Reciprocity Question

[Carnie Bob]

I should know the answer to this but I don't so a little help here would be appreciated.

I understand that in exposing certain films the longer times become problematic for workers as an 1/2 stop change in density can mean long , long times on the camera exposure in dim light situations.

Is there the same problem in contact printing lets discuss Platinum Palladium for example. Right now my time for a good decent print is about 1 minute and if I want to darken the print by 1/2 stop it is within a decent printing time , lets say 1 min 30 seconds.
But I have a lot of friends that find their time for a good decent print is about 7 min using the systems they have, if they want to make a 1/2 stop density increase, do they have to consider reciprocity so their time is actually much longer .?

The reason I ask is that I am mentoring a whole bunch of younger workers , who do not have the types of exposing systems that I have and I have no experience with the slower systems lately and therefore the question.

 

[koraks]

Under the circumstances you mention, no, I don't expect that reciprocity failure within the print medium itself is much of a concern. A timescale of 1 minute vs. 10 minutes is only an order of magnitude, and as chemical reactions go, both are glacially slow. However, you'd have to look into each process specifically and do systematic testing to find out if there's a significant reciprocity failure effect. I don't expect you'll find much for the typical processes we use (cyanotype, gum bichromate, Pt/Pd, kallitype, Van Dyke, carbon etc. etc.) and the typical printing times we encounter (let's say 30 seconds up to 60 minutes).

There are other factors that may spoil the broth and may make things seem to progress non-linearly. You're aware of self-masking, no doubt, and this evidently plays a part in many of the processes we use. Again, it's very process-specific.

Another effect that comes to mind is related to the light source as such; the problem here is that light sources tend to have a non-linear output degradation profile that's also dependent on the type of light source used. E.g. most UV tubes tend to come on in a quasi-exponential way within a few minutes (this varies with the type of tube) and then they remain at a fairly stable plateau. The same for HMI bulbs. LEDs, however, exhibit a negative temperate derating curve, meaning that they start out bright but then light output tapers off as the light source heats up. Again, things stabilize after some time, but this can well take 15-30 minutes to happen, so it's well within the kind of exposure times you're typically working with. Using a light integrator is a fairly good solution to this issue assuming that the spectrum doesn't change along with intensity (which is likely not an entirely valid assumption, but we probably get away with it, sort of, most of the time - especially with LED).

Then there are potential problems with the print medium heating up during exposure, which may or may not affect printing characteristics. I've never noticed any particular problems in this regard, but I can very well imagine that in particular humidity-influenced processes like Pt/Pd may behave differently if the print heats up to a greater or a lesser extent during exposure, causing differences in the humidity of the medium during exposure.

Especially the light source derating effect I mentioned above can and in practice noticeably does 'break' the relation between exposure time and print density, but this is not so much because of a reciprocity failure in the print medium. It's an intensity issue particular to the light source and thus you can't really make good predictions about this in an absolute sense; the best I think you can do is point out the effects that may occur and warn that non-linearities sometimes turn up. As always, systematic testing combined with digging into theory are the way towards getting a grip on those. Lacking such an approach, the best alternative is to stick with rigorous consistency in printing, i.e. "determine what works for you and then try not to change anything, ever."

 

[Sharktooth]

I agree with koraks that your don't really have to worry about the reciprocity deviation for printing papers.

Film is designed to work in hand held cameras with fast shutter speeds to prevent motion blur. Let's say that a typical shutter speed would be 1/125 of a second. A very fast shutter speed would be 1/4000 of a second (5 stops faster), and a very slow shutter speed would be 1 second (7 stops slower). That's still a very big exposure time range where reciprocity still holds. When you get much faster than 1/4000, or much slower than 1 second, then you might have to worry about reciprocity deviation.

On the other hand, photographic paper has much lower sensitivity, and it's designed to work with much longer exposure times. Let's say that a typical exposure time is 2 minutes. A 30 second exposure is only 2 stops different, and a 16 minute exposure is only 3 stops different. That's really not much of a range to worry about, and should not suffer from reciprocity deviation in any significant way.

 

[MTGseattle]

I too think the bigger gulf in what you are trying to do lies in the difference of available exposure sources between you (and what you are using for teaching) vs what the students have available for themselves post instruction.

If we are given a consistent exposure light source and a bunch of the exact same sensitized material, I'm sure one could plot out a reciprocity curve derived from various negatives but holy cow, what a project.

When I finally get off of my behind and get to some Carbon printing, I am basically on my own unless someone else has the exact same exposure light system that I have built. I'm hoping a Stouffer step-wedge will ease my pain.

 

[koraks]

I'm hoping a Stouffer step-wedge will ease my pain.

It will, don't worry. The pain will be in different areas.

 

[Romanko]

I suspect that you are dealing with the shoulder of the characteristic curve of your media rather than reciprocity failure. A simple step exposure test can give you a definitive answer to your question. Which of the effects mentioned by @koraks (including reciprocity failure) contribute to the phenomenon is an academic question. Once you plot your characteristic curve for your media, your exposure system and your processing you have all the information you need to produce beautiful prints (just make sure you expose the last patch for long enough to get the maximum achievable density). If you have a densitometer you can compare your maximum density to that of other workers. Human eye is not very good at differentiating high print densities but you can always use a digital camera for that purpose.

 

[koraks]

I suspect that you are dealing with

I think you need to re-read Bob's post one more time:

The reason I ask is that I am mentoring a whole bunch of younger workers , who do not have the types of exposing systems that I have and I have no experience with the slower systems lately and therefore the question.

He's trying to transfer his knowledge and experience to a broader audience. I think Bob is pretty well in control of his own process; the question is how he can best explain/advice a new generation of printers on how to deal with process adjustments.

 

[Romanko]

He's trying to transfer his knowledge and experience to a broader audience. I think Bob is pretty well in control of his own process; the question is how he can best explain/advice a new generation of printers on how to deal with process adjustments.

Sorry if I didn't make it clear in my previous post. I have no doubts in Bob's skills and experience (which are way ahead of my own). "You" in my sentences should be read as "one" or "they". I was not referring to Bob personally.

All I wanted to say was that a good practical procedure to characterize the media and calibrate the system is more valuable than theoretical knowledge and I suggested that he focuses on that. Bob's audience might not have sufficient background in chemistry and physics to benefit from the theory of alternative photographic processes.

 

[koraks]

OK, gotcha, but even so, I'm afraid I don't quite agree:

Once you plot your characteristic curve for your media, your exposure system and your processing you have all the information you need to produce beautiful prints

Be that as it may, it doesn't answer the question how to deal with increases or decreases in overall density by varying exposure intensity or (particularly) exposure time. Of course calibrating the process is essential. The core of this thread is what kind of problems we (you, they) can expect if exposure parameters are changed.

 

[Carnie Bob]

I think you need to re-read Bob's post one more time:


He's trying to transfer his knowledge and experience to a broader audience. I think Bob is pretty well in control of his own process; the question is how he can best explain/advice a new generation of printers on how to deal with process adjustments.

Exactly, I have trained about 8 people now in tri colour over gum, and as life is not everyone comes to me from the same economic platform or circumstances of equipment . I try not to discourage those who want to learn this process. I get tons of questions and since most of them are not working exactly the same workflow , equipment , materials for me I find myself frustrated not being able to answer every question. In my city there is a massive artist group and at the core of their facility is a McGuiver florescent light box and it is quite slow compared to my unit. I am also trying to use Ian Leakes UV meter outside with students so people can work on their balconies if so desired. They have the elements to deal with and very low light in the winter months , some of the answers above are quite encouraging.

 

[koraks]

Yes, understood Bob. And props for continuing to transfer your craft to the next generation of printers!

One comment about the UV meter (which applies to any device, regardless of manufacturer etc): note that these are dependable only if they're used with the exact same spectrum light source. The reason is that the response of the UV sensor is extremely non-linear and a change in spectrum will have a very big influence on the readings. The implication is that using these for e.g. controlling daylight exposures, they only work dependably if they're used on the same kind of daylight for every exposure - same season, time of day, atmospheric conditions...So in the real world, you'll see variations in exposure when trying to control daylight exposures using a UV integrator. How big these variations are, is difficult to tell. They may be barely significant. They may amount to (much) more than a stop. It all depends.

The same is true for trying to transpose exposure 'units' from one light source to another. This only works reliably if the spectra of the light sources are comparable from the point of view of the exposure controller/light integrator. It's shooting a moving target.

Don't let this stop anyone from trying to use a UV meter/integrator while violating the 'ceteris paribus' control - just be aware of the (significant) caveat.

@Ian Leake also cautions about this in his user manual:

‘Units’ are calibrated to a specific light source, so they are not comparable between exposure units. A 300 unit exposure under a low intensity light source is not the same as a 300 unit exposure under a high intensity light source. And because ‘units’ are an arbitrary measure, they are not comparable across systems made by different manufacturers.

In the context of the use of such a device, the sun should not be regarded as a single, constant light source, but an infinitely variable one.

 

[Carnie Bob]

Yes, understood Bob. And props for continuing to transfer your craft to the next generation of printers!

One comment about the UV meter (which applies to any device, regardless of manufacturer etc): note that these are dependable only if they're used with the exact same spectrum light source. The reason is that the response of the UV sensor is extremely non-linear and a change in spectrum will have a very big influence on the readings. The implication is that using these for e.g. controlling daylight exposures, they only work dependably if they're used on the same kind of daylight for every exposure - same season, time of day, atmospheric conditions...So in the real world, you'll see variations in exposure when trying to control daylight exposures using a UV integrator. How big these variations are, is difficult to tell. They may be barely significant. They may amount to (much) more than a stop. It all depends.

The same is true for trying to transpose exposure 'units' from one light source to another. This only works reliably if the spectra of the light sources are comparable from the point of view of the exposure controller/light integrator. It's shooting a moving target.

Don't let this stop anyone from trying to use a UV meter/integrator while violating the 'ceteris paribus' control - just be aware of the (significant) caveat.

@Ian Leake also cautions about this in his user manual:

In the context of the use of such a device, the sun should not be regarded as a single, constant light source, but an infinitely variable one.

Good Points

 

[Ian Leake]

In the context of the use of such a device, the sun should not be regarded as a single, constant light source, but an infinitely variable one.

The amount of variation will greatly depend on the printing process. For example, silver-based sun printing will be more variable than iron-based sun printing because of its higher sensitivity to visible light.

 

[Ian Leake]

Don't let this stop anyone from trying to use a UV meter/integrator while violating the 'ceteris paribus' control - just be aware of the (significant) caveat.

@Ian Leake also cautions about this in his user manual:

‘Units’ are calibrated to a specific light source, so they are not comparable between exposure units. A 300 unit exposure under a low intensity light source is not the same as a 300 unit exposure under a high intensity light source. And because ‘units’ are an arbitrary measure, they are not comparable across systems made by different manufacturers.

The reason 'units' are not transferable between different Light Counter installations is that they are calibrated to a specific light source and sensor location relative to the light source. One 'unit' is approximately one second of exposure when the light source is at full power. If you change to another light source then the maximum power will change, resulting in a different UV measurement. If you point the sensor in a different direction then it will likely give a different measurement.

 

[Ian Leake]

I should know the answer to this but I don't so a little help here would be appreciated.

I understand that in exposing certain films the longer times become problematic for workers as an 1/2 stop change in density can mean long , long times on the camera exposure in dim light situations.

Is there the same problem in contact printing lets discuss Platinum Palladium for example. Right now my time for a good decent print is about 1 minute and if I want to darken the print by 1/2 stop it is within a decent printing time , lets say 1 min 30 seconds.
But I have a lot of friends that find their time for a good decent print is about 7 min using the systems they have, if they want to make a 1/2 stop density increase, do they have to consider reciprocity so their time is actually much longer .?

The reason I ask is that I am mentoring a whole bunch of younger workers , who do not have the types of exposing systems that I have and I have no experience with the slower systems lately and therefore the question.

In answer to your original question, Bob, I have never considered reciprocity when printing in platinum/palladium. It may exist, but if it does then it has never caused me a problem, even with some very long exposures (I was doing some 30 minutes exposures at the weekend versus my normal printing time of about 3-5 minutes).