HDR Timelapses with the Triggertrap V1

Please note: The following article refers to our first product, Triggertrap V1 and not our current product, Triggertrap Mobile.

I've been getting a few questions from people who want to do HDR timelapses with their cameras. You're in luck, ladies and gentlemen, it is possible, and there are two different ways of doing it. One requires a simple 1-line change in the Triggertrap code, another requires a more extensive amount of hacking. The easy way would be to change the amount of time we are sending the 'trigger' pulse. At default, the Triggertrap V1 sends the shutter pulse for 150 milliseconds. This is enough for your camera to take a photo, but what if you want a set of three timelapse photos?

Well, try this:

The Easy Way to do HDR timelapse bracketing with the Triggertrap

  1. Set your camera to automatic exposure bracketing
  2. Set your camera to continuous firing (as opposed to single shot)
  3. Press and hold down the shutter button

If your camera takes three photos and then stops (that's the normal behaviour for all Canon cameras and most Sony cameras, I'm not sure about the rest of the brands), then you're set.

In the Triggertrap V1 Source Code, find the timelapse.cpp file.

Search for this line of code:

shutterPulseTime_ = DEFAULT_SHUTTER_TIME;

Now, as we mentioned, the default shutter pulse is 150 milliseconds, but we need this to be long enough that your camera will trigger three shots in rapid succession. To find out how this would be, calculate the duration of the exposures, and add about 10 percent as a safety margin (so if you shoot with 1 stop bracketing, and your middle exposure is 1 second, your lower will be 0.5 and your higher will be 2 seconds, the time you want is (1+0.5+2)*110% = 3.85 = 3850 milliseconds. Change the line mentioned above to:

shutterPulseTime_ = 3850;

and upload the new code to your Triggertrap. Every shot of the timelapse sequence, your camera will take a 3-bracket HDR set.

The reason why this version is so simple is that it puts most of the timelapse / bracketing burden on the camera; it still controls the exposures etc, but the Triggertrap V1 simply sends a longer 'shutter' signal to the camera, to enable it to complete a set of brackets for each timelapse cycle. It's a very easy hack that anyone can do.

The more advanced HDR timelapse mode

If you want more than three shots, or if you want the Triggertrap to set the exposure time for each of your shots, you're going to have to do a bit more code surgery on your Triggertrap; You may have to code in an extra mode to the Triggertrap. I'm working on one in my spare time, but the problem is that I don't have a lot of spare time, so I'm hoping that as soon as we release the code for the Triggertrap V1s, that someone who wants this functionality might step up and code up the HDR Timelapse mode.

The version I was working on adds three new settings to the menu:

  • Delay (same as Timelapse mode: Delay before first photo)
  • Interval (same as Timelapse mode: Interval between each photo, although this is now 'between each photo set')
  • #shots (same as Timelapse mode, but now 'number of sets' instead of 'number of shots'
  • Middle exposure (dial in the exposure time for the centre exposure of the photo set)
  • Brackets (either 3,5,7,9 or 13 shots per set)
  • EV (how much of a variance to add to each bracket, adjustable by 1/3 EV steps from EV 1/3  to EV 3)