We take filters, we take all of those, we take our exposures. Now we have to talk about one of the complexity things of actually working with black and white film and all film in general has this problem, but black and white film has this more than other films. And that is reciprocity failure. Reciprocity failure is this really annoying thing in low light. What happens is the silver halide that site within that film actually needs a certain number of photons to hit it, for the silver to activate and cause a slight chemical reaction so that it's able to accept the developer to turn into a silver metal. If the photons don't actually hit the silver, we don't build up enough exposure. Failure of that is reciprocity. So we have to apply a reciprocity correction when we're dealing with low light with film. Each film has a unique reciprocity failure characteristic. Some fail faster and worse than others, other ones hold for awhile and then fall off, but all films as the exposure gets longer a...
nd longer will suffer from reciprocity failure. Interestingly enough they also fail on the upper end, above about 10,000th of a second, 20,000th of a second they experience reciprocity on the upper end but I don't know anybody with a ten thousandth or twenty thousandth per second film, so I never worry about that one. So we just worry about the bottom end. The other piece about reciprocity failure is it has to be the last thing you account for in figuring out your exposure because you determine reciprocity off of the exposure value. So if your exposure on Tri-X is two seconds, because of reciprocity failure you actually need three seconds. If you then apply your filter factors, you're going to be off in the reciprocity failure. So reciprocity failure is always last. I'm explaining that first because we're going to talk about how to calculate reciprocity in a second. I want to make sure you know that's why it's always last because we need to build the total exposure to determine the reciprocity failure. So because of that failure though, what we need to do is we meter and then the meter says I need ten seconds of exposure. We look in our reciprocity charts for the films and it says oh if you meter ten seconds, to actually get ten seconds, you need to expose for 25 seconds. So 25 seconds will give you ten seconds. Now a film like Fuji Acros doesn't suffer reciprocity failure until about 80 seconds. So that's a great film. It's a 100 speed film that rates at about 50. Tri-X at 400 usually rates at around 200. So it's three stops difference there, but if you're shooting at night with the reciprocity failure, you end up being about a wash for exposure on the slower film. So depending on what you do, like I said you look at that data sheet at the very beginning you got to pull all of those variables. But what's going to happen is we're going to figure out the meter, get the time, we add in our filter factor. So my meter says ten seconds, cool, I put a tri red filter on. 20 seconds, 40 seconds, 80 seconds. So my base exposure at that point is 80 seconds. Now I look at my reciprocity failure chart. And my reciprocity failure chart says oh for 80 seconds, that's actually three minutes and 20 seconds. What the reciprocity chart says three minutes and 20 seconds gives you 80 seconds. Now in the bonus material I've given you the reciprocity charts for Tri-X, HP5, and T-Max films. From basically two seconds out, it ends up being about five and a half hours, it goes to 30 minutes. 30 minutes on Tri-X is four and a half hours. Yeah. Meanwhile, if somebody's kicked your tripod, breathed on your camera lens, done anything oh everything's blurry. Hate reciprocity failure, like you want to anchor that in the ground, don't move, nobody touch it. So that exposure though has to be the piece of that and reciprocity failure comes into play, and what happens to people that don't calculate it is they come back and are like wow, I don't know what happened, everything is under-exposed. Reciprocity failure didn't come in. Each film's unique. So I would love to be able to stand up here and be like, oh don't worry about it. Two seconds goes to three, three goes to five, five goes to eight, 10 goes to 22. Every film's different. Color film, interestingly enough, is little bit like black and white film, it's harder to over expose, but there's reciprocity failure with color film as well. It's actually less of an issue, but there is still some there. So like a provious light film, it's about a 1/ of a stop for 20/30 seconds, portrait after about 30/40 seconds, you apply about a third of a stop. So there's always some level of reciprocity failure we have to calculate for. We just really need to get in there and look at that.