Enlarging Paper Reciprocitygreenspun.com : LUSENET : Large format photography : One Thread
Give the fact that many lenses that have been designed primarily for enlarging that are doubling as camera lenses at infinity are recommended for use at f22 and higher to reduce physical aberrations such as coma, I was wondering if there is an optimal f stop for enlarging lenses in general?
Secondly, if f16-f22 is the target versus f5.6 for optimal performance with enlarging lenses, is paper reciprocity with extended exposures an issue worthy of concern or mention beyond the fundamental subject of potential enlarger movement? Is there a general exposure time with current high quality variable contrast papers beyond which reciprocity is initialized?
-- Michael Kadillak (email@example.com), January 30, 2002
The requirements for enlarging lenses are different than those in front of the camera. They are narrow-field lenses very similar to Artars, Ronars and G-Clarons in design and concept (somewhere it's been said that the Symmar and Componon are basically the same lens). Without a wide field of view they do not require closing down more than 2 stops for aberrations. In fact diffraction is a major issue with enlarging because even if you get the "required" 5lp/mm on paper there's still another level of detail being lost when the grain of the film is not rendered to a much higher resolution. That's why the f/9 lenses are not a good bet for enlarging more than ~3x-4x. Lenses for smaller formats tend to be optimized for around f/4-f/5.6 and the larger ones for f/8-f/11 - 2 stops down is a good general rule.
-- Wayne DeWitt (firstname.lastname@example.org), January 30, 2002.
Michael - In the book Post Exposure by Ctein (sorry, don't remember his first name) there is a chart where the author has given the optimum f-stop for enlarging lenses. They are definitely not in the F16 to 22 range, most being, as I remember, in the 5.6 or 8 range. I was pretty surprised, and spent a bit more time making sure my enlarger was aligned after reading that. Given that however, if you're using longer exposure times for b&w papers (and I assume for color) there are reciprocity effects that can be encountered. I believe I have found a document on Ilfords web site that gives the appropriate compensations.
-- Artie Kapell (email@example.com), January 30, 2002.
Mr. Ctein has no first name! He simply goes by "Ctein". My tests have pretty much confimed his findings regarding optimal stops on enlarging lenses. For my lenses, the results are as follows:
Schneider Componon-S 150/5.6 = f/9 Schneider Componon-S 100/5.6 = f/8 Schneider Componon-S 80/4 = f/5.6 Nikon El-Nikkor 50/2.8 = f/4 or 4.5 (a toss-up)
All of these lenses look (relatively) mushy by f/16 or 22; my practice is to never use them below f/11 (5.6 for the 50mm).
-- James Meckley (firstname.lastname@example.org), January 30, 2002.
As a rule of thumb, you should use an enlarging lens at the largest aperture (smallest f number) that will bring the corners into equal sharpness with the centre.
This might not be possible with some cheap lenses, especially if they weren't designed for enlarger use, but any good enlarger lens, on a properly aligned enlarger, should acheive this by a couple of stops down.
There's a very good reason NOT to use small apertures with enlarging lenses: The subject to image ratio is reversed with respect to a taking lens, and the image plane is usually several focal-lengths away from the lens. This means that the effective aperture number will also be multiplied by the enlargement ratio.
For example; a marked aperture of f/5.6, and an enlargement of 4 diameters, gives us an effective aperture of f/22, and the effect of diffraction will be as great as if the lens was stopped down to f/22.
Now, if the lens was stopped down to a marked aperture of f/22 to begin with, then the effective aperture would be f/88, and diffraction alone would limit the resolution on the print to about 15 lppm. This is a low enough resolution to look fuzzy under critical scrutiny.
-- Pete Andrews (email@example.com), January 31, 2002.
Thanks to all for the stimulating responses. One more item on the list for the mind meld when we get together Professor (Richard).
-- Michael Kadillak (firstname.lastname@example.org), January 31, 2002.
Pete, I don't buy it. The f# is certainly based on focal length and extension, but why calculate this from the paper side? The diffraction argument is true, but I like to think of it in terms of the "lever arm" of the rays from the aperture edge where the diffraction occurs. More distance = bigger blur, and it's certainly the opposite situation from the camera, where the rays travel the shorter distance from the point of diffraction. Or is it all semantics?
-- Conrad Hoffman (email@example.com), January 31, 2002.