What's with long exposures using the intervalometer?

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: 

  1. Turn Off Long Exp. NR. 
  2. Take a 30 second shot with the lens cap on (this will be your noise reduction shot later). 
  3. Set the camera to Continuous shooting mode. 
  4. Set your exposure manually, choosing 30 seconds for the shutter speed. 
  5. Using a locking remote, hold down the shutter release and lock it. 
  6. When you're done, unlock the remote and the camera will stop firing. 
  7. Batch process your sequence of images with the noise reference you took in Step 1.   
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