NedL: you are asking a philosophical question to a person not properly versed...
It would seem to me that, here, in this instance, the human brain (flawed as it is) nevertheless sets the 'standard' of perfection as far as proper hues go. But, on the other hand, it would also seem to me that acutance is not relative, but absolute, in measurement. (How could it NOT be, as it is essentially mathematical?) It would be interesting to see if I am agreed with or refuted but I plead ignorance here.
Here is the source of my original query: The Amateur Photographer's Handbook by Aaron Sussman, 8th revised edition, 1973. Page 526, Hints and Suggestions #46:
- David Lyga
With respect to the colour of the light, the issue is that a simple lens (such as a magnifying glass) will focus different colours of light to different distances. More complex lenses can and are designed to correct for this - in some cases exceptionally well.
A basic, three or four element enlarging lens is probably not highly corrected for this, but there is some correction built into the design. Higher quality five, six or more element enlarging lenses will most likely be highly corrected for the problem.
So if you use a good quality enlarging lens (Componon, Rodagon, Nikkor) at typical enlargement amounts (full frame, 5 x 7 through 11 x 14 or 16 x 20) this shouldn't be a problem for you. If you are making very large enlargements, you may need to use the higher range APO lenses to avoid the problem.
You need to realize, however, that even if you use the basic lenses, there is another factor to consider. There is a certain amount of depth of focus at the paper plane available to you when enlarging, and any focus shift that falls within that depth of focus will tend to be almost invisible in the result. So, in most cases, stopping down the basic lenses will also help avoid the problem.
I would guess that the target market of Mr. Sussman's book was most likely the hobbyist photographer in the late 1960s, early 1970s. Top quality enlarging lenses were probably fairly rare in that market, at that time.
No, top quality Nikkors were readily available and not too expensive: about $35 then (1973) for the 4/50, though, of course, it was not apo. The Sussman book was full of interesting information but, true, did not delve fully into theory like the British books did and do.
MattKing: for what I do, the sharpness if fully adequate: My argument was more theoretical than practical and the 'suggestion' that was in the book piqued my interest, as we are all looking for sharper and sharper results. It was worth bringing up the topic, if only to cause thinking. - David Lyga
The demands on a camera lens is somewhat greater than the demands on an enlarging lens, because the visible light spectrum is considerably wider than that which enlarging paper is sensitive to. In my view the more expensive lenses fix other problems, such as field curvature, distortion and vignetting, to a greater degree than the three element lenses. Those are perhaps more important for ultimate sharpness and print quality than the apochromatic design. All paper will be sensitive to green/blue light, and if you can find a narrow band pass filter in that part of the spectrum it would make your results sharper. But that approach cannot fix all the other problems inherent to simple optical designs, and cannot make them look like $500. They will still distort, focus in a curve rather than a plane, vignette etc.
The above notwithstanding, I think there may be darkroom printers who derive enough benefit from such a technique to justify the hassle. For most of use, though, getting a fraction more resolution makes little difference. One has to optimise every part of the image making process to see it culminate in sharper images. That means sharpest lenses at optimum apertures, rock solid support and fast shutter speeds, optimal development i.e. non-solvent, very fine-grained film, and so on. And then printing huge, certainly larger than 16"x20". How many of us really give attention to the point of obsession, to every part of that equation? I doubt many. We all rather focus on the image and its message, and do the best we can under the circumstances with the rest. Some images succeed because of their resolution when printed large, but those are somewhat in the minority in my view. Other images succeed because they have general visual impact.
True, dorff, this never ending search for resolution CAN be obsessive, and, as the eminent Roger Hicks once said: smaller prints are more 'intimate'. How correct he was and is. - David Lyga
David , I think you should think about there is art in designing these papers , films and lenses. If it is designed for wide spectrum bulb , it would be needed to be used as it is. I think blue light use is similar to print a bw advertisement on a luxury magazine with cyan separation. Good art director uses cmyk bw print.