get away with it.
2) Flatness of field. When using a glass carrier, you really can tell the magnification for which an enlarging lens is optimized by observing the focus spread from center-to-edge at various magnifications. For example my 300mm Rodenstock does not have (nor is claimed to have) a flat field at 1:1.
The diameter is the standard unit of measure
Originally Posted by RalphLambrecht
when speaking of magnification. A four diameter
enlargement is twice the size of a two diameter
enlargement. All things being equal.
An example; I've enlarged the negative by
five diameters. Dan
I never heard of that, but thanks. I must get out of that darkroom more often! Last time I came out, they had invented color photography. What will they come up with next? Who knows, someone might try to get rid of film altogether.
Originally Posted by dancqu
Of course you're right. I found it in the dictionary.
diameter |dīˈamitər| (abbr.: diam.)
1 a straight line passing from side to side through the center of a body or figure, esp. a circle or sphere.
• the length of this line.
• a transverse measurement of something; width or thickness.
2 a unit of linear measurement of magnifying power.
diametral |-trəl| |daɪˈømətrəl| adjective
ORIGIN late Middle English : from Old French diametre, via Latin from Greek diametros (grammē) ‘(line) measuring across,’ from dia ‘across’ + metron ‘measure.’