Bugsy Moose writes:
<<What about your Telyt-R 250/4, perhaps on the shoulder stock?
<<The way C-U lenses work is that furthest focus is at the C-U lens FL from the
<<front of the lens.
<<I use a Pentax 6x7 T132 on the 72mm thread 100-400 and Oly 12-100, for a
working distance of 52", but less magnification - and they are few and far
between.
Yes, surprised at the IQ of the T132 on the PL 100-400. The lens itself is
sharpest at shorter FL's and just control the mag by increasing the FL with the
CU diopter. The lens goes to .25mag on its own and wit the crop factor and
T132 goes effectively over 1:1.
Stopping down and keeping thing towards the center helps--often frame to crop
with desired composition. The detail retained is surprising with adequate
working distance for most critters.
The 250/4 goes to 1:5 or 0.2 mag ("apparent" mag more with crop factor) and
will achieve .43 X ("more") again by crop factor even with 0.76 diopter.
Lower power ones tend to do well with longer FL --more mag achieved and better
IQ than higher powered---see this for calculator. There is no guarantee how one
achromatic diopter may behave on a given lens. There is ample evidence though
for some older zooms using a CU diopter led to better IQ than using the lens
itself at MFD. I am not 100% sure but may be the case for the center stopped
down at some mags with the T132 and PL 100-400.
https://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm
I wonder how they calculate the "effective FL"? The FL is really the FL of the
diopter, no? For lenses with FL shortening for higher mag the "EFL"= MFD/
(2+m+1/m) The IS is a bit fooled by the diopter but as working distance is
less but mag more perhaps not as bad
as it could be as these variables tend to cancel the effects on how much IS is
needed for any given displacement.
Curiously some CU diopters may benefit from reversing!?
http://www.coinimaging.com/add-on_macro.html
Up close and personnel for some critters, but not tooo close, Mike
--
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