Reciprocity misbehavior.

[Leigh B]

As for this thread: I want to nail down the tc,1 coefficient, but I'm not sure exactly how I'll know whether my negatively was "properly" exposed at 1 second with any given aperture. I figure it's important if I'm trying to accurately map the reciprocity failure of the film.

I think your goals far exceed the capabilities of the system.

Shutter speed accuracy for mechanical shutters is spec'd at +/-20% for medium and slow speeds and +/-30% for fast speeds.
Modern electronic shutters are better, but you don't often find them.

And the aperture accuracy is not specified at all. It should be pretty repeatable for a given lens, but not necessarily an accurate f/#.

- Leigh

 

[Doremus Scudder]

Check out these threads first:
(there was a url link here which no longer exists)
http://www.largeformatphotography.i...r-film-reciprocity-failure-X-Ray-Film-results

As for testing: I'd just cobble together a reciprocity table either from info from the above threads and/or tables for conventional films and use this as a starting point. Then, take an exposure with the adjustment from your table and then another one and triple the time (barely one stop over given the additional reciprocity failure). See how this does. If you need even more time, make up another chart and test again. Keep good notes and zero in on your times.

Reciprocity failure happens at different rates depending on the amount of luminances in the scene. Empirical testing like the above is going to be faster and easier than just about anything else.

Best,

Doremus

 

[baachitraka]

Any idea how to get the correction factor for Fomapan 200? Please.

 

[Maris]

Any idea how to get the correction factor for Fomapan 200? Please.

Reciprocity failure testing by doing stepped exposures is easy but very boring.
First exposure is an evenly lit target at the longest shutter speed where there is no reciprocity correction expected, say 1/2 second.
Now reduce the light on the target so the meter suggests a 5 second (say) exposure. Make a series of exposures at 5, 10, 20, 40,....seconds.
Reduce the light again so they meter asks for 20 seconds. Make a series of exposures 20, 40, 80, 160, ....seconds.
Continue reducing the light and making stepped exposures until your patience or the film runs out. Keep accurate notes. Develop all the films together. By matching negative densities to the one you got at 1/2 second you can see how much reciprocity correction needs to be applied to meter readings of dim subjects.
You can easily use up a whole day doing this. The reciporocity corrections for Fomapan 200 are not as extreme as the manufacturer suggests.

 

[Doremus Scudder]

Most manufacturers publish reciprocity data for their films. Such tables can be used as starting points for (similar) films that don't have published data. Even for x-ray film, I'd likely take a reciprocity failure correction table as a starting point and then make one other exposure significantly longer than the already corrected one to try and out-flank the correct exposure. Even if the starting corrected exposure is overexposed, it shouldn't be by enough to prevent making a print from.

Then, after evaluating two exposures (instead of 4-8) you should be able to make corrections that get you in the ball park. Do this for, say 2 seconds metered, 10 seconds metered and 100 seconds metered. Find your extrapolated corrected exposures for each of these (6 negs), plot these on some graph paper and connect the dots. Voila, a reciprocity correction table that should be fairly accurate and can easily be refined.

Doremus

 

[senderoaburrido]

Most manufacturers publish reciprocity data for their films. Such tables can be used as starting points for (similar) films that don't have published data. Even for x-ray film, I'd likely take a reciprocity failure correction table as a starting point and then make one other exposure significantly longer than the already corrected one to try and out-flank the correct exposure. Even if the starting corrected exposure is overexposed, it shouldn't be by enough to prevent making a print from.

Then, after evaluating two exposures (instead of 4-8) you should be able to make corrections that get you in the ball park. Do this for, say 2 seconds metered, 10 seconds metered and 100 seconds metered. Find your extrapolated corrected exposures for each of these (6 negs), plot these on some graph paper and connect the dots. Voila, a reciprocity correction table that should be fairly accurate and can easily be refined.

Doremus

Sorry, let me rephrase and then have verify because I'm having a hard time understanding what you're suggesting (pardon my idiocy, I was recently concussed).

So I take the table, and grab three different exposures of some distance from eachother (2,10,100s) and then, using a chart for a similar film, take another three exposures with the adjusted values, and then compare the results across the two sets to narrow it down?

How do I determine which films would be closest to EB/RA Carestream Fast?

 

[Ryuji]

I'm trying to figure out the reciprocity failure for the EB/RA Carestream blue-sensitive x-ray film that I use in my 4x5 because I'd love to do some night work with it. I use a Wratten #44 on my spot meter to approximate what the film is sensitive to. Wondering if there's a way that I could use my equipment to experimentally figure out the important number for this formula. Would I need a densitometer to figure it out?

I'm not familiar with that film but if it is made for green fluorescence screen exposure, the emulsion should be reasonably optimized for long exposures, in the range of a few seconds. I'd guess even longer if it's modern film with tabular grains. Unless you are doing extremely long exposures, reciprocity failure is probably not going to be a major issue.

 

[Doremus Scudder]

Sorry, let me rephrase and then have verify because I'm having a hard time understanding what you're suggesting (pardon my idiocy, I was recently concussed).

So I take the table, and grab three different exposures of some distance from each other (2,10,100s) and then, using a chart for a similar film, take another three exposures with the adjusted values, and then compare the results across the two sets to narrow it down?

How do I determine which films would be closest to EB/RA Carestream Fast?

Not sure which film(s) would be closest to your x-ray film. Ryuji's comment indicates that it may have better reciprocity characteristics than many conventional films.

As for the test. Find a reciprocity table for a tab grain film like Delta or TMax. Use it as your starting point for tests. Find a real-life subject that you can either vary lighting with or stop down to get a variety of exposures. Find an aperture where you can get a 2-second exposure. Consult the table to find the compensation factor (there will be little if any adjustment here, likely an aperture change and no change in shutter speed). Take one exposure at this setting. Now, without changing aperture, triple this exposure (i.e., 6 seconds). That's set one.

Now, adjust light or aperture to get you a 10-second exposure. Consult your table. Say it says 25 seconds. Take an exposure at 25 seconds and at 75 seconds. The third exposure should be even longer, at the extreme of your table. Ilford tables only go to 35 sec. metered exposure, Kodak gives date for 100 seconds, so lets do the first: 35 seconds metered = 200 seconds adjusted so take one exposure at 200 seconds and one at 600 seconds (10 minutes).

You'll end up with six negatives. Make proper proofs on grade 2 paper and pick the best value for adjustment for each of the data points. You may have to extrapolate an intermediate value. Now, using graph paper, make a matrix similar to that on page 2 of the Ilford fact sheet here: http://www.ilfordphoto.com/Webfiles/20106281054152313.pdf , with indicated times along the horizontal axis, corrected time along the vertical. Enter your extrapolated adjustment times (yes, you'll only have three, but that's enough to get into the ballpark). Now connect the dots. You can just use a ruler and get a pretty close approximation to correct. You can use a French curve if you feel fancy

You now have your own table. Use and refine this. If you find that you consistently over- underexpose, enter other data points taken from your copious and carefully made field notes and redraw your curve.

Overexposure is rarely a problem in such cases since you can just print through extra density. The build-up of contrast, however, can be troublesome. You may find that you need to reduce development by greater amounts the longer the exposure. If you end up printing at low-contrast settings a lot, build a development time compensation into your table.

Best,

Doremus