>So 200mm internal focus will give about the same as 135mm with extension.
depends on the type of internal mechanism. internal mechanisms like zooms
give lenses that shorten alot as they focus more closely. the nikon
200/4 micro-nikkor shortens to 153mm at 1:2. On the other hand, other
200mm macros focus by having a helicoid extension between a front lens
and rear teleconvertor. These lenses focus by increasing the amount
of extension between the front lens and rear teleconvertor. This leads
to some shortening of focal length, but not as much as the first type of
lens described above, and it won't be as short as a 135mm lens with
enough extension to get to 1:2.
Joseph
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