
In a nutshell, this comes down to quality of equipment and process, to provide results which are reliable for assessors and decision-makers. The guidance (currently in draft) is very much leaning towards use of full-frame DSLRs with a 50mm focal length lens. This arrangement provides consistent, reliable results, with the minimum of optical distortion. Obviously, there is a bit more to it than that...

Explaining some of the different camera sensor sizes
Thanks to Ben Gurney for attending and taking the great photo :)