T3D Re: Film plane mark on SLRs (was A Dull Double #1)

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Bob H writes to my dull question #1.
> 
> RE: Bruce Springstein's question...the measuring from the film 
> plane is only correct WHEN USING THE DISTANCE MARKS in ft. 
> or meters on  the lens mount. Optical formulae still want the center 
> nodal points which on some lenses can be considered at the 
> diaphragm. BobH

This is useful, because when a recent P3D discussion of how to
implement the Spicer/Bercovitz formulas was under way, Tom Deering
assumed, as I did, that this advice to measure from the film plane was
applicable to the main discussion - how to get the general case math
into *practical* use.
Tom even used this advice in some of his helpful charts, and it seemed
to me to subvert the formula - especially at short subject distances.

My "fallacy" was mainly tongue in cheek - to suggest that this way of
measuring would make using the formula seem a little pointless.  The
article I referred to was of course the one John Bercovitz co-authored
in "Stereoscopy" a couple of years ago, and which appears at Steve
Spicer's web site.  John B's stab at my question seems not quite warm
;-), but his confusion at just what I was getting at is completely
understandable - I'm even more oblique than obtuse sometimes. :-) 
This is partly a side effect of the topic's banishment from the list
he doesn't see to the one he does. Sorry to sneak in that way. 
 
The nub of my "fallacy" is the phrase "would seem to reduce a' to
zero".  Variable a' is whatever the camera and lens determine,
regardless of what fictional lens-to-subject values you put into the
equation.  The problem of applying the film-plane advice to the
generalized base math equation is just one of "lying" about the
number.  Provisional Moral:  Don't measure subject from film plane
when trying to use the Bercovitz/Spicer recipe.

Do I have it right yet?  ;-)

Bruce Springsteen.  
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