Measuring film resolution

Bill Burk · Jun 3, 2015

Rudeofus said:
Attached to this posting are three graphs which compare exposure vs. resolution for POTA and D-19....

If the very sharp peak in resolution versus exposure for D-19 is typical of regular films and developers (in a very general sense)... Then it means there is very little to no exposure "latitude" if you are seeking highest resolution.

And it could drive you crazy trying to evaluate it, but even the correct exposure... where you achieve this peak resolution... would only be correct for a very limited part of the subject - the part of the subject that is in the correct light and has the right contrast.

That's where microfilm copying of documents like newspapers and magazines is probably the best kind of subject (where you can adjust the exposure and processing with some trials) to try to achieve the pinnacle of resolution, lock it down and get some work done with it.

In an everyday photograph, you may have a lot more trouble hitting the highest peaks of resolution deliberately... But you might hit it accidentally in some parts of some photographs.

I was hoping for that in my pictures of Oak and Mistletoe, Black Diamond Mines. It would have been great if the leaves of the mistletoe bunches was clearly visible on the print... So it would have been great to have focused on the bunches and selected an exposure that would place the average of the bunches at the peak of the "exposure vs resolution" graphs. (I don't think I even achieved 10lp/mm in those shots. So I will have to re-do that).

Rudeofus · Jun 4, 2015

Bill, the extreme peak as published by Levy appears to come from document film plus high contrast developer. I have no experimental data to back this up, but I do think with normal film and normal developers we should have a resolution vs. exposure characteristic more like Levy published for document film plus POTA.

The main intention for posting these graphs was to highlight that exposure is not irrelevant when measuring resolution, and using the same exposure that one uses for step wedge exposures will like give poor results.

Photo Engineer · Jun 4, 2015

Ahh, but we learned that fill and flare take place at different places on the scale and thus with different exposures and that is why we use both positive and negative charts for measurement. This is a point you keep ignoring here. I doubt if many companies did that type of test.

PE

Rudeofus · Jun 4, 2015

With contact printing I see no advantage from using positive and negative versions of a resolution target. Given the number of misleading resolution test results I have seen due to focus problems, poor lens/aperture choice or other issues I would only contact print such a target if at all possible.

Photo Engineer · Jun 4, 2015

They can be used with both contact and enlargement printing to advantage. In fact, my charts are 4x5 and I use them in contact printing to test various surfaces of papers. It works well and there are 3 examples in my book on this subject.

PE

Bill Burk · Jun 5, 2015

I think you could make a film positive for contact printing, of a small portion of a resolution target (example the inner three-sixteenths-inch of the 1954 target) step-and-repeated every three-sixteenths of an inch in a single row... You could put this in a contact printing sandwich, (emulsion in contact with the emulsion of your test film), with a Stouffer step wedge and get 21 results to examine to make an exposure vs resolution graph of your own.

Even if your step and repeat pattern had its own resolution limit of, say, 100 lp/mm, you could at least see and report results in terms below the best that your step wedge has, for example you could say that you got results "better than 80 lp/mm".

Bill Burk · Jun 5, 2015

To continue the idea, making such a "positive" (with a step-and-repeat pattern) would begin with a "negative" - so it makes the case for having a negative master to begin with.

Rudeofus · Jun 5, 2015

It will be a lot cheaper to replicate the positive master on Fuji Velvia than to obtain an affordable negative pattern (transparent stripes on black sheet). I do like the idea with the repeated inner pattern to match the progress of the step wedge. It will be tricky to make such a complete pattern on one test strip, but it will save a lot of time during actual measurements.

Bill Burk · Jun 5, 2015

You could make a suitable Master negative like this: Print-out a paper chart and photograph it from a distance with the best lens and film combination you can devise. Expose the film in-camera using several attempts performing focus, aperture and exposure bracketing. Develop this film the best way to achieve high resolution. Maybe you will use CMS 20 or Technical Pan or other high-resolution microfilm. Maybe you will use POTA developer.

From the best negative that you got, you could contact print to make the Second-Generation positive step-and-repeat.

Anyway you should start by deciding up-front how much resolution you want to measure.

My gut tells me this might get close to 80 lp/mm in the final positive, which may be good enough for "process control" purposes.

Photo Engineer · Jun 5, 2015

You don't seem to get the fact that the paper surface affects resolution and sharpness.

PE

Bill Burk · Jun 5, 2015

Photo Engineer said:
You don't seem to get the fact that the paper surface affects resolution and sharpness.

PE

You mean photographing a paper target won't produce a usable Master Negative from which to make a 2nd generation positive?

I'm only thinking of coming up with a reasonable resolution reading for process control purposes for everyday film and developer combinations, at a very low price.

I don't have any illusion that this would create a research laboratory quality master.

What I'm wondering is... What resolution could I achieve with this plan?

Athiril · Jun 5, 2015

I get over 150 lp/mm on CMS20 (from a lustre paper target), with the Canon FD 28mm f/2.8 @f/5.6 and f/8.

I've never had a missed focus shot reducing resolving power at this focal length and aperture.

I try to set a distance that gives me the range of frequencies I want on the first half of the scale (1-10 rather than 11-20), as the black lines become thicker than the white lines on the print size I made.

Bill Burk · Jun 6, 2015

OK so this exceeds most of the other films you would be testing, seems you could use that negative to make a step-and-repeat contact positive on another piece of CMS20 to create a single strip... there is a chance you would still be above 100 lp/mm even by that point.

Another possibility is to shoot a "negative" lustre paper target designed to contain several 3/16 inch step-and-repeat patterns. Assuming you get four or five good repetitions in a single frame you could splice four or five of these frames into a single strip to use in contact with the Stouffer scale.

Rudeofus · Jun 6, 2015

My impression is that some of CMS20's resolution comes from its rather high contrast - and that's exactly what I look for with contact printed Velvia 50, and without the double reversal step and with no lens causing flare and other defects.

Alan Johnson · Jun 6, 2015

Microfilm-types are not high contrast if developed in the appropriate developer for normal range subjects. Here are mtf curves for some Kodak products:
http://www.mikrofilm.hu/pdf/negativfilm.pdf
Some time ago I tested my Canon EF 50 f1.8 at f5.6-8 and developed Rollei ATP in a normal range developer, don't recall which one, got 125 lppm on the negative.

Athiril · Jun 6, 2015

I just shoot the target directly with each film.

Photo Engineer · Jun 6, 2015

Bill Burk said:
You mean photographing a paper target won't produce a usable Master Negative from which to make a 2nd generation positive?

I'm only thinking of coming up with a reasonable resolution reading for process control purposes for everyday film and developer combinations, at a very low price.

I don't have any illusion that this would create a research laboratory quality master.

What I'm wondering is... What resolution could I achieve with this plan?

The master negative will be quite degraded and will probably not give suitable results.

PE

Rudeofus · Jun 6, 2015

Alan Johnson said:
Microfilm-types are not high contrast if developed in the appropriate developer for normal range subjects.

But that's not what people seem to do here with CMS 20, at least not in this thread here.

Alan Johnson · Jun 7, 2015

If you are trying to make a line target by photographing a resolution chart on microfilm-type it is possible the lines might be less degraded if the film is developed to high contrast, IDK, have never seen a comparison.
I would think that purchasing a purpose made chart as you are doing will give a better result though.

Rudeofus · Jun 7, 2015

Alan Johnson said:
If you are trying to make a line target by photographing a resolution chart on microfilm-type it is possible the lines might be less degraded if the film is developed to high contrast, IDK, have never seen a comparison.
I would think that purchasing a purpose made chart as you are doing will give a better result though.

I have such a target, and will try to implement Bill Burk's idea of repeated resolution patterns which follow the step pattern on my Stouffer wedge. Having such a special resolution target would allow me to see (and measure) density, grain and resolution versus exposure in one single test clip, which gives me much more reliable results than determining the three properties from three separate test strips.

Since we have reason to believe that higher contrast yields better resolution unless grain grows out of control, I think I will try to create this resolution strip with repeated patterns on Fuji Velvia 50. Not only is this film plenty sharp enough to make a resolution target for my purpose (Tri-X, HP-5+, Delta 3200), it also saves me the second copy step or the (very expensive) inverted target.

RobC · Jun 7, 2015

Many years ago my photography tutor said something to me which I thought was garbage at the time. He suggested that an exposure of 1/30 or 1/60 second would always give the best result but didn't explain why. I thought at the time that he was talking nonsense. I don't know whether it was just his observation from experience or whether he knew something of what has been talked about here but maybe I'm a little wiser now.

Photo Engineer · Jun 7, 2015

Hand held at those low shutter speeds would not be very good.

PE

RobC · Jun 7, 2015

Photo Engineer said:
Hand held at those low shutter speeds would not be very good.

PE

Agreed but tripods aren't excluded and most of us on the course were doing landscape using tripods.

Photo Engineer · Jun 7, 2015

Yes, I understand, but you made it as a general statement and didn't qualify it so that statement as is, made to students, is misleading.

I hope you see my point.

PE

Athiril · Jun 7, 2015

CMS20 has more than enough contrast in it's dedicated developer. It has a much lower drop in contrast and maintains more contrast as it approaches the resolution limit put down by the lens until a sharper cut off occurs, unlike with a regular pictorial film that may be more gradual.

Still not sure why you want to contact print sources to the films, rather than image direct by lens?

Measuring film resolution

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