Resolution of photopaper

[Michael S. Briggs]

The resolution of photo paper is rarely discussed because it almost always exceeds the needed resolution. Photo paper is much slower than film, so it should be easy to make it with resolution at least comparable to film -- but the film is typically enlarged, and paper typically isn't, so film (and lens, etc.) will typically be the limiting factor in resolution, almost never the paper.

The book "Photographic Sensitivity" by Tadaaki Tani has a graph from 1970 (Fig. 1.7) showing the typical properties of Sensitivity and Resolving Power for various photosensitive materials. B/W papers are shown as having resolving powers ranging from 80 to well over 100 line pairs per mm (lp/mm). The next figure places two not-identified color papers at resolutions of more than 100 line pairs per mm. These resolutions are all far beyond the resolving power of the human visual system, so unless one wants to magnify the print, the resolving power shouldn't be an issue.

Ctein in "Post Exposure" reports that the discontinued Ekaflex prints only resolved 20 lp/mm and that some people could notice the difference. He also states that the typical paper for color-negatives can record about 65 lp/mm, while B&W papers can reach 125 lp/mm.

The only modern (non-alternative process) material that I know of in which the print resolution is perhaps discernable is Polaroid prints. Careful examination (e.g., peering at them with my near-sighted eyes) will show them to be slightly softer in sharpness than conventional prints. This fits in with data provided by Polaroid, e.g., the pdf datasheet for Type 52 B+W film has an MTF graph which shows the modulation response falling to 50% by 4 spatial cycles per mm.

The Polaroid figure shows how far most print materials exceed the required resolution. When I look at a Polaroid print I don't notice poor resolution. Perhaps if one had a comparison print of the same subject made with conventional film and paper one might easily notice the difference.

[Steve Baggett]

In Photo Techniques Mar/Apr 2002 Ctein has an article on this very subject. The article's title is "Is Your Print Paper Sharp Enough?". The first sentence in the article is "In a word: Yes". His conclusions are that all other factors that can affect the sharpness, except the paper, are the limiting factors to the sharpness of a print. See this link.

[Steve Baggett]

In my reply the link to Photo Techniques was wrong. This link is the correct one.

[Emmanuel BIGLER]

Armin. When I was a student 25 years ago I contact-printed a USAF test target (a glass/chromium photomask) on a regular b&W enlarging paper. What I found is similar to what is mentioned by Michael. Observed with a 50X microscope lens and probably a 10X eyepiece, the estimated resolution was better than 5 microns, 10 microns of bar+space. So I'm not surprised by the figure of 100 lp/mm published by Mr. Tadaaki. A limit of 6-7 lp/mm is often considered as what is required for a good quality by visual inspection. But not on this group of course, where everybody insits on a minimum 50lp/mm on any final print, achievable only with technical pan or Gigabit plus a Zeiss "Starlith" wafer stepper lens ;-);-);-)

[Peter Galea]

Emmanuel, I thought you were kidding about the "Starlith" till I looked it up.

'Starlith 400 optical system is designed for use in ASML's i-line step-and-scan machine. The optical system provides a resolution of 0.28 µm.'

I can't wait untill dagor77 offers one of these babies up.

[Emmanuel BIGLER]

Peter : sorry but I was *actually* kidding ;-);-) the problem with photorepeater or wafer stepper lenses, besides the fact that they are designed for a specific UV mercury wavelength line and fixed magnification is that first you should reverse them to use them as an enlarger lens. In theory the performance is preserved for a lens used backward, but the usable field for the negative would be very small according to large format standards, something like 3x3 cm maximum, in any case the size of the biggest silicon chips fabricated in one pass (recent microprocessor chips or 24x36mm image sensors could be the actual record as of 2003). Here at the university we have such a photorepeater lens to generate photomasks, from a previous Zeiss 0.8 micron technology (~1970-1980). You could use this lens as a 10X enlarging lens with 1x1 cm negatives and final images of 10x10cm. 0.8 micron (1.6 micron of bar + space) on the neg would be enlarged as a 8 micron resolution (16 micron of bar+space) limit on paper... many B&W papers are even better than this limit !!!!

I'm pretty sure that californians could find photorepeater lenses from obsolete equipement just by looking inside garbage bins in the Silicon Valley ;-);-)