Quote Originally Posted by ted_smith View Post
Holy smokes! Several hours?! I've heard and understand to some extent the reciprocity failure rule for 30 seconds+, but how do you work out the exposure for scenes requiring exposure of several hours?
If you've an iPhone there are some reciprocity calculators available. I have one called called film timer.

My iPhone is fast becoming my most indispensable tool in my photo bag.
Light meter, Depth of field calculator, golden hour calculator, posing app.
Not to mention the massive dev chart app which is brilliant as well.