Would you believe 30 is really 32?
For some reason Nikon (and others) seem to have not used the right doubling numbers to deal with long exposures. Double 1 second is 2, 2 to 4, 4 to 8, 8 to 16, and 16 begets 32.
Even that's not quite precisely correct. As I point out in my books, a "15 second exposure" on a Nikon DSLR is nominally 16.5 seconds for some reason. A 30 second exposure works out to something over 32 but not 33 for yet more unclear reasons.
But the point is this: if you set the "interval" to 30 seconds, that means 30 seconds between the start of each shot. But if the 30-second shot takes 32 seconds, you're going to lose shots. Moreover, if you've got Long Exp. NR turned On, a 30-second exposure can take as long as 65 seconds on some Nikon bodies (or as little as 45 on others). Ugh.
So if you're trying to do a sequence of long exposures, for example to create star trails, what should you do? Skip the Intervalometer.
Here are the steps:
- Turn Off Long Exp. NR.
- Take a 30 second shot with the lens cap on (this will be your noise reduction shot later).
- Set the camera to Continuous shooting mode.
- Set your exposure manually, choosing 30 seconds for the shutter speed.
- Using a locking remote, hold down the shutter release and lock it.
- When you're done, unlock the remote and the camera will stop firing.
- Batch process your sequence of images with the noise reference you took in Step 1.