Paper Resolution

[Patrick Troccolo]

Actually, it was Ctein's article in Phototechniques that started this chain of events for me. I realised that my prints were an order of magnitude away from the resolutions he stated and I began a study to understand final print resolution, using first principles and 1/Rt**2 = 1/R1**2 + 1/R2**2 etc., to see where the weak link in my chain of MTF degradation was. It wound up with the paper resolution. Funny how life is about these things.
If the limit to "sharpness" on the print is acutance, I'd like to be the one making the choice. I've made choices on the quality of lenses I've purchased, the film that was chosen to minimize grain and maximize resolution, the rigid enlarger (Durst 138s and not cheap) and large format view cameras (5X7), why not the paper.
I don't know what a high acutance paper would produce, wrt a high quality print, and maybe someone's already done that work and has handed us the result, I'd just like a choice, or at least to know why we're limited to 12 lp/mm.

[Oren Grad]

Emanuel -

Thanks very much for posting those details - exactly what I was looking for.

Paul -

Thanks for your further elaborations on this topic. I should say that my own "scientific" interest here is not primarily in what is the maximum spatial frequency that produces a visible effect. Rather, it's in accounting for why contact prints, at least in my experience, look different from even the most careful projection prints. I'm still inclined to believe that the phenomenon can be explained in terms of MTFs, but which spatial frequencies are the critical ones in accounting for this particular phenomenon remains an open question for me.

One familiar example of MTFs greater than 100% for at least one stage in the imaging chain: Kodak's current published MTFs for TMax films and Tri-X show modulation > 100% for "lower" frequencies (specific range varies by film, but it extends to relatively high frequencies with TMX). I think the usual explanation for this is edge and adjacency effects.

This reminds me of a question that may be related to the contact print issue: why does TMX tend to look soft, even when it is objectively rendering the subject with good contrast out to fairly high spatial frequencies?

[paulr]

Oren, I've been curious about the contact print phenomenon for a long time. You might be right that the difference has something to do with the shape of the MTF curve. I'm pretty sure it's about much more than just maximum sharpness ... I've seen some amazing contact prints that had "that look" but on close examination weren't really that sharp. I've also had a few strange experiences of being able to recognize something as an 8x10 contact print at a glance from across the room. One time I saw reproductions in a book over a friend's shoulder. They were candid looking, formally loose portraits. But something struck me and I asked my friend if those were 810 contacts. Turns out the book was by Nick Nixon ... handheld 8x10s of his family. I don't know what it is that jumped out at me, but something did.

The t-max question is an interesting one, too. I switched to tmx from agfapan 100 several years ago, and noticed a slight overall softer look ... in spite of the new film recording much, much more detail. Part of it might be the grain--the apx had a very crisp grain structure, which was lurking just beyond the range of visibility in my prints, but in tmx the grain is nowhere to be found. I doubt this is the whole story though. In the end, the slightly softer, smoother look grew on me, so i don't regret the move. I have to say the difference has been subtle. I get some very sharp prints from tmx ...they're just more likely to emphasize smoothness over edginess, which wasn't as much the case with the old film.

Incidentally, the reason film can be developed to 100%+ MTF is slightly artificial in most cases. It just means they're developing it to a higher contrast than whatever a neutral reference would be. Edge effects would be a more legitemate example of >100% MTF.

[Emmanuel BIGLER]

I totally agree with Paul R. that edge sharpness and MTF resolution for small modulations are different issues. The original message by Patrick Trocollo refers to a resolution limit in terms of estimated 10/12 lp/mm, i.e in terms of line pairs on a periodic test object. The original question refers to paper resolution. To both question an answer in terms of the USAF test target and separating groups of bars+spaces is IMHO, meaningful. We know that for extremly small modulations silver halide grains can be so small that 5000 lp/mm can be achieved on an holographic plate ; coating the same stuff on paper would certainly degrade actual resolution !!!! And we also know that holographic silver halide media are totally irrelevant to conventional photography, but it gives an idea of the potentiality of ultra-slow-sensitivity and ultra-fine grain silver halide processes.
I agree that 'contrast' and 'edge sharpess' are physiological notions to which we try to associate objective measurements, but this is difficult. Like in music, Fourier analysis of sound and images is not the ultimate key to solve all visual and physiological effects.
For example, some contact prints made from negatives developed in pyro exhibit a very special quality in terms of edge sharpness and "subtle feeling" which is amazing and for which I prefer not to elaborate any theory : some day I should try it myself by hands. This is also the joy of LF printing.
A musical similarity is : record a piano with a good microphone but cut by digital processing all the transients i.e. the very beginning of the sound. In terms of Fourier Analysis very few things are changed if you cut the starting transient signal and consider the quasi-steady state decay of sound, but the resulting sound without the transients is exactly like.... an accordion !!!! Hence, for the ear, edge sharpness ( a non linear effect) is probably more important that Fourier spectrum (in the model of small, linearized signals)!!! I'm quite sure that this also applies to the assesment of print sharpness.

[Oren Grad]

Paul -

> Part of it might be the grain--the apx had a very crisp grain structure, which was lurking just beyond the range of visibility in my prints <

Ah, but that's part of what's so tantalizing here - in your APX 100 example, we can't quite discern the grain as such, but there's some mechanism by which it's having a visible effect. In this respect I think TMX vs Delta 100 is a very interesting comparison - both look grainless in the small enlargements I make, but Delta 100 looks "edgier" to me and TMX looks softer, although both are superb in rendering fine detail.

> Incidentally, the reason film can be developed to 100%+ MTF is slightly artificial in most cases. It just means they're developing it to a higher contrast than whatever a neutral reference would be. <

Excellent point! I didn't think of that - it's like introducing an amplifier at one stage in the chain.

Emanuel -

> Like in music, Fourier analysis of sound and images is not the ultimate key to solve all visual and physiological effects... A musical similarity is : record a piano with a good microphone but cut by digital processing all the transients i.e. the very beginning of the sound. In terms of Fourier Analysis very few things are changed if you cut the starting transient signal and consider the quasi-steady state decay of sound, but the resulting sound without the transients is exactly like.... an accordion !!!! Hence, for the ear, edge sharpness ( a non linear effect) is probably more important that Fourier spectrum (in the model of small, linearized signals)!!! <

An alternative interpretation is that the Fourier spectrum really does capture the differences that matter; although they may be small when examined on a graph, if the difference occurs at frequencies to which the hearing system is disproportionately sensitive, the subjective effect can be substantial. Anyway, audio is more complicated in one respect - comparisons of live instruments versus recordings played through speakers are affected not only by effects of the recording/playback system on the frequency spectrum, but also by spatial effects associated with binaural hearing and the coupling of the sound transducer to the room - live instruments just behave differently from speakers in that respect. It would take a 3D Fourier spectrum - a record of how the Fourier spectrum varies through the listening space - to capture that, something that's theoretically straightforward but practically a real bear to implement and interpret.

I still want to think that the optical effects we're talking about, however subtle, have an explanation in terms of MTF - or, more properly, in terms of the coupling of MTF to the perceptual physiology of our visual system.

[paulr]

Emmanuel,
the sound analogy is a good one. But an even tougher one to comprehend. We should count ourselves lucky that we don't have to deal with human psychoacoustics when making pictures. As murky and complex as visual phenomena seem, the corresponding audio ones make them seem simple in comparison.
I'll bet that right now, in some dark parallel corner of the web, a bunch of recording and mastering engineers with nothing better to do are yelling their very similar, equally unproveable theories at each other!

[Oren Grad]

Forgot to add that sound is further complicated by being time-varying as well as space-varying. Paul's right - there's much more voodoo in audio...

[Patrick Troccolo]

I went back to the darkroom this morning and made contact prints of the 150mm Super Symmar HM lens resolution tests. Remember they had a resolution of 75 lp/mm and an enlarged print resolution of 12 lp/mm. I printed them with 3 different Ilford contrasts, 0, 2 & 4. The contact print resolutions were 28 lp/mm, 35lp/mm & 40 lp/mm.
Add that to your contact print vs. enlarged print comments.
BTW, I've never been able to make a print from an enlarger that matched a contact print.

[Patrick Troccolo]

Do you guys ever do any experiments, or is it all about a social medium?

[Sal Santamaura]

Contact print numerical paper resolution were you use the identical negative on Bergger VC and/or graded paper would be most interesting. Please post your results if you do run either of those tests.