In many of these threads, we have all expressed the joy of such high resolution on our chromes. We have received some great resolution input from many different perspectives. I am a little confused of exactly how this chrome resolution translates to printing paper resolution.

For example, it is my understanding that good papers can only hold 3 lpmm of detail. Also from these threads, I have learned that normal human vision can detect "out of foucs" subjects on paper of 5 lpmm or less. So my first question is, is this apples to apples we are talking about? If all of our eyes consider 5 lpmm or less "not sharp", than why don't all prints look "not sharp"? There must be something missing here. Is there any advantage to resolving 70 lpmm to a 4x5 chrome vs. 40 lpmm, if the maximum enlargment desired is 10x? The lower 40 lpmm can resolve this to paper 40/10 = 4 lpmm. So where exactly is the advantages of all this expensive extra resolution? For example, if you shot 2 4x5 shots side by side, with the same film, one with a new modern LF lens and another with a older lens resolving much less, then made only a 5x enlargement, (well within the reach of each chrome to resolve 3 lpmm to print) could you tell the prints apart when viewed side by side? If so,how?

-- Bill Glickman (bglick@pclv.com), February 13, 2000

Answers

It would be very convenient if paper could accurately reproduce a certain number of lpmm, and no more. Similarly, if human eyes could see a certain resolution and no more.

Unfortunately (or perhaps fortunately), life isn't that simple. As the frequency increases, the ability for paper or our eyes gradually tapers off.

The next problem is that 'looks out of focus' occurs at a much higher lpmm than our ability to resolve lines on test charts.

Here's an experiment you can try. Take a photo focusing exactly on a brick wall. Take another photo, this time with the wall slightly out of focus, but well within the official depth of field. Then another exactly at the limit of the depth of focus. Look at the prints. Can you tell which one looks sharpest? The simple, conventional theory says you won't be able to. I've tried this and I can, and so can the eyes of rather elderly people.

I haven't pinned this down exactly, and there are too many variables to give a single number, but I suspect that 'looks as sharp as can be' occcurs at around 20-40 lpmm on the paper. Yes, really that high.

-- Alan Gibson (Alan@snibgo.com), February 13, 2000.

This general topic has always seemed complex and confusing to me. Whenever people who really seem to know what they're talking about start talking, they lose me in a morass of optical science. However, at my extreme layman's level, I do think it's worth pointing out that among the numerous variables involved here is the distance at which you view the photograph, which in turn ties in with the size of the circle of confusion. Obvously, the farther away from the photograph you are, the larger the circles of confusion can be and still have everything look "sharp." So I don't think it's as simple as being able to say that anything under 5 lpms will not look sharp. Still and all, there must be a point beyond which lens resolution is in fact "wasted" (i.e. when it exceeds the ability of the paper to capture it and the human eye to see it) but I have no idea how to determine exactly where that point is for all lenses, films, papers, and viewing distances.

-- Brian Ellis (bellis@tampabay.rr.com), February 13, 2000.

Bill and I have been discussing this for some time. The issue of frequency response is difficult to square with actual experience. The result seems to go something like this. What our eyes perceive as sharpness is contolled by tonality and crisp edges of objects in the image that are much larger than the "resolution" limit. Few of us take photos of tiny patterns of lines. But signal analysis tells us that sharp edges come from high frequencies in the signal. Here is an analogy from the world of sound. When we buy stereo gear, we are interested in flat response (100% MTF in the optical world) out to 20 Khz. Few of us listen to recordings of dog whistles, and most of us probably couldn't hear a pure monotonic 20 Khz tone (if we ever went to a rock concert), so why do we care if our gear can reproduce 20 Khz. The reason is that 20 KHz puts the sharp edges on other sound. The crisp attack of a trumpet note, the clash of a symbol, or the gnash of the snare drum. Optically, we can't see nor do we care to see lines on paper at 5 lp/mm, but our lens-film-lens(scanner- Lightjet)-paper chain needs to maintain some response at 5 lp/mm to put sharp crisp edges on those 3 mm wide blades of grass we want to look sharp.

-- Glenn Kroeger (gkroeger@trinity.edu), February 13, 2000.

I have not seen resolution figures on other color papers but Ilfochrome(Cibachrome) specs 50 lp/mm for their low contrast paper and 63-100 lp/mm for their other paper and transparency materials. This is quite a bit more than you could ever hope to see with the unaided eye. I suspect other papars are of a similiar resolution, at least far beyond what the unaided eye could see. Where did you get the 5 lp/mm figure?

-- Gary Frost (gfrost@home.com), February 13, 2000.

The best reference on this subject is Ctein's 1997 book "Post Exposure." In Chapter 1, he argues that for a print to appear "perfectly sharp," it must exhibit 30 lp/mm. He goes on to point out that typical RA-4 color-negative papers can reproduce around 65 lp/mm. In the Chapter 5 discussion of how sharp an enlarging lens has to be, he states that, if one is seeking perfect sharpness, work must be done in 4x5, and preferably 8x10. Finally, Ctein shares his decision to live with less than perfect sharpness in exchange for a more portable and versatile setup, adding that he's never deluded himself into believing that bigger wouldn't be observably better. He primarily uses a Pentax 67 system.

The most important aspect of all this, in my opinion, is that there are very few images where perfect sharpness is highest on the list of important characteristics. Fuzzy concepts sharply rendered inundate us daily. Although I sometimes (when the subject matter demands) shoot with an 8x10, more otherwise outstanding images which are less than perfectly sharp would be welcomed by this viewer.

-- Sal Santamaura (bc_hill@qwestinternet.net), February 13, 2000.

Some thoughts:

1) in none of the steps of perceiving sharpness (taking lens, film, enlarging lens, paper, human vision) does the MTF abruptly terminate. Instead is it slowly falls off at higher frequencies. This means that the taking lens having good response at very high frequencies corresponsing to viewing frequencies where the eye has low response still has some limited benefit.

2) Most sources state the limit to the spatial frequency response of the human eye as 5 line pairs per mm. The book "Applied Photographic Optics" by Sidney F. Ray gives a graph of the MTF of the human eye (Chap. 22, Depth of field and depth of focus). It peaks at 1 line pair per mm and the response is down five-fold by 5 lpmm and tapers to zero around 7 or 8 lpmm. So the commonly quoted 5 lpmm for human vision may well be conservative in the sense the visual response to 5 lpmm is quite low.

3) Another poster quoted the argument of Ctein that 30 lpmm is needed. This is based on the fact that the Fourier decomposition of a square wave of N cycles per mm needs higher spatial frequences (= finer detail) to make a good edge. He may very well be right, but his viewpoint is not accepted by many experts, so don't automatically accept his idea as true.

4) As an example, Polariod datasheets for their instant print materials give MTF curves (http://www.polaroid.com/service/filmdatasheets/4_5/index.html ) . The curve for Type 53 (similar to Type 52, except coaterless) shows a rolloff that begins at a spatial frequency of about 1 cycle per mm and that falls to 40% at 4 cycles per mm. When I look at my photographs on Type 53 material, at a comfortable viewing distance of about 1 foot, they look sharp to me. If I take my glasses off and view them from 4 or 5 inches (I am nearsighted) under a bright light, low contrast (e.g., Zone IV grey against Zone VI) look a little soft in sharpness from grainness. High contrast edges (Zone II vs Zone VII) still look sharp.

4) Part of this perception may be relative. If I had a print on a higher resolution medium, perhaps my opinion would change. The viewer normally views one print and doesn't have a print of perfect sharpness to compare it too and so will perceive the actual print as having good sharpness.

5) The perception of sharpness is controlled by several factors. Besides resolution, increasing the light/dark contrast of an edge will create the impression of sharpness. Increasing the paper grade should make a print look sharper. (I suggest choosing your paper grade to get the tones you want, not on the basis of sharpness.)

6) Don't worry about all this. Go out and take photographs, then work in the darkroom!

-- Michael Briggs (MichaelBriggs@earthlink.net), February 13, 2000.

As usual, some great responses, I will try to comment on some.

Sal, I appreciate your reference to the book Post exposure This finding of 65 lpmm on paper has eluded many of us, specially me. I have heard 3 lpmm on paper for the longest time, and never questioned it BIG MISTAKE, it seems we need to question everything in this field! You quote that a print must exhibit 30 lpmm to be sharp, however, if the human eye can only discern 5 lpmm, this seems very high? Once again, mystery, what else is new? 30 lpmm seems to make sense, now I have something to start experimenting with I will also buy the book you reference.

Gary, I must sadly admit, this figure of 3 lpmm, I have read several times in many books. It never referenced a single paper, however, in conversations with people, no one ever seemed to dispute it. I just took it for granted. One of my big mistakes is accepting things that you read in books 10 15 years old. Possibly the information is very outdated, and back in the mid to early eighties this was a good rule of thumb at that time?

Alan, yes I have noticed the same as you in the experiment you propose , and that is why I always question the cc max. in the DOF formulas used for so many years. Is it possible these formulas were all made when paper really could resolve only 3 lpmm? Then the formulas never kept pace with the increased resolving capabilities of the papers? If so, then maybe the flawed DOF field formulas are a result of us not updating our ccs to match what papers are designed to handle today. Your second point is maybe one of the big missing links in this equation. Somewhere along the lines, everyone assumed taking human test chart figures for resolving capabilities and assumed the same applied to viewing a photograph.. however, I agree also this is not the case. I wish a test was performed with different prints at different resolutions to see at what point people see things out of focus. I like your 20  40 lpmm guess, I would have guess about the same.

Brian, I am sorry I did not include the viewing distances this with my original post. It should have mentioned a given viewing distance. I am quite aware that the viewing a print at distance equal to prints diagonal will hold perceived sharpness, assuming the first small version, say 8x10 had the perfect sharpness to begin with. But for simplicity sake I was trying relate this discussion to where exactly things appear not sharp at short viewing distances, like 12  15, i.e. what lpmm was perceived as not sharp.

Glenn, I appreciate all your input on this subject, you have been very helpful to many of us on this forum. You offer extra ordinary knowledge into this subject and are very gracious in sharing it with us. Your minimum 5 lpmm of blades of grass on print, seem to differ from Alans 20  40 lpmm to print, for sharpness, and I think this difference is why there is so much mystery in this area?

Of all these responses, one thing for sure has opened my eyes - papers of today resolve much more than many of us suspected. This probably comes as a result of a disproportional amount of time taking pictures vs. making prints, or at least taking a more active roll in the print making process. Does anyone know when paper actually could only resolve 3 lpmm? Was this something from the 50s or much more recent? Im sure this was once the case, and this conventional wisdom hasnt died yet! With this new information in hand, it also raises another question. Why do LF lens makers only quote MTF curves up to 20 lpmm? As mentioned above, if we need to resolve say 20 lpmm to print for perfect sharpness, then with a 3x enlargement (which is very small) we need to shoot for 60 lpmm to film. Is it possible this also is simply behind the times?

-- Bill Glickman (bglick@pclv.com), February 13, 2000.

I think that the often quoted limit of 5 lpmm (or 3 lpmm, whatever) came about when the materials (film and paper) and equipment (camera lenses, enlarger lenses) could only achieve that. So a circle of confusion of 0.03mm on a 35mm negative gave a print that was indistinguishable from a sharper print, simply because the 'sharper' print wasn't actually sharper. If you use a very fast film such as Delta 3200 then your apparent Depth of Field will be greater.

With modern materials and equipment, our Depth of Field has actually decreased, because our definition of 'sharp' has become more critical.

Two measures of sharpness have been used on this thread. One is 'the print look perfectly sharp'. The other, more critical method, is 'this print isn't as sharp as that one'.

Of course, we don't often compare the sharpness of two otherwise identical prints. However, it does create a problem in portfolios.

I often disagree with the reported comment of Ctein (I haven't read his book), but he and I seem to be in agreement here.

-- Alan Gibson (Alan@snibgo.com), February 13, 2000.

To clarify: I often disagree with what people in the forums report Ctein as saying in his book(s).

-- Alan Gibson (Alan@snibgo.com), February 14, 2000.

One of the areas of confusion in this discussion is whether we are talking about color or B&W paper. Color paper is lower resolution, but the eye's color resolution is also lower.

-- Glenn C. Kroeger (gkroeger@trinity.edu), February 14, 2000.

This was an interesting discussion to read. I only wanted to add that I believe (but I could be wrong) that Polaroids have much lower resolution than standard processes. I have never seen a polaroid that I considered to be "sharp".

-- Lanier Benkard (lanierb@leland.stanford.edu), February 16, 2000.

Bill, here is something else to consider that is not directly related to the discussion, but I feel is important and can impact the sharpness of the final print produced. The general discussion implicitly assumes all premium papers are equal. This is most definitely not the case.

The surface characteristics of the paper can have a large impact on the sharpness of the photograph. From my test matt and luster finishes will produce a image that is less sharp then glossy. Equally important is how smooth is the paper surface. Fiber based papers and resin coated papers I have found to be course and too rough for my needs. Papers made of opaque asetate (sp) backing are extremely smooth and can produce amazingly sharp images. I suspect that is why most films use this material (in translucent form) as well. The Fuji Super Gloss papers (Fujiflex) are made of acetate. My 16x20s using Fujiflex "appear" to be sharper than any of the 8x10s I have made using RC papers such as Kodak Portra and Ultra. There "appears" to be little or no degradation in image sharness between my 16x20s and 20x24s using Fujiflex papers.

Here is an easy test you can do to validate my claims. Take a sheet of Fujiflex paper and hold in front of you adjusting it until you catch a reflection. You will quickly see a clear mirror like image. Now try this with other kinds of paper and see what you get. I suspect you will be very surprised to see just how "poor" the other papers perform. Of course, you should make some prints as well to convince yourself that spending 3-6 times more for Fujiflex papers is worth it.

Hope this helps.

-- Stephen Willard (willard@lvld.hp.com), February 25, 2000.