Re: Closeup lenses

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Eric G. writes:
> John B wrote:
>> I use a closeup lens the way you imply: I set the lens at infinity,
>> screw on the closeup lens, and move the camera until the object is
>> in focus.
>
> One benefit to this approach is that you should be able to focus easily
> by measurement (with 50s rangefinder cameras, for instance). A +1 should
> focus at 39 inches, a +2 at 19-1/2 inches, a +3 at 13 inches, a +4 at a
> bit under 10 inches, etc.

Exactly.  But as always, the devil is in the details: from where do we
measure the 39.37 inches?  And that's where the lens combination formula,
which tells us where the combination principal points lie, comes into play.
I've really got to come up with a sensitive test for this thing.  Maybe 
over the holidays I'll have a little time.

John B


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