OK, sports fans, this weeks test involves the Canon 300 f/4L IS.
Specifically, I wanted to see how it would compare against the 400 f/
5.6L (which sadly lacks IS).  I did a series of shots with each lens
at the official TN Canon Testing Grounds (shot out of my bathroom
window).  No tripods were used in the test (tripods are for wimps!),
but I was able to stabilize each lens on a copy of Percy Boomer's
1946
classic, "On Learning Golf" which I keep at the test facility.

Here is the test site with the tested area shown in the red box:
http://www.pbase.com/bret/image/75187704

I won't post all the images, but first I shot with the 300 f/4L
without using the IS.
Then I took another shot with the IS on.  This setting is not
recommended for tripod shots, and it doesn't do much better when
balanced on Percy Boomer's 1946 classic, "On Learning Golf."
The pic without the IS was noticeably sharper. This is because the IS
mechanism is "hunting" trying to stabilize an already stable image.
So
from now on, only use the IS when you are handholding.  Got that?
Ok,
let's move on.

Next I tested the same lens using a Hoya UV filter to see if I could
tell the difference.
Many people buy expensive lenses and keep UV filters on them at all
times for protection.  Surprisingly, I was able to tell a difference
with the UV filter on and off.
The shots without it were cleaner and slightly sharper.  However,
this
difference was not as great as the IS vs. non-IS shots. But even with
only a slight improvement, it seems silly to me to buy the expensive
"L" glass and then cover it with additional glass.  You wouldn't
expect to get a great photo shooting through a window would you?

Next I tested the 300 f/4L against the 400 f/5.6L.
I was very interested in seeing how the 300 with the Canon 1.4x
(giving an effective focal length of 420mm) would match up against
the
400 f/5.6L.

Here are the comparison images:
http://www.pbase.com/bret/image/75187706/original

As you can tell, the image from the 400 f/5.6L is noticeably better.
This is not surprising since we are putting more glass between the
lens and the sensor. But I was hoping the results would be closer.
So
comparing these lenses one must decide if the benefits of IS (on the
300 f.4L) when handholding these lenses outweighs the inherent
sharpness advantage of the 400 f/5.6L.

The bottom line is: you just can't cheat physics. Buy the biggest,
fastest primes you can afford and forget the teleconverters.

Just for fun, here's what the 400 f/5.6L looks like with both a 2x
and
a 1.4x stacked on it.  That gives a FL of 1120mm.
http://www.pbase.com/bret/image/75187707