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# Thread: Calculating exposure for extremely long exposures

1. ## Calculating exposure for extremely long exposures

I'm thinking of trying my hand at extremely long exposures, in the order of days, weeks, months and maybe even years.

The plan is to exploit the reciprocity failure of some films to get the long times I'm looking for, especially Kodak 400TX since it's the worst of the of "standard" films.

My question is this, what to do with varying light conditions. For example, as a test, I intend to shoot a shot of my apartment for couple days. For maybe 12 hours per day, the apartment will be lit by sunlight/artificial light, and during the night it will be dark. In my head, I'd figure that if I meter for the light part of the day, and then overexpose one stop, it should make up for the darkness half of the time. Is this right, close to the truth, or completely off?

And how accurate must I be when doing these exposures? I mean, if I expose a piece of film for say a month, how much does it alter the exposure if I cut the time short by a couple days, or forget about it and stop the exposure a week too late?

2. If you calculate the correct aperture for a month long exposure, and leave it for an extra week, I'd call that about 1/4 stop overexposed. I think the math still works pretty much the same again once you've gone that deep into the reciprocity failure end of things.

Who was that guy who took the years-long exposures of building projects and stuff? Oh yeah, Michael Wesely:

http://www.unfinishedman.com/the-lon...michael-wesely

Must be a heck of a stack of ND filters involved :-)

Duncan

3. Okay, so if it's dark half the time of the day, that would mean a 50% decrease in light, which would require a one stop increase in exposure from the computed exposure at daylight?

4. I guess, but most places even when it's dark it's not really DARK... and across a bunch of days you might need to figure out more precisely what percentage of the day it's really dark, etc. I hope you have a lot of patience, to zero in on the proper exposure, it may take quite a bit of "bracketing"!

Duncan

5. It seems to me that the idea of metering goes against the grain of extremely long exposures. If it were me, I would pick an arbitrary exposure time, develop the film, and see if I had blank film, some image, or a completely opaque negative, then adjust exposure of the next negative based on the outcome.

I am curious as to how you are going to restrict the light to permit such long exposures.

6. If you have an old box camera you don't care about you might want to try setting it up at the same film, aperture, etc. for a day and develop it and see what you get and go from there. Heck, you might even want to start with an hour of exposure during daylight to make sure it isn't already completely white and if it comes out then do a 4 hour exposure to see if it actually captures anything more than 1 hour.

7. The plan is to load up my Canon FTb with Tri-X, lock the shutter open, and stack ND-filters infront. My calculations have resulted in that if the average light is around EV6 I would need a -16 stop decrease to reach a 347 day exposure with 400TX after compensating for reciprocity failure (if the effect computed with the formula T=Tm+a*Tm^b still works for these insane times, i.e. 2048 minutes of metered exposure)

8. 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.

Are you going to do any shorter tests first? It sounds like an interesting project but I think it would be a bit disappointing to get a completely black or completely white frame after the year because no real world tests were done first. I guess you want to get started but I'd use one camera for the year test and set up some more for shorter tests to develop early in case the calculations were off.

9. Originally Posted by frobozz
If you calculate the correct aperture for a month long exposure, and leave it for an extra week, I'd call that about 1/4 stop overexposed. ...
To be more precise, and assuming reciprocity, the f/stop error is calculated as:

f/stop error = log(actualTime/targetTime)/log(2)

which gives us an overexposure of 0.32 stops for a time increase of 25%.

However, this example is far into reciprocity failure, and therefore, all bets are off.

10. My intentions are to start off at a couple of days, then do more tests until I think I got it right. The only problem is how to test for 1 year exposures, if the calculations are off, then it takes a year until you notice, and if the formulas for reciprocity failure are off, then how am I supposed to go from there? One way would be to just set up a bunch of pin-hole cameras, and do 10-15 simultaneous tests, but then it becomes problematic to make sure that all the cameras are identical, and that the apertures actually are the size used for calculations.

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