a focusing laser beam into the rear of the objective will produce an
output of minimal divergence when the focus is some distance past the
flange. Sometimes the only way to find out is to do this test and measure!
+
+ A specific method for measuring the position of the back focal plane
+ follows. Place a tube lens in line with the objective lens and shine
+ collimated light into the tube lens from where the camera would normally
+ go. Adjust their relative axial position until the divergence of the
+ light out of the objective (e.g. spot size on a far surface) is minimized.
+ The tube lens is now focused at the back focal plane; if you are
+ telecentrically aligning these lenses you are done. If you need to know
+ the location of the back focal plane relative to the objective, you can
+ remove the objective and determine where the beam is focused, for
+ instance using the visual speckle technique demonstrated at
+ https://www.youtube.com/watch?v=h5y1h_DWeEM.