Re: closeup lens focus

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Eric G. wrote:
 
> Come to think of it, I used to do a lot of panoramas through
> compositing, but did have difficulty finding the right spot to pivot
> around with SLRs. As most of my close focus work is at +1 or +2, I guess
> I'm still working with enough DOF to cover the difference between the
> film plane and the actual RNP.
>
> How about retrofocus wide lenses, John, like Canon 24, 28 or 35 mms?
 
Well, it's the same thing - the spreadsheet still applies.  The front 
principal point will be a bit deeper into the lens but otherwise things 
are the same.  BTW, I'm not convinced that the front principal point is 
exactly where you want your pivot, although I know conventional wisdom 
says it is.  It may instead be that the pivot should be at the center of 
perspective which is the  entrance pupil of the lens.  Of course the 
entrance pupil and front principal point are located at about the same 
place - in fact at exactly the same place in the case of a symmetrical 
lens.  Anyhow, it's something I'm thinking on.
 
If you want to know how to find the front principal point of a lens, it's 
easy.  Let's take your 35 mm focal length lens.  Focus light from a 
distant object onto a piece of paper but have the _back_ of the lens 
pointed at the object.  The piece of paper is now located at the front 
focus.  Now head into the lens from the piece of paper a distance of 35 mm 
(in this particular case) and that's where the front principal point lies.
 
John B


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