Hello, Geo,
There are no restrictions on the focal length of the used lens, as long as the following conditions are met:
1. The distortions of the lens can be well modeled using the Brown's distortion model:
http://en.wikipedia.org/wiki/Radial_distortion2. Cameras are not observed by each other (at least the majority of them).
Manual lens calibration is usually required when focal length information is not available in the EXIF meta data. As PhotoScan assumes 50mm focal length in this case, the alignment procedure can fail or produce wrong results if the real focal length is too different.
If you can send a sample photo set for your lens to support@agisoft.ru, we could investigate the problem more thoroughly.
With best regards,
Dmitry Semyonov
AgiSoft LLC