Aperture control during enlarging

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Some basic question on b/w printing

If I can the aperture during printing. Is it ture that is only affect the 1) time requires to exposure the print,and 2) the DOF of the image ( i.e. the sharpness of the print )

Does it affect the contrast of the print? i.e. Does it have any influence on filter selection?

-- Phyllis (phyllam@yahoo.com), May 06, 2002


Basically, no. Bad lenses though can lower print contrast when used with wide open aperture by blurring the image (a slightly blurred image shall appear less contrasty) or when used in their smallest aperture by diffracting light and so adding stray light (fogging) in the picture. You shall always get better prints when using your lens with the aperture being stopped down a couple of clicks.

-- George Papantoniou (papanton@hol.gr), May 07, 2002.

Let me clarify the pervious response. For most lenses, the optimum f stop is 2-3 stops closed from wide open. For example if the lens is an f/4, the optimum f stop is usually between f/8 and f/11. Counting click stops can be confusing since some lenses have clicks for half stops. There are exceptions to this, particularly APO lenses which tend to be at their optimum at 1-2 stops closed from wide open.

When we say optimum, we mean lens resolution and contrast (and a few other parameters). The difference in contrast at various f stops is probably not significant, but it does exist. At the very smallest aperture opening (highest f stop) you will obtain maximum depth of focus and the most even image illumination (freedom of light fall-off at the edges). But the maximum resolving power of the lens starts reducing once you go past 2-3 stops closed. This is due to the lens design and diffraction limitation effects caused by the aperture at the smallest openings.

-- Michael Feldman (mfeldman@qwest.net), May 07, 2002.

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