Chris, on extension tubes, wrote:
> Does the close-focusing ability of the lens affect this? That is, would I
> get the same magnification using a 50mm extension tube on a 100mm macro as
> I would on a regular 100mm? If not, then differences in close-focusing
> ability must affect the magnification ratio.
Yes, even w/ "regular" lenses, there is intrinsic extension
when close-focusing. Notice that the barrel extends as you
focus closer and infinity is "no extension". Besides their
specialized flat-field optics, macro lenses incorporate a
"built-in" helicoid extension tube of sorts. There are moving
group optical tricks on some of the more modern ones, as well.
But these schemes alter the effective focal length, as well.
The lens' own extensibility is why I used the "approximately",
any precise magnification calculations would combine lens
and tube extension measurements. In most non-scientific imaging
it is not necessary to "target" some precise magnification factor.
Creating a consistent series of images of various sized objects
would be the sort of thing that would benefit from calculations
as opposed to trial & error.
Your 100mm macro most likely has ~50mm of "extra" (macro)
extension built into it's barrel to give the more common
1:2 capability or ~100mm if it is a 1:1 capable one. The old
100mm BellowsTakumar had no helicoid at all - it was all
done via the bellows.
Bill
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Bill D. Casselberry ; Photography on the Oregon Coast
http://www.orednet.org/~bcasselb
[EMAIL PROTECTED]
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