Can anyone tell me from where on a large format lens (or any 'thick lens' for that matter) the lens plane is measured from? If I have a 90mm Super Angulon mounted on a lens board, where is the lens plane? ie, from where will it be 90mm (nominally) to the film plane when focussed at infinity? The possible answers I can think of are: -The closest bit of glass to the film -The front of the lens board -The rear of the lens board -Some other point nominated by the lens manufacturer. Note: If I had a 90mm Super Angulon I may be able to work this out. Needless to say, I don't. All help appreciated. John Ford.