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# Thread: Calculating exposure for extremely long exposures

1. Originally Posted by colourgeek
... That's why Ansel Adams's Zone system doesn't really work with present day materials.

2. I have to agree with the other Keith- the proof is in the pudding, you just have to try. If there are big doubts about exposure, my suggestion would be to develop test strips and determine the best dev conditions experimentally. Snip off a bit and develop and see how you're doing. Can you set up to expose a few extra pieces of film simultaneously? If so then this is going to be easy.

3. Ach now I see Ralph's post #11, sorry. My suggestion is almost identical to his except I am suggesting to "bracket" your development, not the exposure.

4. Okay, this goes a little against my nature, I'm the kind that likes to do a lot of calculations beforehand and then go and try it afterwards.

My original idea was to use a film that doesn't take well to reciprocity failure, i.e. Tri-X, since it allows for longer exposures with more light than for say T-Max (if the reciprocity failure calculations still apply at 72 hours of metered exposure, then T-Max need a little under a year while Tri-X needs over 3 years. )

5. Originally Posted by Stuggi
Okay, this goes a little against my nature, I'm the kind that likes to do a lot of calculations beforehand and then go and try it afterwards.

My original idea was to use a film that doesn't take well to reciprocity failure, i.e. Tri-X, since it allows for longer exposures with more light than for say T-Max (if the reciprocity failure calculations still apply at 72 hours of metered exposure, then T-Max need a little under a year while Tri-X needs over 3 years. )

If you're mathy try thinking in integrals - if your shot is light and dark in the same exposure then its unlike %99 of the still photos you'd normally take, where the light stays pretty much constant throughout the exposure (ignoring motion blur)...

Draw a graph of light intensity over time - your exposure is a sum total of the area under that graph...

Also remember long exposure reciprocity is a function of low light intensity not long exposure time, it just so happens that long exposures are required for low light. But just be aware of your cart and horse ordering, even if your horse is in front, it might be backwards, and your cart pointing up - (pedants lets ignore that reciprocity issue with short exposures also). Anyways, this will confuse your issue in that you're going to have more reciprocity 'lag' to deal with when its dark than when it is light... A side by side test over the same duration with two cameras, one shutter closed when it is dark would probably have a very similar image as the one that was open the full time

Its a bit of a mess mathematically - 2nd year university stuff ?

I'd do all the math I wanted, come up with a number - try it out, but have about zero faith it'd be nailed first time

6. Yeah, I'm starting to think that whatever I calculate, it isn't going to be right. But since reciprocity failure would result in me getting less exposure than I want, I don't think I run any risk for overexposing...

7. Originally Posted by Stuggi
Yeah, I'm starting to think that whatever I calculate, it isn't going to be right. But since reciprocity failure would result in me getting less exposure than I want, I don't think I run any risk for overexposing...
I think your method of combining math and testing is sound. If you do a reciprocity test, you should be able to develop an equation that allows for a reasonably close extrapolation. To me, math gets you into the ball park and testing nails it down.

8. Yeah, but correct exposure is still going to be hard to nail down without any means of measuring the negative. What does one of those meters that measure negative density cost?

9. Let us know how the first exposure test come out next year!

BTW - what ND are you going to use for the filter?

10. densitometer,

But, well - I'd be happy with just some clear bits and some dark bits on my neg - not enough dark then increase exposure by x2 or 4 or take off ND

too much dark so add ND or shorten - once you're in the realm of some ok exposure then print and see how the range is looking

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