Back Focus

[B. Sakthidoss]

What is the relation between Back Focus and Flange Focus and Focul Length and Focul Plane ?

[Nick Mulder]

First off can you please outline your particular knowledge of each of these concepts and what particular aspect of it that is important for you to understand - ie. for what reason ...

 

Its such a big open ended question not many people will give you the time if a. they think you are being lazy or b. have no direction in which to go with the enclopedia of possible replies ...

 

;)

[Adam Frisch FSF]

Back Focus and Flange Focal Distance is for all intents and purposes the same thing. It's simply the distance from the lens seat (or any reference point on the lens and mount) to the image capturing device. This is a critical measurement, since the focusing scale of the lens depends on it completely.

 

Back Focus has become something associated with digital cameras where it has to be checked regularly. Film cameras also need this checked, but less often. The reason is simple: video cameras are often constructed poorly (lots of plastics) when it comes to hardware and the mounts are not solid enough. They're also more sensitive to heat, which means the mounts in relation to the CCD will move a lot more, which screws up BF. If they built them correctly, you wouldn't have to check BF constantly.

 

Mind you - Flange Focal and Back Focus are all part of a relative chain: ANY lens ever made will focus, or defocus, as you move it forward or backwards (anamorphic lenses being the only exception). I.e. you could have a lens with no focus ring at all, but a mount that moved forwards and backwards instead and achieve the same thing. This is how projectors or many medium and big format stills cameras work. So the main purpose of the FF or BF-setting is to line the lens up so that the focusing ring marks on the lens correspond with reality.

[chuck colburn]

What Adam said.

And keep in mind that a zoom lens has a front AND back focus adjustment that must be set up right or the lens won't hold focus as you zoom.

[Michael Nash]

Moderator's note ("Cinematographers" forum) -- I removed a post that said "this is posted in the wrong place," before I moved it here. So if you don't see your post, that's why! Nothing else was said in that post...