T3D Re: Where?

[Tom Deering <tmd@xxxxxxxxxxx>]

On 10/1/98, john bercovitz wrote:
>> 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! {;>)

>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.



In order to explain MAOFD to non-technical people, I've looked for an easy
to understand simplification.  How far off would I be if I said "measure
from the iris"?  I also like idea of measuring from the body, although I've
not analyzed the numbers.  I'm looking for the best approximation.

When I measure the cameras I own, the MAOFD error introduced by the "iris"
simplification seems minimal.

(I'm trying to dream up something simple to explain how to use my MAOFD
spreadsheet.  Only 1 stereo photographer in 100 is going to bother shining
light backwards through his lens in order to use my spreadsheet.  I'd like
to reach the other 99.)




------------------------------