Or, you can use modulation transfer curves and make decisions based on how
you will most often use a particular lens:

http://www.usa.canon.com/consumer/controller?act=ModelDetailAct&fcategoryid=150&
modelid=7445

By the way, the sharpness freaks can lead one astray.  I have run tests
between two similar Canon zoom lenses, one of which was L-glass, and was
underwhelmed with the difference in sharpness.  It was definitely there ...
I could see it; but honestly the difference was modest.  Just me.

I'm not saying that L-glass is not worth it, by the way.  Auto-focus
acquisition time and ruggedness are certainly worth having and paying for if
one has deep pockets or is a pro.

Not sure if I missed it in the other posts, but a tripod is a must for
such a test!
- Take photos at smallest F stop and F8 (at least)
- At min, max and middle of the zoom range

Shoot a brick wall/ facade or the like and review the results at 100%
in photoshop.

yes, I also always loved the "Watch out for bad copies."";-)) However,
there are as many results as testers. Someone had a kit lens that was
sharper than his new 24-105 F4L IS if I remember the post correctly. I
would be pi#%d off ;-))

T
http://tr-photo-blog.blogspot.com/

At least you realize the difference. That's 90% of the problem solved!

If you really want to "measure" there are charts and tools that can
help:

http://www.imatest.com/
http://www.normankoren.com/Tutorials/MTF5.html
http://www.graphics.cornell.edu/~westin/misc/res-chart.html

The main problem with these is that they are not used under the same
lighting/distance as most real-world photographs.

Note that these focal lengths are a bit long on an EF-S 1.6x crop
camera. The 17-85mm is the EF-S equivalent.

And guess, what? All of these opinions may be true!  A lot depends on
how you use a lens and what you expect (and even what camera you use it
with!).

There are two solutions for this:
1) buy where you can return if not satisfied
2) be able/willing to send in the lens for calibration repair (ideally
under warranty).

You can also often rent lenses you cannot afford to buy - even via the
internet in the U.S.
http://www.rentglass.com/

--
Erik

I don't really know the details, other than it is a rather well
done permanent conversion to a Nikon F mount on some kind of a
contract for a number of lenses (both Canon SSC 600mm f/4 and
800mm SSC f/5.6) by Tempe Camera Repair in Tempe AZ.

I'm not sure when was done, but assume that it would have been
modified in the late 70's or even early 80's.  The lense was
designed for the 1976 Montreal Olympics, and was in production
until 1981, when Canon came out with a newer L version.

Adorama sold four or five each of the modified 600mm and 800mm
lenses on eBay.  I've seen other odd lenses that Tempe Camera
had modified too, so apparently they did more than just a few.
All of them have been obviously well used, though also well
maintained, lenses.  The one I have is in good condition, but
the case had clearly been shipped around the world a few times
and suffered the consequences.  The outside was beat to pieces
while the inside was in very nice condition.

I considered it quite a find, because it sold for not only far
less than any other decent quality Nikon mount lense, the ones
modified for the Nikon mount sold for less than other unmodified
Canon 800mm lenses had sold for.  It isn't like having an
autofocus lense, but that of course would cost thousands of
dollars more.

What is really astounding is that it works well with a selected
pair of telconverters, a 1.6x and a 2x, or with another specific
2x (I tested it with a number of different combination and
individual teleconverters to find ones that make a good match,
because most of them simply do not work well at all).  Can you
imagine how hard it is to focus an 800mm lense with a 1.6x and a
2x teleconverter, making it into a 2560mm focal length!  It
requires a *sturdy* tripod.

(Right now it is in the box, stored away, waiting for the sun to
come back up in late January. :-)

Tempe Camera Repair is still in business, and is a very
interesting outfit.  I have no idea if they would consider doing
such modification today or not.  Do a web search for them, it is
interesting reading!