Obviously, one of them appears a little sharper than the other and has
different DoF. But I'm thinking about something I learned many years
ago from, I believe John Shaw's "Close-ups in Nature" book.
In macro photography, if we put the teleconverter immediately behind
the lens with no focus by extension on the lens, it functions as a
longer focal length lens. A 50mm lens + 2X teleconverter has the same
general characteristics as a 100mm lens. If you then place the
extension tubes between the lens/teleconverter combo and the camera,
you achieve closer focus.
But if you place your extension between the lens and the
teleconverter, the teleconverter acts as a multiplier of the image
from the lens. So the characteristics of a 50mm lens remain the same,
but the teleconverter is effectively "cropping" the image.
If you focus the lens by extension, have it mounted to a
teleconverter, and then the stack put on extension tubes, you get a
combination effect where the lens acts something like an IF (internal
focus) lens and the effective focal length may shorten, depending on
the optical design of the lens and teleconverter.
So, to rephrase the above:
50mm lens + 2X teleconverter + 100mm extension = 100mm lens focused close.
50mm lens + 50mm extension + 2X teleconverter = 50mm lens focused
close, but the image is magnified.
50mm lens + 25mm extension + 2X teleconverter + 50mm extension =
should be about the same as above but will have a slightly reduced
effective focal length.
Where this is important is working distances. If you must stay farther
away, it's best to place the teleconverter immediately behind the lens
and then extend the combo out from the camera. But if you need to be
close in, extending the lens and then using the teleconverter next to
the body will give you the magnification of the longer focal length,
but the working distance of the shorter focal length.
Modern zoom lenses that focus very close, achieve it through magic,
and some combination of above by moving optical groups around.
AG Schnozz
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|