RESOLVING POWER OF ENLARGING PAPERS; FURTHER DISCUSSION

Thank you all who have replied to my question. It has become an interesting thread with many people wanting the answers.

Many have given excellent references; Ctein, Jacobson, Kodak, etc, but these are unfortunately not available to me at this time, even though I have an extensive library of old books.

Some further thoughts..... the resolving power of the enlarger-lens-negative combination coupled with the paper need not be large or anything like the resolving power of the original film. The film is enlarged but the paper is not. Film has resolving powers of 100-150 lines per mm and can be easily worked out using an appropriate target and examining the film through a microscope. I do this a lot to test my lenses and commonly get results of 60-80 lines per mm in my testing. We know that film has the capacity to resolve the upper limits of lenses resolution. So we have negs with resolution of , say, 80 lines per mm. This is enlarged say 10 times and this gets us to the limit of unaided eye resolution, which is about 10 lines per mm. Lets be generous and say the eye can resolve 15 lines per mm at closest focusing distance, without lens magnification. Therefore paper does not need to be any better at resolution than this number.

I suggest that paper has much lower resolving power than film and would be in the order of 10-20 lines per mm. It does not need to be any finer and would be expensive in terms of more silver.

Idealy one would print a target which had been placed in the enlarger, and then look at the print under a microscope. This would give a rough idea of the resolving power of the printing process, of which the enlarging paper is part. But exactly what target, in the enlarger?

Lets keep in mind the difference between what the eye resolves, and what , exactly, is the resolving power of the paper.

Please look up your references and give us some answers. But be careful of authors who are making "guestimates" and those who have done the definitive testing.

For those who are pedants like me, please add to the discussion by means of general observations, fact and philosophy.

I will be contacting Kodak, Agfa and Ilford to get some more info.

Thanks again to all, some have really got the idea of what we are trying to find.

David Strachan in Orange, NSW, Australia

-- David Strachan (strachan@cww.octec.org.au), May 29, 2000

Answers

Of course it is a moot point when the eye sees an image on any paper and given a decent enlarging lens it "looks" sharp. What is the point of searching high and low for a paper that renders more than the eye can percieve anyway? James

-- james (james_mickelson@hotmail.com), May 29, 2000.

James, the point is to increase our knowledge. It might not have photographic significance, or it might, but until we can put a number on it, we don't really understand it and can't make any definitive statements one way or another.

-- Conrad Hoffman (choffman@rpa.net), May 30, 2000.

>> I suggest that paper has much lower resolving power than film and would be in the order of 10-20 lines per mm. <<

David, I suggest that it's silly to think this without having some kind of supporting basis. As I pointed out in the other thread, typical paper has much lower silver content than film, thus can, in principle, have a thinner emulsion which is a major factor in resolution issues. Do you know of a way the manufacturer could cause a material to lose resolving capability (relative to another similar material) while giving it a thinner emulsion and smaller silver halide crystals?

I can't supply a good reference on the silver loading numbers, but Kodak's website indicates that you can call their Environmental Services at 1-716-477-3194 for estimates of "recoverable silver" from their materials. In publication J-210, they give some calculation examples using (for color materials, not B&W): Gold 100 film at 16 troy ounces of silver per 1,000 square feet of film and Ektacolor Royal III paper at 2 tr. oz./1,000 square feet. So in this case, the film has about 8 times more silver than paper. Note: for further calculation purposes, there are about 14.583 troy ounces per avoirdupois lb (453.59 grams), so a troy oz is about 31.10 grams.

Just as a comment, in his book (page 3), Ctein indicated that dye transfer does better than 15 lp/mm (line pairs per millimeter) and that typical RA4 paper could reproduce about 65 lp/mm. Also, that photographers were able to notice a difference in sharpness between a product at about 20 lmp/mm (obsolete Kodak Ektaflex) and RA4 at 65 lp/mm even though both resolution ratings are probably beyond the resolving capability of the normal human eye. Presumably due to sharpness effects?? PS, although I referred to Ctein, he is actually not a very good formal reference in this respect as he has not described results of a well controlled experiment nor given other specific references as a basis.

>> Idealy one would print a target which had been placed in the enlarger, and then look at the print under a microscope. This would give a rough idea of the resolving power of the printing process, of which the enlarging paper is part. But exactly what target, in the enlarger? <<

May I suggest purchasing a USAF 1951 target* on a high resolution film or glass plate, then contact print that onto paper for resolution numbers? On the other hand, since such targets are fairly expensive, how about use your film test procedure on paper. That is, simply load your camera (in a darkroom) with a single piece of photo paper and shoot your resolution target. I would caution you about possible focus problems due to differences in spectral sensitivity between films you have tested vs typical papers. I would suggest first using a fixed grade paper and shooting a focus test with a tilted target to verify where best focus is. If you find that variable contrast papers "slump off" from the fixed grade resolution numbers, I would suggest there is probably a focus discrepancy introduced by the two spectral sensitivities of the emulsion layers. You could isolate these layers with the appropriate filter and test independently.

* for a contact printing target, try Rolyn Optics stock# 70.6031; list price was $156.95 in 1995. It appears to go down to 228 cycles per mm (roughly equivalent to line pairs/mm, I believe).

Maybe you can put these questions about paper resolution being a limiting factor in print sharpness to rest.

-- Bill C (bcarriel@cpicorp.com), May 30, 2000.

Bill, Thanks for your reply. But your comments regarding..... "Maybe you can put these questions about paper resolution being a limiting factor in print sharpness to rest." This is not entirely the point; 1. I am interested just for the sake of the knowledge, and to further my research 2. What is the point of people always rabbiting on about lens and film resolution, when the enlarger-paper combination is just as important; as this is the final result of photography (not transparencies).

I thnk many people get swept up in research arguements without looking at the forest, they just concentrate on one tree. My research is part of a big jigsaw that has been consuming me for 5 years. ie basic photographic principles, optics, etc

One of my main thoughts is that the final print being viewed can be optimised in terms of tone and resolution and sharpness, through a practical understanding of all the variables. I think the depth of field - circle of confusion - viewing distance arguments have some errors which have carried through the decades of photography. I do not really want to start this as a seperate thread until I have some more grist for the mill.

Cheers David Strachan B App Sc , M Env Sc

Anyway let me have my interest....it does no harm.

Thanks for your discussion of recoverable silver. Do not know for sure of silver content being the main determining factor for resolution. I reckon it would be though. And also how the silver is dispersed through the emulsion in the paper seems important ie well sorted, small or large grains, etc as with slow vs fast films.

Am pressing on against the odds, determined in my quest to go where no man has gone before :=) and besides its been fun with all the discussion and funny arguements, some others are as interested as me. David Strachan

-- david strachan (strachan @cww.octec.org.au), May 30, 2000.

I have been reading this with great interest for the last few days. I think you are all right. Some of us are less inclined to look into the esoteric than others but I believe that both points of view are valid. Photography has always had the equipment people, the scientists and the shooters.Personally I'm more interested in getting the image and working in the darkroom on that image to care about the resolution of my printing paper. If my cropped 11X14's look sharp, I'm happy.However I'm glad someone is thinking about things like this because it is important to not only know how to do something by why you do it.

-- Robert Orofino (rorofino@iopener.net), May 30, 2000.

OK, heres one small data point on the subject. I contact printed a glass USAF target on some Seagull RP, F surface, grade #2. This is a glossy graded paper. Made a dark, medium, and light test (densities of 1.89, 1.15, and 0.53), then looked at em under a microscope. The light one gets hard to see due to low contrast, but it clearly resolves 64 lines/mm or better. The medium one hits about 71 lines/mm, and the dark one bleeds the lines together at 50 lines/mm. I suspect with a vacuum frame, a chrome instead of film target, and a point source, the numbers would be even higher. This was a very low tech midnight effort, but the results are surprisingly good. I wasn't terribly worried about paper sharpness before, but now I have something solid to base my beliefs on. Floats my boat ;-)

-- Conrad Hoffman (choffman@rpa.net), May 31, 2000.

Pheweee! Thank goodness for that Conrad. I had visions of having to buy Technical Pan in 20 x 16 sheets to do my prints on. :^)

-- Pete Andrews (p.l.andrews@bham.ac.uk), May 31, 2000.

Good work Conrad Now this would be a target usually photographed from a distance? Would it? Therefore the actual numbers of lines per mm are not realistic as the definitive answer? Is this right? Interesting getting different resolutions at different exposure densities....which IS what we would expect. The same thing happens with film, eh? Am rushing this morning with the usual pandabloodymonium, getting kids off to school and work. So am thinking further on your tests.

Will get back (:^) David

-- David Strachan (strachan@cww.octec.org.au), May 31, 2000.

Target is a two inch square piece of glass with a 1951 USAF target repeated in 13 rows of 13 targets- 169 of the tiny things. Not having data on it, I checked the calibration under a microscope against several other standards. Keep in mind that these kinds of resolution numbers are subjective, as different people may pick different points at which they feel the bar pattern breaks up. Regardless, the paper is certainly adequate for my aging eyes!

-- Conrad Hoffman (choffman@rpa.net), May 31, 2000.

Further on......

Conrad...thankx for doing the contact prints. Should your print sizes (of your targets) be reduced? Say you normally use these targets at 50 times the focal lenght of your camera lenses, the neg size of the target is therefore constant on the neg if using a 50mm lens or a 200 mm lens. (That is how my targets work). On the emulsion they might be say, 3mm across for all focal lengths, (aside: a microscope is used to examine the finest lines which can just be resolved; these lines have a number next to them which represents lines/mm which the camera-film combination is resolving). So if your contact print is 30mm across, for example, should you proportion your resolutions similarly. So, usually 3 mm on emulsion the way the target is used normally (?), then divide by 30mm (contact print size) and then multiply by the resolution you got. So 3mm divided by 30 mm multiply by 71 l/mm, equal 7 l/mm on paper. (lengths given of negative target and of the contact print target are only demonstrative examples.) Is my point clear, is this thinking correct? I aim to do similar tests but are these simple ratios the right way to get an approximate answer.

If appropriate targets can be made to: 1. Contact Print and/or another target made to.. 2. Enlarge to a certain size, and if we know the theoretical manufacturers resolution of their paper, we can open a whole new way of testing enlarging lenses-enlargers- enlarger light source combinations. Have you noticed in photo forums how enlarging lenses are always discussed with anecdotal evidence rather than measured and definitive numbers? eg the Schnukker Confrac F4 80 mm El "...is sharper and better...." than the Nukkor F2.8 80 mm.

If some targets and facts were available, we could do much better comparisons, and easily, in our own darkrooms.

I've contacted Kodak, Agfa and Ilford to try and get some paper resolutions. Agfa looking hard.

We are nearly there......... David S ;?)

PS just got your latest re:details of USAF target 2" across. my discussion still relevant though.

-- David Strachan (strachan@cww.octec.org.au), May 31, 2000.

A series of posts that turned into an exchange of personal attacks have been removed from this thread.

-- Mason Resnick (bwworld@mindspring.com), June 01, 2000.

Thank you for editing the replies. I like this Forum; it is just amazing how we can further research through the web.

THE DISCUSSION CONTINUES Still looking for the definitive answer, with supporting evidence, regarding the resolving power of B&W enlarging papers; Multicontrast RC and Fibre in particular. Conrad has done some good work and I feel we are nearly there. Also Bill C has some good practical ideas. Will let you know when I get info from manufactureres regarding resolution. Have written to Kodak, Agfa and Ilford. I hope they come through with the definitive answers. If they do not then practical testing of targets is needed. But unfortunately this home grown testing is not as good as a rigorous test platform from a manufacturer, which would give us the best data, from which our own setups can be compared. Thanks everyone. Cheers David Strachan of Oz ;/)

-- David Strachan (strachan@cww.octec.org.au), June 01, 2000.

Bill Just rereading some of the Forum and you mention that IYO the paper will have resolving power so high that it just doesn't matter. My thesis is that "..the paper may be manugfactured to only resolve somewhere around 10-20 lines per mm, this is what good eyes see, at viewing distance of 250mm (10"). So there is no need by the manufacturers to make the resolution any higher (?)..."

On another part of your reply, you imply that I may have probs getting nice sharp prints. No my prints are beaut and have no problems with sharpness. Use Nikkor EL's 50mm and 80mm and an Opemus IIA enlarger, old but good.

No my aim only at this stage is to find out the resolving power, for further thinking about print size vs veiwing distance vs circle of confusion, etc. Once I have the numbers my thinking will be clearer! ....like Teflon invented for the space exploration was used for other things.

Cheers David

-- David Strachan (strachan@cww.octec.org.au), June 01, 2000.

Hi again, David. On your immediate previous post, (ie, no need for mfctr to make paper resolution any higher), I get the impression you think they have to work harder to improve resolution. This is not so! The B&W paper emulsions should inherently have very good resolution characteristics. This is due to the paper typically having (compared to film) a very thin emulsion and smaller silver halide grains.

I agree that there is not a NEED for the paper to have really high resolving paper; rather that other characteristics of its manufacture tend to cause this to happen.

Regarding unsharp prints, I don't mean that you, specifically, are having a problem. I am trying to say that a person might find a problem where their prints on one type of paper (variable contrast) are slightly unsharp, while a different type of paper (possibly fixed grade) IS sharp. Even though both papers have a resolving power way beyond the perception capability of the human eye. A very plausible explanation for this is given by Ctein (in his book, Chapter 10). In essence, he found some VC papers had a near UV sensitivity for one of their layers while the typical lens has a focus difference between human vision and that near UV sensitivity. In other words, a person might do their best to focus by eye; this actually puts the short wavelength-sensitive layer of the paper out of focus. Their natural, but erroneous assumption, would be that this paper has lower resolution capability.

Conrad's resolution test using contact prints eliminates the possibility of being fooled by this spectral sensitivity mismatch (BTW, good job, Conrad!).

PS; teflon is said to have been invented in the 1930s by DuPont scientists working on refrigerants, not space exploration.

-- Bill C (bcarriel@cpicorp.com), June 01, 2000.

The accepted reason for different print "tones" i.e. cold, warm, neutral, is that the grain of the emulsion interferes with the reflected light, and so acts a crude dichroic filter. Smaller grains absorb more blue light and so give a warm tone to the image. Now blue light has a wavelength of ~400nm (0.0004mm), so one can only presume that the grain clumps of a paper emulsion are of this order of size. This ought to be fine enough for anybody.

The only physical reason that a paper emulsion would have a low resolution is if the surface texture was so coarse that it broke up the image before it could enter the emulsion. With glossy paper this just ain't the case.

-- Pete Andrews (p.l.andrews@bham.ac.uk), June 08, 2000.