Inspired by Gary Reese's recent posting I measured the usable
working distances of several macro lenses. This is the distance
from the front of the lens, with no filters or hoods etc., to the
object (an aftershave box with sharp printing). The magnification
given is that on the lens scale.
1/2 1/3
OM 50/2 macro 4-3/8" 6-1/2"
OM 135/4.5 ---- 20"
Vivitar Series
1 90-180 at 180 17-1/2" 24-1/2"
OM 50/3.5 4-5/8" 6-5/8"
Tamron 90/2.5 9-3/4" 13-5/8"
50/1.8 with
Vivitar macro
teleconverter 9-1/4" 13-1/4"
All of the above are consistent with very little focal length
shortening. The ideal for a single thin lens would be
3*focal length for 1/2, and 4* for 1/3. The above values tend
to be a little short, but that's because the front nodal
point is back in the lens. The 50/2 is reputed to shorten its
focal length slightly. It shows a little less working
distance than the 50/3.5 even though its optics aren't as
deeply recessed. The CFC mechanisms of some OM lenses may
slightly alter FL, but they are mainly for aberration and field
curvature correction, rather than internal focusing. The
lack of FL shortening in the 90-180 is quite unusual for a zoom.
Most macro zooms shorten FL dramatically from the marked value.
Anybody have another maker's 180-200 macro for comparison to the
135?
Paul Farrar
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