"Chris Brogden " writes:
> On Wed, 23 May 2001, Bill D. Casselberry wrote:
> > 1:2 & 1:1 are easy calculations --
> > 1:1 is achieved w/ extension tubing ~= to the focal length
> 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.
As I understand it, yes and no. If the lens doesn't use any internal
ofcusing elements, then the lens helicoid is mainly just extension, and then
yes, it merely adds to the extension by tubes. If the lens uses has
internal focusing elements, then its helicoid also affects the internal
elements, which typically do their thing by effectively changing the focal
length of the lens, and in that that case, it has an effect, but it isn't as
easily figured since both extension and focal length are simultaneously
changing.
However, in practice, if you are trying to achieve an exact magnification
on film, then just include a reference object, such as a ruler. That
eliminates the need to control exactly how large the resultant print or
projected slide is. For everything else, which is mostly what I do, knowing
how to approximate what extension is needed is sufficient. In fact, I
always set up my camera with enough extension to allow the lens to not be
fully extended. That allows me to tweak the focus using the lens's focusing
ring.
hope that helps,
patbob ([EMAIL PROTECTED] [EMAIL PROTECTED])
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