I've owned both at the same time. I tested extensively against each
other. I was never able to see any difference in sharpness when viewed
as a 6' projected slide, a 13"x19" print or through a 10x loupe on a
light table.
IMO, the non-IS had no advantages other than price.
OTOH, the IS version had the following advantages:
Very close focusing without the aid of tubes or extenders. Close to
Macro. Much closer than non-IS ver.
Manual focusing much smoother. The non-IS was more a metal-to-metal
feel. The IS ver. was much smoother.
Tripod Collar. IS version smooth as silk. No slippage. Non-IS ver.
more metal-to-metal. Kinda Klunky. Hard to keep tight. A nuisance if
you've ever attached a flash to the camera body and had to shoot in the
vertical position without a Tripod Collar.
AF seemed faster with the IS ver.
Even if you do not hand hold, the IS works great when using a beanbag or
monopod. If you ever do hand hold, the advantage is obvious. While
Canon does not recommend using IS in mode 1 with a tripod, I found I
could tell no difference if I used a tripod which is very stable and
sturdy and used proper techniques. I did not do this on purpose. I
would forget to check.
There were other minor reasons too. Hope this helps.
Ray Amos
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