--- Andreas W <[EMAIL PROTECTED]> wrote:
> As far as I know the number is not the close focuing
> distance, but the focal
> length of the lens. Normally, lenses of this kind
> are marked with their
> diopters, and a +1 diopter lens has a focal length
> of 1000mm, the +2 is
> 500mm, and so on (focal length=diopter/1000mm). The
> shorter the focal length
> of the close-up lens, the greater the magnification.
> Magnification will also
> be greater on a longer "host" lens, and I guess this
> is why canon recommend
> the 250D for short lenses, and the not so strong
> 500D for longer lenses.
I think this is not correct. The diopter does only
change the focusing distance but not the focal length.
That later is given by the lens itself. A diopter +2
has a focusing distance of 1m/2=500mm. So Canon calls
it 500(D). A diopter +4 has a focusing distance of
1m/4=250mm. Which is why Canon calls it 250. Now for a
long lens a maximum focusing distance of 250 is pretty
short. So unless you need relatively high
magnification you are better of with a 500 on a long
lens.
Robert
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