Wednesday 28 May 2008

When I publish my dive photographs, I like to include an approximation of the depth at which the photographs were taken. I can do this because the camera records the time a photograph was taken and my dive watch and dive computer log their depth at 5 second intervals. Combining this information allows me to determine the depth.

Before each dive, I take a photograph of both my dive computer (Citzen Cyber Aqualand Nx) and my older dive watch (Citizen Hyper Aqualand). This allows me to synchronise the watches with the camera because the I can apply the difference in the time shown on the watch with the time the photograph was taken (according to the camera) to all photographs taken (within a few days). For example, in the photograph below, the dive watch shows 5:48:26 pm. The EXIF indicates a time of 17:48:14. This means the camera is 12 seconds behind the watch and so 12 seconds needs to be added to the time of each photograph so that it matches the watch time.

A photograph of my dive watch (left) and dive computer (right) taken before a dive for synchronisation.

I wear the dive computer on my wrist during the dive and I mount the dive watch on my camera rig to help hold the strobe sync cords in place as well as being close to the camera to give me fairly accurate depth readings.

After the dive I upload the dive data from both the dive watch and dive computer to my personal computer for my dive logs. The uploaded data can then be used for the depth determination. I normally use the data from the dive watch as it is mounted on the camera. If for some reason the watch didn't record the data, I have the dive computer as a backup.

I log my dives with a home grown Lotus Notes database. Within each dive log I record the following information:

1. Start and end times of the dive (to the seconds) as recorded by the watch I am using.
2. A pointer to the file which stores the depth data from the watch.
3. The time on the watch in the photograph.
4. The time the photograph of the watch was recorded as per my camera.

After loading the depth data from the file (into a hidden field), I am able to determine the depth of a photograph taken on the dive simply by clicking a button and entering the time (hh:mm:ss) that the photograph was taken. The code behind the button takes the time of the photograph entered, adds the difference between 3. and 4. above, determines the number of seconds from the start time in 1. above, divides by 5 and uses the result as an offset into the table of depths loaded from 2. above.

This system works very well, letting me get an accurate approximation for the depth of my photographs.