Hello again Marx,
I know Python, haven't touched C for so long I wouldn't care to now - life's too short! 🙂
I'm very interested in the GPS module! (depending on cost, as ever).
Regarding detecting shutter press, it looks like Canon is too clever by half. It seems to check whether a valid flashgun in fitted before it enables the contacts. However, there may be scope for me to use Gphoto2 or something similar which detects an event.
Actually, I'm not that bothered about declination. Just to be able to know the general heading would be huge bonus when using map software to display where the picture was taken.
Many thanks for your help! I'd be happy for you to post about it if I manage to make it work!