The below post is beyond über-geeky, and was meant to be an entry in our Triggertrap Mobile FAQ. However, it turns out I have quite a lot of things to say about the topic, so I'm posting it here as a separate post as well. It's worth a read if you're getting unexpected results with our long-exposure modes, as it can help explain what is going on.
Q: I am getting unexpected results using HDR and Bulb Ramping. What's going on?
Some users are experiencing some curious behaviour when they are using our long-exposure modes; this includes all modes where you have to set your camera to 'bulb' mode to make them work: Long exposures in the Cable Release mode, Long exposure HDR, Bulb ramping, Star Trail mode, etc.
To understand why this happens, I have to make a short detour and explain how our long exposure triggering works. Since we are using the remote control port to trigger the camera, we cannot influence the shutter speed directly. In other words, we do not tell the camera, "Ok, for the next exposure, we want you to take a 2.4 second exposure... Go!". Instead, we send a long shutter signal. When the camera receives it, it opens the shutter, and when we stop sending the shutter signal, the camera ends the exposure.
Now, in theory, this means that if we send a shutter signal that lasts 1.6 milliseconds (that's 1/600th of a second), the camera should open the shutter, and close it again, to give us a 1/600th of a second exposure.
Unfortunately, due to various shutter electronics, and the way the camera processes the signal from the remote control port it doesn't work like this. Instead, it appears that as soon as the camera receives the remote control signal, it opens the shutter. Only later does it start listening for the end of the signal.
In practice, this means that different camera models have different minimum exposures. Many entry-level cameras, for example, operate with a 1-second minimum exposure in bulb mode (test it yourself: Without plugging in your Triggertrap, set your camera to Bulb, and press the shutter button as briefly as you can. If the shutter goes for significantly longer, your camera probably has a 1-second minimum). However, even cameras without a 1-second minimum exposure tend to have a minimum exposure. The Triggertrap Canon EOS 5D mk3 for example, appears to be unable to take exposures shorter than 1/10th of a second or so in Bulb mode (that's 100 milliseconds), even if we send a 1-millisecond shutter signal.
In summary, one way your HDR or Bulb Ramping photos can come out in unexpected ways, is if your camera decides to take longer exposure, even if we send it the correct exposure duration. Unfortunately, there's no cheap and easy way around this; it's a limitation in the way the camera's remote control port and Bulb mode work.
If we work around the minimum exposure problem above by sending signals longer than one second, you would assume that the functionality is something as follows: If we send a 3.8 second exposure followed by a 4.1 second exposure, and then a 4.5 second exposure, you would expect that you end up with three different exposures. For most camera models, this is exactly what happens.
However, a very small minority of camera models, do exposure rounding when you are working in bulb mode (we are aware of some cameras from Nikon who do this). In practice, this means that it appears that certain camera models, instead of ending the exposure as soon as we stop sending the exposure signal, it continues the exposure, rounding up to the nearest 'step'.
In practice the 'steps' in question are often the nearest 1/125 of a second, but if we for the sake of example imagine a half-second step: In the above example, instead of 3.8, 4.1 and 4.5 seconds, this camera will round up to 4.0, 4.5 and 5.0 seconds. This is annoying, but at least you're getting three different exposures, and it may be difficult to trouble-shoot what is going on. If we instead wanted to take 3.1, 3.2, and 3.3 second exposures, it turns out that this camera would take three identical exposures of 3.5 seconds each.
Obviously, if your camera is rounding off the exposures to the nearest step, you're going to get very odd results when you are trying to do Bulb Ramping or HDR. Unfortunately, I don't know why Nikon started using exposure rounding on their Bulb modes, and it is surprisingly difficult to find out which cameras are affected by this. If you are getting strange results when you are trying to use our long exposure modes, it could be that you are one of the people affected.
What can I do?
Unfortunately, there's not a lot we can do about this on the hardware side of things, as it is entirely out of our hands. Sadly, Nikon isn't returning our phone calls where we are telling them how to create their cameras.
However, this is not an unknown problem, and some clever software geeks have been able to create Lightroom adjustments (based on Raw files) that can do the exposure compensation in software instead. As a good starting point, check out the Nikon Bulb Ramping Measuring Test thread on the excellent Timescapes blog.