Today's Science Lesson: Reciprocity Failure
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So, silver halide crystals tend lose their sensitivity the longer they're exposed to light. This means that after a certain threshold, stops are no longer measured in halves and doubles.
Humanspeak: Under normal conditions, a shutter speed of 1/100 is one stop more light than 1/200.
But with film, this gets tricky with longer shutters. A shutter speed of 1/2 a second is not necessarily one stop more light than 1/4. Because at half a second, the silver halide crystals start to lose sensitivity. So you need to account for this "reciprocity failure."
This may seem like a drawback with film, but not if you know how to use it. Here's about a 3-hour exposure that Obie Oberholzer took in South Africa—it's not an image you could easily take with a digital camera unless you had some ND filters to simulate the reciprocity failure. (Fun fact: he set up his camera, then drove this road himself to create the light streaks from his van. You can see his van in the shot because he was also drunk at the time and parked for awhile to sober up.)
Anyway, I took some photos with a Holga at night to see if I could get some decent images. Because why the fuck not.
As previously discussed, Holgas are awful. Their aperture is f/??? (you have to do some testing to get a sense of what it actually is as there's no quality control), you have to tape the sides in the right places to prevent light leaks, and the focusing is also ??? and requires a lot of testing.
I used Ilford HP5 film for these photos, which has a reciprocity factor of 1.31, which kicks in after a shutter speed of half a second.
So, if you know, based on your spot meter readings, that to get the EV you want for your shot you need a 10-second long shutter, you need to raise 10 to the power of 1.31 instead. This comes out to roughly 20 seconds.
There's no way to know if you got it right, and medium format film currently costs about $3 per photo. So it sucks when you fuck up an entire roll.
Turns out, I didn't fuck up the roll.
This was a 26-second exposure.
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Very cool pic!!
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“Don’t make me repeat myself”
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@jon-nyc said in Today's Science Lesson: Reciprocity Failure:
I’m going to do my own research, thank you very much.
You should you lazy bastid.
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@mark said in Today's Science Lesson: Reciprocity Failure:
Love this. Very cool photo.
I haven't heard that term in many moons.
Wanna know something cool? Kodak brought ektachrome back a year or two ago, and it has a LONG time for reciprocity to kick in: 10 seconds. It's a freaking slide film. How?!
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@taiwan_girl said in Today's Science Lesson: Reciprocity Failure:
Very cool pic!!
Thanks, TG. I took it on top of a parking garage, adjacent to a casino. Only it had these massive lights in the corners of the top deck so the entire damn town could see what I was up to. Including some casino rent-a-cops who walk around the parking lots. They started coming up the stairs as soon as I started the exposure. They asked me to leave but I was already packed up by the time they caught up to me.
