> "Ken Durling" <[EMAIL PROTECTED]> wrote:
> 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?
Hi Ken,
If you put the +2 Diopter in front of any lens, you make the
FAR FOCUS point at 500mm.
The CLOSE FOCUS point is also affected by the +2 Diopter, it
depends on how close the lens could focus without the diopter.
An easy way to solve the equations is to use my Javascript
lens calculator: http://members.home.net/jul.loke/
The actions require just three mouse clicks and three
keystrokes. However, the explanation takes the rest of this
long e-mail !!!
Canon specifies the 28-135 IS USM as having close focus of
0.5m without any adapters such as diopters or extension tubes.
So, set the "focal length" to 135mm, and "Closest So" to 0.4m
(an approximation for the minimum working distance, allowing
about 0.1m [10cm] for the length of the lens barrel).
The calculator now shows minimum magnification (MAGmin) of 0 at
infinity and maximum magnification (MAGmax) of 0.50943 at 0.4m.
If you change the close-up diopter setting to +2D, the calculator
now shows a MAGmin of 0.27x at 0.5m (the FAR FOCUS point), and
MAGmax of 0.91698x at 0.222m (the NEAR FOCUS point).
This means that at 135mm, the working distance ranges from 22cm
to 50cm producing a magnification between 0.91x and 0.27x
Furthermore, the diopter has increased the effective focal
length of your lens to 106.3mm!
The calculations for the 250D (which is a +4 D close-up lens)
are left for your amusement :-)
Cheers
Julian Loke
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