how do i work out what lens length to use in blender when compositing 3d onto an image when i don’t know what lens was used to take the photograph?
try to place a cube or a plane in the scene, and match something in the photo, by the hand. :spin:
Last time I tried hand-matching (with the use of the BLenses script) a short video sequence that didn’t work at all.
I could match part of the sequence (with 2 reference cubes vs. 2 buildings) nearly perfect, but as soon as more geometry appeared in the video (i.e. the camera made a bigger turn to show another part of the scene) they just didn’t match the lens. Aapting the lens to match that would just screw it up for everything. Maybe there was too much distortion, but I gave up after wasting too much time with this 
It certainly can be done, but only with a lot of work and patience.
On a related note: The development on the libmv [1] project seems to still be going - lets hope we’ll finaly see match moving in Blender at some point in the future 
Werner
PS: Also the Camera Matcher script (aka. “object2background” script - don’t worry about the german forum entries, simply download the last .zip file in that thread ;)) might be of help if you already have a solution for the lens/camera.
[1] https://code.launchpad.net/~mierle/libmv/libmv2
https://code.launchpad.net/libmv
http://wiki.blender.org/index.php/BlenderDev/ReconstructionAndMotionTracking