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#11
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Quote:
Ctein (is that his whole name?) goes into diffraction issues in his book Post Exposure, which you can download free off the internet. He gives formulae for calculating the theoretical limits to sharpness caused by diffraction in an enlarger lens. As I understand it, the main difference from what Reginald S laid out above is that you need to consider the degree of magnification as well as the physical size of the aperture (and arguably also the resolving power of the human eye at the intended viewing distance). However, I wouldn't rely too much on what theory says. There is, for instance, a huge difference between mainstream theoretical calculations of depth-of-field for camera lenses and the very persuasive arguments and empirical evidence put forward by Harold Merklinger in The Ins and Outs of Focus. So I think the choice of enlarging aperture is a balancing act, and we each have to make that choice by trial and error for our own lens and printing style. A few years back I made careful copies of the same print at whole stops from f/5.6 to f/16. I was surprised to see that those at f/5.6 seemed marginally sharper than those at f/8, and those at smaller stops were distinctly below par. I use a glassless negative carrier to avoid the annoyance of Newton rings, so negative popping during long exposure times is definitely an issue. I don't tend to do intricate bodging or darning, so an exposure of around 20 sec at f/8 for a 12x16 print is a workable compromise, provided I let the negative cool and check focus before each print. |
#12
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Prints made.
Will get them scanned up tomorrow. |
#13
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If in doubt, some empirical work like Colin hopefully will do is all to give an answer to the starting question. But I have found knowing about the little things behind could be exciting and helpful. Magnification: Normally I am not affected by magnification problems. Typically I get 2x or 4x enlargements (linear) of my negatives but it is of course an interisting point for friends of smaller negative sizes. Stopping the lens down further than usual may become of more importance than aspects like diffraction. |
#14
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I have tried a sharpness test in the recent past, with my most expensive mini zoom lens, that was specifically designed for my Pentax 'digi' camera and as far as I remember, was quite impressed by the results across the board.
I have never done a test of my current two Nikon enlarger lenses though. Given time, I think I will make a test, out of sheer curiosity. I will say though, that they are a large leap better than the two that I had previously, and that I thought were quite good, at the time. I now quite happily print quite dense 35mm negatives from the 1980's, with the lenses wide open at F4, which give me lovely sharp prints from corner to corner. For more recently taken negatives, I usually go to F5.6, or occasionally F8. I don't think that I have ever used anything lower, so it could be interesting. Terry S |
#15
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6x6 negative
4.3 X magnification 80mm f4 Componon S enlarging lens First print made at f8 for 6 seconds (should be 'ideal' aperture) Second print f22 for 48 seconds (No movement in darkroom during the longer printing time) Paper is Foma RC at Grade 2 and-a-half Both prints developed for a timed 2 minutes. My understanding from school-days (yes, a while ago now) is that diffraction is directly connected to the absolute size of the opening the light has to pass through. F stops though are a ratio, of aperture size to focal length. This means f22 on a 50mm lens is a lot smaller than f22 on a 150 (or 300) mm lens. Here I've used an 80 as I was printing 6x6. With both prints in hand, using magnification, I really can't see any difference in sharpness. That doesn't mean there isn't, but I can't see it. What I think I can see is a very slight loss of contrast in the f22 print? From this test all I can conclude is that I would happily print this lens at any aperture. To go any deeper would need a test target, shot on a tripod with fine film, then printed using a 50mm lens. The smaller apertures on a shorter lens are more likely to reveal diffraction effects. |
#16
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Thank you Colin for sharing your results.
Probably a scan doesn't say much concerning sharpness and quality of your prints , but to me the right prints looks less sharp a little bit which could depend on missing contrast there. So in my case the next question would be "will give stopping down less contrast as usual with that lens?":-) In details the left screw of the right sign really looks unsharp, especially compared to the left print. If you haven't see this with the magnification lens, is has to be the scan himself. |
#17
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Enlarging lens
I went to print this evening from 120 colour negs onto a 10x12 Sheet of Fuji (Odd the kept that size when all other manufacturers changed to 12 x9.5 years ago)
The enlarger lens was a 80mm Rodagon APO and had to stop down to F16 because the printing times were too short (8 seconds @ F11) Perhaps it was the enlarging ratio was small but I could not see any difference that would matter. |
#18
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I measured the changes with an USAF1951 chart. There the diffraction effect was clearly visible / could be measured. It is possible, the one cannot see it in real photographs.
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#19
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I'm not sure if stopping down always gives this diffraction effect.
In my opinion with stopping down one just leaves the ideal zone of the lens before any diffraction will come in place. As said before and as said from Colin, too - visible disturbing the straight march of light will need a very small aperture, measured in diameter. Uwe, there is a photographer who measured changes with USAF 1951 too, while using enlarger lenses as repro lenses. I'll try to dig this site.. |
#20
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Ok, here's an interesting comparism of 3 enlarging lenses - a bit away frome their typical usage, but of course with some truth concerning different apertures.
https://www.photoinfos.com/Scanner/v...ografieren.htm The tables with pictures will explain themself. The results are described with few words: Table 1, measuring the behaviour of the lenses in their center while observing the tiny group in the center of the test negative. For table two the negative has been shifted so that the corners of the projection could be observed. For the Componon S / 80mm it became obvisious that the performance in the lens center will show best results at f 5;6 but also with weak corners then. Stopping down to f/8 increases the corners dramatically but it is accompanied by loosing the centers quality a bit. Concerning center AND corners the Apo performs best at f/5,6 !, but then will loose at f/8. Don't hit me but I can't imagine a turn at f/16 :-) And i am really excited to find my way back to my darkroom - which is at work, but I am in holidays for 2 weeks - and making own tests with my new to me LED light which surely has to speak some words, too. A friend who meanwhile enlarges his high quality 8x10" negatives with LED light once has said: " With the LED light now it remains only one aperture giving best performance".. Last edited by Reginald S; 26th May 2022 at 02:20 PM. |
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