T3D Re: Where?

[john bercovitz <bercov@xxxxxxxxxxx>]

> O.K., I'm STILL confused about where to take my base measurement 
> (i.e. the camera) from...
[...]
> Be gentle; I've had a rough week! {;>)

Always; but thanks for the warning.  ;-)  8-)

I hope I'm not presuming too much when I presume you mean for the
MAOFD spreadsheet.  If this is the case, you want to measure from
the front principal point unless you have a highly asymmetrical
lens in which case you don't want to deal with it, believe me.  (See
the perspective paper Tom Deering put up on his web site for details
if you're a masochist.)

How to find the front principal point?  Simple, really.  Just take 
the lens off the camera and run light from a distant object through
it _backwards_.  Where the light from that object focusses is called,
somewhat unimaginatively, the "front focus".  Note where this point 
is relative to the body of the lens.  It will often be right near 
the threaded part of the lens which accepts the filter.  Now all you
have to remember is that the front principal point is located one 
focal length deeper into the lens.  As Bob H says, the front principal
point will be about where the camera's lens joins the camera body.
In other words, at the front of the body.

I hope I'm not being too tedious (though I usually am) but here's an 
example.  You run light through the lens backwards and you find it 
focusses 3 mm (1/8") off the front of the lens.  It's a 50 mm lens
so you measure from that point 50 mm back into the lens.  In other
words, you go 47 mm (1 7/8") into the lens from its front ring.  And
there lies your first (aka "primary") principal point.  That should 
also appear to be where the iris of the lens lies when you're looking
into the front of the lens.  If it does, the lens is symmetrical enough
from a perspective standpoint.

John B


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