To determine type and plate length please follow these short guidelines
Determine the number of screws you need. If your lens foot has more than one mounting hole, it is recommended that more than one hole be used to mount the plate in order to keep the plate from twisting on the foot. If your lens foot has only one mounting hole, see the anti-twist section below.
If your lens has only one hole, you can prevent the plate from twisting on the lens foot by positioning the plate ridge on the back of the plate, against the back of the lens foot. If you need or prefer to have the plate extended behind the lens foot, you can set the plate ridge against the front of the foot. You also have the option of purchasing the plate with a removable ridge. In that case the ridge can be mounted in various positions along the plate.
To determine the length of the plate needed for your lens please measure your lens foot. In normal circumstances that is the length of the plate you will need. If you are using optional flash brackets and you wish to mount it on the lens plate you may prefer to add an additional 3/4 of the inch to lens plate. Also you may opt for one that is a bit longer than your original lens foot, especially if you are using a gimbal head where you have to move the camera and lens backwards and forwards to get optimal balance.
If your lens foot has a 3/8” mounting hole (very rare) install a reducer bushing into this larger mounting hole. This allows use of the 1/4” plate fastener in the larger 3/8” hole. You can purchase reducer bushings in the miscellaneous section if you have a 3/8” hole, the center slot is not wide enough to use 3/8” screw.