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# Thread: Calculating exposure for extremely long exposures

1. How about setting up a few cameras and leaving them out for as long as is practical to do a test at, let's say, 1, 2, 4, 8, 16, 32 days? Then use the achieved densities and extrapolate them out for up to a year on a 2^x scale. That should get you pretty close, and in one year you can adjust the curve to be more perfect next time.

2. Leave it to you, Ralph, to suggest a "stop" solution to his problem. This is why I think your book is so useful.
Dan

3. Originally Posted by hpulley
With that tiny amount of light my main concern is that the reciprocity failure will be so severe that you'll get absolutely nothing on the film ...
Not so. There's a long history of extra long exposure photography. Reciprocity doesn't take away all the exposure, just a huge fraction. If you get a blank. you used way too much neutral density.

A few misc. thoughts on very long exposures...

Metering and calculations are pretty useless other than for providing a start for exposure guesstimates.

To shoot your apartment with exposures ranging from months to years, you don't have to worry about a few rainy days or whatever, since it will all average out.

How would I come up with a "film speed" for, say, a one month exposure? Well, I wouldn't even worry about the math or trying to rate a speed other than starting with nothing more than a rough guess. I think I'd end up with an empirical / rote formula that would look something like - Tri-X f/8, 2 months, two stacked ND such and such zillion factor fiter.

Since over the course of a month or whatever, having enough light is not a problem to put it mildly, why not choose whatever is the optimum fstop for sharpness for your lens, probably f8 or so.

Length of day might be a variable, since winter days are shorter than summer, and in high latitudes probably average less bright. That might be a one or two stop seasonal factor.

I'd start with a very, very wide range of exposure for a test. And I'd rely only on a test since there are no published tables of reciprocity losses for days, weeks, and months...

Were cost no object, to make the test process quicker I'd probably just buy perhaps 5 cheepo 35mm cameras off craigslist or such for \$10.00 each, load them with the 400TX or whatever you plan to use, and set them up side by side on a table with whatever neutral density filters (and that's the expensive part because ND filters just aren't cheap, especially the really dense ones) you plan to use, and have very wide ranges of exposure - a middle exposure, a second exposure of a hundred times less exposure than the middle exposure, a third with hundred times less than that, then the same sequence on the plus side; a plus one hundred times more, and a hundred times that. All the exposure variation would be through through neutral density.

Why such a huge exposure range? Since, pardon the pun, effective film speed for months and years exposure is a complete shot in the dark, the first thing is to just get somewhere in the rough neighborhood of the right exposure. If you get within a couple of stops in the first run through, you are doing very well. The next test can refine that value.

If you get a total blank, up your exposure a thousand fold. If the film is totally blackened, drop exposure by a thousand fold. there's little point in muddling around with half stops. You'll spend the rest of your life sneaking up on the right exposure if you take little steps. Each step will set you back a week or a month or a year. If you establish right off the bat that three stacked 1000x ND filters cut too much exposure, and two stacked 1000x NDs blow you away with a totally black negative, finding the middle ground won't take that much longer.

4. Originally Posted by Dan Henderson
Leave it to you, Ralph, to suggest a "stop" solution to his problem. This is why I think your book is so useful.
Dan
Let's not hijack this interesting thread, but it should be said that many processes only show significant change if variables are at least halved or doubled. One exception may be a film test, where development time is increased by square root of 2.

In the OP's example, it makes no sense to increase the exposure time by one day at a a time. Always double the exposure to create meaningful data relatively quickly.

5. A couple of examples to consider.

Adding just 1/3 stop to a 365 day exposure means adding 12 weeks, and that doesn't count the additional reciprocity failure introduced by the longer time.

Adding 1/3 stop to a 4 week exposure means adding 6.5 days, also not accounting for additional reciprocity speed loss.

If you expect to get any meaningful data from testing, you'd better have a good densitometer and working habits.

Manufacturers state that reciprocity failure may vary significantly among different emulsion batches of the same film. Some of that is just denial of responsibility for using the film outside design parameters, but even a 1/3 stop variation, as can be seen above, will be significant to you in adjusting time.

Lee

6. Originally Posted by Lee L
... but even a 1/3 stop variation, as can be seen above, will be significant to you in adjusting time.

Lee
On the bright side, a day, a week, or a month more or less won't matter much.

7. Hmm, this does give some interesting problems. I'll think I'll start of with my intended plan, since that doesn't require me to invest a pile of money in a stack of ND-filters, and see where the 1-2 week exposures leave me. That's fast enough for me to alter parameters, while still giving me the effect I'm looking for. It also let's me see if reciprocity failure calculations are even in the ballpark for these exposures.

The stop-system suggested by Ralph is also very interesting, by plotting the differences it shouldn't be impossible to calculate the effect, and if all the frames are shot on the same roll of film, it would take out variations in processing as well. Only problem is that I do not own a device for measuring film density (what are these called and how much do the cost anyhow?). An alternative would be using my epic excel sheet for reciprocity failure (based on measurements by another APUG:er who's name escapes me at the moment) to calculate the needed times as the time doubles, and then see how deviation it would get in the film densities.

BTW, would pulling affect reciprocity failure? I'm thinking of pulling the 400TX down to EI100 to decrease the amount of filters I would have to acquire. Or I could just use a ASA25 film instead and wing it, for example Rollei Ortho 25, but then again I'm not sure if there are any reciprocity failure data on that film at all...

8. If you want good reciprocity and 100 ISO, T-Max 100?

9. ## All bets are off

With super-long exposures, all bets are off. Suck-it-and-see is the only thing to do. I would choose a slow or medium speed pan film, because reciprocity effects are the least. (B.t.w., astronomical plates offer the lowest reciprocity failure and stand long exposure times.) I would use cut film in a plate camera, which allows a stepped exposure to be done, by withdrawing or inserting the darkslide "sheet", just like making a test strip in the darkroom.

Pulling isn't a good idea, nor is pushing, for that matter. With this project, pulling adds further uncertainties, but more importantly you loose shadow detail and contrast. All modern films, b&w and colour, are "minimum silver loaded", which means the D-max is set by the amount of silver in the emulsion, and development is taken to "unity". This isn't absolute, but compared to pre WW2 materials, speed and dynamic range is set by the manufacturer. That's why Ansel Adams's Zone system doesn't really work with present day materials.

10. There is no way to really tell with exposures that long until you just try something and see if it works. I'd start with negative film, and lean toward overexposure. Then let us know what happens. We look forward to your post on 1-12-12!

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