Calculating very long exposures e.g. Several hours?

[l2oBiN]

I would like to leave the camera open for several hours to get some dusk to dawn exposures...

How can I calculate the correct exposure/film/reciprocity factors?

[Vaughn]

Well, I have a chart given out by Michael Kenna at a workshop years ago. For Tri-X, if the meter reading said 40 minutes, the chart recommended an 8 hour exposure. (and a 20% reduction in development times).

So if this is correct, just use Tri-X, and you could use an f/stop that would normally give you a 40 min exposure. Leave the lens open for 8 hours (or 10 or 12 hours -- not that big of a difference).

That is a starting point, anyway. One could extrapolate out from existing charts put out be the film manufacturers, also.

TMax and Acros films would have much less of a reciprocity failure rate, of course. For the 4 hr time MK recommended for Tri-X for a total darkness landscape, he recommends only 90 minutes for TMax 400 (both films at f/5.6)

PS -- MK was referring to roll film.

[BrianShaw]

I have (somewhere) a calculator that was made for long exposures. It is called the Black Cat Calculator.

http://www.blackcatphotoproducts.com/guide.html

There is also a calculator on the internet known as The Fred Parker Untimate Exposure Calculator. It gives some ideas for long exposure photography.

http://www.fredparker.com/ultexp1.htm

[l2oBiN]

Would be great if people could post their charts here?


What about other films?

In B&W I am using

Acros 100
Fomapan 100
TriX320

In Color

Velvia 100F

[grahamcase]

I usually just stop down my aperture as far as I can go, and open up the lens. I don't usually think about it, especially for really long exposures. For an exposure of 6-8 hours you're pretty much guaranteed to need a very small aperture no matter what, so why worry?