Time Lapse

Interval Exposure

April 2011

This is the set-up I use. My camera (an elderly Olympus compact) has a built-in intervalometer, so no extra hardware is required. The camera must be tripod mounted and on manual focus. Because of its age it uses an obsolete memory card with only a small capacity. Consequently, I set the image quality relatively low so I can record enough images. Fully charged batteries (or a mains power adapter) are also a must.

The interval period is determined by the length of time the event takes, how many images required to record it and the finished video's running time.

When sufficient images have been recorded, shut the camera and intervalometer down, remove its memory card, plug it into the memory card reader and copy the images across to a suitable folder on the PC. I now have a choice: if I intend to use all the images recorded and want the video to run chronologically then I will go straight to Virtualdub. However, if some of the images need to be deleted or I want to run the sequence in reverse order, I will use PhotoLapse first.

PhotoLapse gives the option of omitting images if needs be and of reversing the sequence, which opens up many creative opportunities. The frame rate and codec can also be selected. I have used frame rates as low as 5fps but it is probably best to stay with 25fps as per UK standard. Choice of codec depends on how the final video will be used. If further editing on Virtualdub is anticipated or the sequence is to be part of a DVD presentation then I use "Uncompressed". If the sequence is destined for use on a PC or website then one of the higher compression codecs may be more appropriate. I often use "Xvid" but this is not well supported on older machines. Be aware that generally speaking, the higher the compression, the poorer the picture quality.

Virtualdub is a free video editor. Not perhaps the most flexible or the easiest to use but, once its idiosyncrasies have been mastered, more than up to the current task. I do not propose to give an in-depth tutorial on its use, there are whole websites out there for that. For it to compile a video from the image sequence it is only necessary to open the first image. For this to work, the numbered images must be in an unbroken sequence. If this is not the case then either use another tool such as PhotoLapse or use Virtualdub to compile separate videos and then use its "Append AVI" facility to link them together. If required, a sound track can be added. An audio tool such as Audacity can be used to create elaborate music and commentary files that can be used by Virtualdub. Please note that there are considerable copyright issues with music intended for commercial or public use. Again, the use of the final video decides the choice of codec. My comments above apply here as well. If its destined for DVD then "Uncompressed" is a good choice, otherwise use "Xvid" or a more widely supported codec. Once the video and audio codecs have been selected, and any editing completed, the final video can be created. Virtualdub can only create AVI video files. You may want to use another video converter to obtain other formats, particularly if it is destined for a website.

The Calculations

Assuming a frame is recorded every 30secs for 5 hours. This gives 5x60x60/30 = 600 frames.
Compiling the sequence into a video of 25 frames/sec (fps) gives 600/25 = 24 secs running time.
The time dilation factor (tdf), that is, the original duration of the event divided by the video running length of: 5 hours/24 secs = 750.

Generallising, if a frame is recorded every R seconds for D hours then there will be:
60x60xD/R frames.
Compiling these at T fps gives a running time of:
60x60xD/RxT seconds, and a tdf of:
60x60xDxRxT/60x60xD which simplifies to tdf = RxT

Block diagram of interval timelapse video creation