AG writes
<<Modern zoom lenses that focus very close, achieve it through magic,
<<and some combination of above by moving optical groups around.
This is by focal length shortening and thus reduction in working distance--not
always conducive to chasing critters but results in less light loss than
extension.
Excellent summary of the various extension tube /extender combos to achieve
same mag. In the table of trade offs should include stops of light lost, dof
at "aperture setting" on lens, working distance ( function of effective FL)
and hand holdability index----
For macro really is 1/focal length with (1+M)**2 where M is the mag as
modifying factor. So at 1:1 require 4 times the shutter speed to hand hold at
same focal length for a sharp shot. This requires some geometry to prove, but
focal length DOES matter not just mag for macro. The handholdability index
still applies for IS but modified by a few stops depending on effectiveness.
Axial stability not helped by IS becomes a major factor as mag is increased.
One would assume extension tubes with no glass would be sharper than a TC at
same mag in all circumstances but doesn't hold true for some lenses with
"matched optics." I experienced that years ago with the Tammy 80-200 with
matched TC. May be true even more with the exactly matched TC's required for
lenses with aspheric optics. Just have to experiment and see.
Steady as she goes, Mike
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|