I'd done some long exposure photography on digital recently, here and here, and had lots of fun doing it. The intent had always been to try it on film, and I'm finally getting around to it.
Even doing this digitally has its challenges. In most digital cameras the light meter looks through the lens, and if you've screwed on an ND filter, what it's seeing is pretty dark. Running the shutter speed down to the 30 second maximum might not show you anything on the back of the camera. It might be impossible for the camera to achieve focus, meaning either focus manually, or set the focus before screwing on the ND filter and hope nothing moves. The usual route is to guess at the camera settings and try it. Look at the results, tweak as necessary to achieve your artistic vision for the scene. This might take a while but it's good practice. Just make sure you know which way the tide is going.
Film is more difficult because the try till you get it right approach isn't going to work. One solution is to figure it out with a digital camera first, then translate the results to the film camera. Except, just because you're using ISO 100 film, for example, doesn't mean setting your digital camera to ISO 100 gets the same sensitivity result. An f stop setting on the digital camera might not mean the same thing as the f stop on a different lens on a film camera, and almost certainly won't if it's a medium format film camera.
Except I found a formula. Shutter speed times 2 to the ND filter number power. There. All clear? Good to go? Maybe not.
So, assuming I'm using my GW690, which has no light meter. I use my trusty phone app to discover that for a scene, the settings are 1/125 second at f16, which is not a surprising setting for a normal exposure of a subject on a sunny day. To do a long exposure of that scene I screw on an ND1000 filter, which is a 10 stop filter. Don't ask me why it's called 1000. A whole lot less light gets through the filter to the film, unless I leave the shutter open longer. How much longer? 1/125 times 2 to the 10th, which is 8.2 seconds. The point 2 doesn't matter. I'd probably do 9 seconds, maybe 10, because film tends to deal with over exposure better than under exposure.
Now let's pretend that after experimenting with the digital camera, you know you aren't getting the effect you want, and you need 16 to 20 seconds of exposure. That's fine. Change to f22 to halve the amount of light coming into the lens, and leave the shutter open twice as long, 16 seconds in this case.
Except, in another film complication, there's a thing called reciprocity failure. For most normal exposure settings on a camera, the shutter speed and aperture f stop are related, in terms of how much light goes through the lens. A small opening for a longer time, can be the same as a bigger opening for a shorter time. (Yes, for the pedants out there I'm ignoring depth of field issues.) Double one, half the other, and it's the same amount of light, for most practical purposes. Except for film, when the light starts getting dim.
Film needs a certain amount of light for the chemical reaction in the film to work. This varies with different film stocks, but after a second or so of exposure, (which would typically mean a night time exposure, or an ND filter exposure, or some funky intentional camera movement photo) this needs to be taken into account. Going from a 2 second to a 4 second exposure might not be twice the amount of light, you might need 6 seconds to give the film enough light to react.
In case you missed it, long exposure photography works best if there's something that moves, typically water or clouds, and something that's fixed, like a pier or a building. I wanted to get out and try it, and had a spot in mind. The only hitch in the plans was that it was clear blue sky. I got involved in something else for a while, and headed out when it started to cloud over. I was thinking some cloud motion with a blue sky background would be good.
Except I left it too long. By the time I got there, got set up, tested the exposure and composition with the digital camera, it had pretty well clouded over and was getting windy and cold. I was glad I wore the parka. At this point I didn't know the formula, and used the digital settings plus a bit of by guess and by golly, knowing I could do some compensation during the digitization process. The film is Acros II ISO 100, which doesn't especially have a reciprocity failure issue, which does simplify things.
As a minor technical note, there's two ways of the shutter staying open. One is Bulb mode, where it stays open as long as you hold the button down, which had better mean you're using a cable release or else you're moving the camera and destroying your image. The other is T mode, meaning push the button and it stays open till you change the shutter speed. Which is what my GW690 has. The trick is to cover the lens with a hat, then reach under it to change the shutter. AND THEN DON'T FORGET TO CHANGE IT BACK FOR THE NEXT EXPOSURE. Learned that one the hard way.
1. Here's what the scene looks like with a normal exposure. This is the Bow River near the Ivor Strong bridge. Not going to win any awards with this, but that's not the point.
2. This is actually a minute long exposure at f22. I wanted to smooth the water out as much as possible. See how much better the reflections show up in the water? It would be a much more interesting photo if the clouds had cooperated.
3. Under the bridge was much darker, and with some math in my head figured I needed 4 times as much light, so went for a 4 minute exposure. Neither of these images needed exposure tweaking in NLP or Lightroom so I guessed about right.
4. Then over to Mallard Point to finish the roll. By now it was getting colder and darker, so I hand held the digital for 30 seconds just to see what the light would be like, and figured the film would need 90 seconds. This looks a bit dark, and yet there's no detail in the snow. Maybe if I wanted to be really fancy I could mask it in Lightroom and play with it a bit, but the odds of anything good coming of it are small. Notice you can't see the birds that flew through the scene.
I'm thinking about other places to try long exposures on film.