At 06:23 PM 8/13/01 -0400, you wrote:
>That's an interesting tidbit. So 180-20%=144mm, 200-20%=160mm?
>
>Do you have anymore details on this? I'm surprised that the 50 & 90 fall
>into the same category. If you put a 50/2 or 90/2 on a bellows focused at
>infinity, would it suffer from the same focal length reduction? Or is this
>a characteristic of the changes in relative positions of the lens elements
>to accomplish close focusing in a helicoid design?
>
>BTW, all the long focal-length Macros mentioned are internal focusing
>designs.
>
>Skip
>
Most modern macros use a combination of internal focusing (shortening focal
length) and unit focusing (moving the whole assembly.) I believe the Nikon
180 shortens to about 150-155mm or so. Instead of having 90mm of extension
to go to 1/2x, it shortens the focal length and uses about 75mm or so.
With short lenses, internal focusing makes it easier to go to 1:1 without
additional extension tubes. One Sigma 180 macro was purely internal, so at
1/2x, it had a focal length of about 120mm.
Just about all macro zooms use internal focusing. An extreme case is the
Nikon 70-180mm macro zoom, which at its closest "180" setting has a focal
length of about 65mm! Outdoor Photography helpfully noted that that would
be an advantage in tight spaces!
The Zuiko 135mm deals with the helical length problem using a slide and
lock auto extension tube. It's a real 135mm, so we are not disadvantaged
very much relative to other makes at the closest distances.
Paul
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