----- Original Message -----
From: "Ken Durling" <[EMAIL PROTECTED]>
> I'm going to get one for use on the 28-135, but only the 500D comes in
> the 72mm filter size. I understand that the number is the close
> focusing distance in mm? I know people have posted about using the
> 500D on the 28-135, but does the above statement imply a limitation
> when using the 500D on the 28-135?
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.
Well, that's about most of my knowledge on this matter.
/Andreas
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