Enlarging Meters and Reciprocity Failure

I’ve been reading about the Darkroom Automation light meter / timer system. It looks interesting, and I rather like the idea of never having to make another test strip again.

As a force of habit, I have already for a number of years been selecting exposure times in terms of “stops” when exposing paper (in other words, I tend to choose exposure times of 125, 60, 30, 15, 8, or 4 seconds just like in-camera.)

However, like film, paper suffers from reciprocity failure. So, do these “stops” actually mean anything? And therefore, does the theory behind the Darkroom Automation system actually work?

In other words, if I make a print using f/5.6 and 30 s, and then the midtones of the resulting print look too dark (say, zone VI instead of zone V) then a naive photofinisher might assume that an exposure using f/5.6 and 15 seconds would be one stop lighter.

However, because of the reciprocity failure of photosensitive materials at long exposure times (greater than 1 second) this won’t actually work. So what value does printing in “stops” really have?

Reciprocity failure doesn't kick in with paper until light levels get much, much lower than the levels that require 1 second exposures.
It varies with different papers, but I wouldn't worry about exposures until they get into the several minute range.
Very, very intense and fast exposures - such as the ones used in the digital enlargers can cause the other type of reciprocity failure. If your exposures are extremely bright and in the fraction of seconds range, you have to be careful. The roll papers designated as being for digital exposure are designed with that in mind.
Ilford says this in their FAQ section of the website:

"DOES RECIPROCITY AFFECT PAPER?
Paper products are designed for much longer exposure times than film and are less sensitive to reciprocity failure. Whilst there may be a small effect over very long times it is generally not necessary to make any compensation for reciprocity failure on paper products."

waffles said:

I’ve been reading about the Darkroom Automation light meter / timer system. It looks interesting, and I rather like the idea of never having to make another test strip again.

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Oh, dear. The system doesn't work that well. Expect test strips every now and then along with work prints. I often make 5x7 work prints to get the lay of the land and then go on to 11x14 final prints - the meter/timer work superbly when resizing a print (but, then, so does a ruler http://www.darkroomautomation.com/support/stopsruler.pdf).

As to reciprocity failure - there really isn't any. That is, of course, key to making the Darkroom Automation system (or any other system) work. For results of a test see http://www.darkroomautomation.com/support/appnotereciprocityandintermittency.pdf

In general, the slower the emulsion the less the reciprocity failure. It is possible, under very low light conditions, to have Technical Pan (nominal ASA 25) require less exposure than Tri-X (ASA 400). Hydrogen hypersensitized Tech Pan was a favorite of amateur astronomers http://www.scienceandart.com/photom31.htm - not bad for an 8 hour exposure with an 8" telescope (well, truth be told, 3x8 hour exposures, one each through red, green & blue filters); I have a 20x30 print from when they sold them that big and the detail is stunning.

waffles said:

In other words, if I make a print using f/5.6 and 30 s, and then the midtones of the resulting print look too dark (say, zone VI instead of zone V) then a naive photofinisher might assume that an exposure using f/5.6 and 15 seconds would be one stop lighter.

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And he would be right - be he sophisticated or naive.

Photographic paper has higher contrast than film. To get a one zone change - one stop of exposure in the camera - you only need a 1/2 stop of change when making a print; this varies, of course, with paper grade/contrast filter.

Nicholas Lindan said:

Photographic paper has higher contrast than film. To get a one zone change - one stop of exposure in the camera - you only need a 1/2 stop of change when making a print; this varies, of course, with paper grade/contrast filter.

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Wait, what? You need a 1/2 stop change in exposure to make a 1 stop change in density on paper? That doesn’t make sense to me. If so, it would mean paper has less contrast than film, not more. Right?

waffles said:

Wait, what? You need a 1/2 stop change in exposure to make a 1 stop change in density on paper?

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Sorry, I stated that a bit confusingly.

In very approximate terms, and playing very fast and loose with the numbers:

Film will take a 10 stop scene brightness range and turn it into a 5 stop density range in the negative (told you this was fast and loose with the numbers) - for a contrast of 50% (if that number means anything).

Paper takes that 5 stop range in negative density and turns it into a 5 stop (1.5 OD) range in print density - for a contrast of 100% (continuing this post's tradition of not paying much attention to the fine points of numbers).

End-to-end the contrast is 50%: Film has 50% and paper 100%, so paper has more intrinsic contrast than film.

To mimic the effect of a 1 stop change in scene brightness in the print (a 1 Zone change) you would change the print exposure 1/2 a stop.

Yes, papers experience recip issues just like film, but not typically under a minute. I just do test strips regardless. No need for fancy programs, readers, or formulas. Yes, I am equipped with some fine darkroom enlarging meters which came in handy back in Cibachrome days, but have seen little use since. Papers are not all the same in this respect anyway. They differ from one another, and even change somewhat in different developers. Therefore generic answers tend to be unrealistic. It's all about the specifics, product and methodology wise.

Nicholas Lindan said:

Sorry, I stated that a bit confusingly.

In very approximate terms, and playing very fast and loose with the numbers:

Film will take a 10 stop scene brightness range and turn it into a 5 stop density range in the negative (told you this was fast and loose with the numbers) - for a contrast of 50% (if that number means anything).

Paper takes that 5 stop range in negative density and turns it into a 5 stop (1.5 OD) range in print density - for a contrast of 100% (continuing this post's tradition of not paying much attention to the fine points of numbers).

End-to-end the contrast is 50%: Film has 50% and paper 100%, so paper has more intrinsic contrast than film.

To mimic the effect of a 1 stop change in scene brightness in the print (a 1 Zone change) you would change the print exposure 1/2 a stop.

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Really?! That's the first I've ever heard of this. So, if I understand you correctly. Photographic paper can produce much more varied tones than we will ever be able to capture in photographic film? Even modern T-grain films??

waffles said:

Really?! That's the first I've ever heard of this. So, if I understand you correctly. Photographic paper can produce much more varied tones than we will ever be able to capture in photographic film? Even modern T-grain films??

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I think Nicholas Lindan has the basis of an interesting way to think of something fundamental. But wrote it too fast and muddled it up. I know exactly what he’s trying to say. But I don’t know if I will make it better or worse…


A light meter may find 7 stops of difference in the darkest and lightest reflected light readings in a normal original scene. Depending how hard you look for brighter and darker keys it’s ok to say there’s ten stops difference in subject luminance range may exist in a normal original scene (Zone System names 10 Zones).

When you develop film you never develop to 1:1 contrast (except for special purposes). You develop to about half the density range compared to the original subject luminance range.

So the range of light you want to hit the paper with when you print will be about half the range of light in the subject. You won’t hit the print paper with 7 to 10 stops of enlarging light range, more like half that. Figure 3 to 5 stops.

That 3 to 5 stops takes the paper from white to black.

Later if you read the print with a light meter you would only find 5 stops of difference. That’s all you can get with 2-dimensional flat prints. (The original 3-D subject had some stuff in the shade).

Original 10 stops.
Negative 5 stops.
Print 5 stops.

waffles said:

...if I understand you correctly. Photographic paper can produce much more varied tones than we will ever be able to capture in photographic film? Even modern T-grain films??

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Sorry, but I do not understand this statement. Try Bill's restatement, above.