Backfocus measurement question

[Swoop1]

When measuring to achieve back focus, where are the measurments take from and to?

 I am working out what spacer I may need to get the 55mm for my RVO Horizon 72ED field flattener and Sony a6300 combo.

Thanks,

Matt

[Elp]

From the last lens at the bottom of the connecting thread, to your sensor. Basically the total thickness of your extension rings and adaptors where they come to a physical stop should be the length of the backspacing taking into account (including) the front of your camera to sensor distance (Sony fe, likely e mount also is 18mm). It helps to have a Vernier caliper at hand if you need to go sub mm precision with delrin or equivalent fine spacing rings.

If you've got filters in the system, you typically add 1mm (1/3 of filter thickness) to any backfocus distance but it depends on the filter glass thickness.

Edited January 13 by Elp