Quote Originally Posted by Lee L View Post
It depends on the approach you're taking. If I tell you that film A needs three stops of compensation at 30 seconds exposure you can do one of two things. You can open up the aperture three f-stops and you're fine. But if you try to double the exposure three times, you're going to get compound reciprocity failure because at the adjusted exposure of 240 seconds, you've got additional reciprocity to deal with, not just the adjustment for 30 seconds. You can calculate that adjustment with one of several formulae for adjusting for reciprocity with exposure time (as opposed to adjusting with lens aperture).
That is not correct. Reciprocity failure is due to the film responding poorly to low luminous flux, i.e. the loss of sensitivity is a function of flux not exposure time therefore the correction factors listed by manufacturers are a time-extension factor.

Say the table that comes with the film says you need a 1-stop correction, you double the exposure time. If you were to open the aperture by one stop instead, you would double the flux hitting the film and thereby improve the reciprocity performance, which would result in a more-than-one-stop increase in exposure, and an over-exposure (fine for negatives, bad if you're shooting chromes at night).

There's a further wrinkle in that reciprocity failure increases contrast, therefore increasing the exposure isn't a complete fix. I believe the manufacturers tables aim to maintain the mid-tones, but it's possible that that's not universally true.

Anyway, the easiest solution is to buy a film with excellent reciprocity performance like Acros, TMY2 or (for colour) Provia; they require almost no correction at all and therefore have very little contrast expansion, even for exposures of half an hour.

And for metering, the best solution is to use a DSLR set to a very high ISO setting e.g. 3200 and wide-open (about f/2). Even if the DSLR's meter is not accurate in the dark, the sensor is itself a very powerful and sensitive light-meter, particularly if you learn to read the histogram.

Say for example you get a well-exposed image (according to histogram) at ISO1600, f/2.8, 2.5s and you want to shoot your film at ISO100 (Acros) and f/16. That means you need +4 stops for the ISO change and +5 stops for the aperture change, so +9 stops from 2.5s is 1280s. At that point you'd add about 1/2-2/3 stop of reciprocity correction for Acros, so about 2000s (30-35 minutes) total exposure depending on how bored/fidgety/cold you get.

If you use a film with poor reciprocity performance, you can expose it all night and still get less image than the Acros or TMY2 collected in half an hour.