Primarily the diameter of the tube. A T-mount has about a 37mm clear internal diameter (42mm x 0.75 threads), a C mount tube is smaller and has a 22mm clear internal diameter (1 inch or 25mm thread size). The diagonal on the 4/3 sensor is 22.5mm so with a C mount you're cutting it pretty close but you should be OK. A C mount is really too narrow for larger sensor cameras. The T-mount diameter would present no vignetting concerns at all with a 4/3 or APS sized sensor (or even full frame 24x36mm in most cases). In the past, T-mounts were more commonly used with SLR cameras and the C-mount was more common with movie and video cameras.What is the difference between using a C-mount and a T-mount with my micro 4/3 GF1?
What objective(s) do you have with the Brock?
