The worst trait of a pessimist is his profound belief of him being a realist.

Category: Photography

Timelapse with a dSLR

You all have seen timelapse clips. Shooting your own timelapse with a dSLR can be an easy or а complicated task, depending on your goal, and the attention to detail. Here's something like a checklist of the process:

  1. Choose what you'd be filming (of course, there are endless possibilities here). If shooting outdoors, be sure to use a sturdy and heavy tripod, as the wind tends to make the frame twich in an unpleasant way otherwise.
  2. Framing is doubly important here, so be sure to get it right. It's very frustrating to shoot 500 frames having all the same annoying flaw.
  3. Turn off anything labeled 'auto' on the camera. Manual WB, manual focus, auto image enhancements disabled if you have them, and, if possible, use manual exposure settings. The last one depends on what you're shooting - e.g., in artificial lighting, or when the scene brightness doesn't change a lot, manual is all fine. But, when the lighting shifts dynamically (e.g., sunrise/sunset), you'd be forced to use some of the semi-auto modes. The downside of that is the necessity to postprocess the photos afterwards, as you'd almost surely need to adjust the frames a bit. The problem comes from the fact, that as the scene brightness changes, the camera changes the exposure settings (f-stop, ISO, and shutter time). These are all accessible in ⅓ stop increments, so there's a ⅓ stop inherent quantization present. When the brightness comes out halfway between two such stops, the decision of which of them to choose is more or less random. For example, with fixed aperture and ISO, when the brightness is between the adequate for 1/100s and 1/125s, consecutive frames of the timelapse sequence can be randomly assigned one of the two speeds. This results in brightness flickering in the resulting clip, a very noticeable artefact. To cope with this problem, I've added a Timelapse tool (demo), which does the suitable post-processing.
  4. You have to choose the frame interval. A quick rule of the thumb here would be to choose it in a way that would achieve 400 to 1000 timelapse frames for the expected duration of the shooting. Less than that, and the clip is probably too short (or needs low FPS), and if higher, the clip may be too long and boring. For example, an interval of 1 minute could be suitable for a whole-day timelapse.
  5. The other parameter, which is more difficult to control, is the exposure time, or, more precisely, what percentage of the frame duration would be exposed in the shot. Movie cameras operate by shooting, e.g., 24 frames per second, and each frame could be exposed for 1/50 s, so around 50% of the frame time is exposed. This results in a nice motion blur, as the movement is smoothly "joined" in the consecutive frames. In a daylight timelapse, the sutiation is a bit different: 1/400s (typical) at 15 s frame interval... that's 1/6000 (about 0.017%) of the frame duration, which is exposed. This gives the clip the notorious clay animation look.
    To solve this issue, one should target for intentionally long exposures, so the exposure takes a significant proportion of the time interval, usually by employing a shutter priority mode on the camera. This is easy to do at night or twilight, but in full daylight, such exposures would be usually inaccessible. Here the natural density filters come handy, and in fact, you'd need quite a heavy ND filter (ND256 or greater). These tend to be expensive, so another solution is to stack two polarizing filters: a linear polarizer in the front, and a circular polarized after it. The phase between those two controls the remaining light, so it's kind of an adjustable ND filter. I'm still waiting for my filters to arrive, and I'll share my experiences when I get them.
  6. The actual interval photography is enabled differently, depending on the camera model. A lot of simple and cheap cameras do have this little function built in, but dSLRs usually don't (oh, a pitiful win of marketing over engineering ò_ó). There are a few solutions here. You can install an hacked firmware addon (e.g., CHDK, Magic Lantern). You can also use your vendor's software and command the camera from a laptop, if possible. The third way is to use the shutter relase cable input, with an hardware interval trigger. This is what I do, and I've even designed two such triggers: the photodude, and it's smaller brother, the minidude.
  7. A good bonus would be to add slight movement during the timelapse - a few milimetres per frame, for example. This would invoke the feeling of a ordinary, realisting movement "in the real world" in the resulting clip, but the time would appear to run unnaturally fast, to an astonishing effect. To do this, in terms of hardware, the camera is usually placed on a tripod head, which, in term, is moving along some rails, usually employing stepper motors for the precise tiny steps between the frames. I'm too thinking of building such a system, but as a prototype, I designed something much simpler. Stay tuned for future developments here :)
  8. After the photography, it's time to stich the frames into a movie. As a linux-user, I find the command-line mencoder way to be the easiest:

    mencoder "mf://@/tmp/listfile" -mf fps=25 -o output.avi -ovc lavc -lavcopts vcodec=ffv1

    This would read a file list (e.g., JPEG files) from /tmp/listfile and stich them into an uncompressed .avi. You can then compress it with your favourite program, e.g. HandBrake, and you're all done.

That's it - happy shooting!

Posted in category Photography -- clock 27 May 2012, 04:35, 1 comment




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