Re: Closeup lens where focus from? (TECH3D digest 60)

[T3D Bob Howard <bobh@xxxxxxxxxxxxx>]

RE: Eric says there great as you just measure the diopter distance 
(reciprocal meter=power), but John B. says "devil in details" where 
do you measure from? He says he will look into calculating nodal 
point?
In the old days, one measured from the closeup lens..or if trying to 
be more accurate from about where the diaphragm was located or some 
other "center" of the lens. 
When SLR came in with bulky auto-diaphragm lenses and helical mounts, 
the glass in the lens was hard to find....SO, most SLRs distance 
scales are calculated to measure from the FILM PLANE not the lens, 
which is usually marked on the top deck near the back edge..sometimes 
a circle bisected by a line. This of course confused the above where 
do you measure a closeup lens from...e.g. meausre from the focal 
plane index line and then subtract the body/lens thickness? It is 
easier to go back to the diaphragm..and leave the index mark for the 
footage markings on the lens..e.g. 18 inches minimum on most slr 50mm 
lenses. BobH


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