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Old 24th May 2022, 08:35 AM
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Reginald S Reginald S is offline
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Quote:
Originally Posted by Nat Polton View Post
I always close down the enlarger aperture one or two stops when printing.
Looking at the specs of enlarging lenses this seems to be the comfort zone of these type of lenses, but let's regard the physical optic a bit.

Enlarging lenses are designed to project a flat image with even Illumination and sharpness over a wide field (size of the print) without being too dark in projection.

It's a physical characteristic even of simple meniscus lenses to project nearly perfect in the middle of the lens (stopped down) but loosing quality in sharpness wide open, which will need some stopping down.

I do believe in diffraction, but diffraction depends on the diameter of the diaphragm, which appears, in my memory, at diameters less than 2mm.

Apertures/aperture values are not simple numbers - they are "formulars" depending on the diameter of the entrance pupil and the focal lenght of each lens, themselves expressed in the writing onto the front of old lenses.
Let's say the writing " f:4,5 , f= 250mm" on my old Tessar lying on my table means "aperture 4,5 devided through the focal length 250mm" will show us the entrance pupil .

Like usual with formulars we can use them viceversa which will help to find the wide opening value of let's say a meniscus lens of 50mm diameter and a focus point / focal length of 300mm..

Once given the wide opening value as a number one could do some logarhytmics for finding the necessary diameters for f/ 8 or f/5,6 and so on..

Anyway, my Tessar shows an opening of 5mm at f/32 which will not bring any sweat to my neck.
Checking the stopped down diameter of a 50mm lens otherwise could be a good idea concerning diffraction.

Sorry for boring you, but I have to state that Iam totally fascinated from the math and phsics behind my lenses :-)

So, this all theory will say something but not all about the performance of your lens at different apertures.

That's the reason to me for testing my enlarging lenses with test negatives and at typical print size, with different apertures.
This will give great impressions of sharpness from center to the edges but it will show the performance of the enlarger, too (given or not given right angle of the axis enlarging base and enlarging head).

Don't be surprised with the results of your setup :-)
And it could happen that you will use only one specific aperture in the future, if you are after quality of your prints.

Last edited by Reginald S; 24th May 2022 at 08:41 AM.
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