Buying a used 17-40 F/4L on Friday, have some questions!

This is my first post here, great forum you guys!

• What is the best way to test if I got a sharp copy?
  
• At what mm / aperture is it the sharpest?
  
• Do I need the lenshood on my 1.6x XT Rebel?
  
• Recommend me a nice circular polarizer (77mm)

TIA

Felix

See the review below. Looks like peak overall performance is at F8, although the centre is already very sharp wide open. Looks like a great lens and I want one too!

Sorry I can't answer your other questions.

http://www.photozone.d​e …es/canon_1740_4​/index.htm

Oh and welcome to the forum too. I've been using it for a few months now and find it an excellent resource.

You and anyone else that uses a lens on a DSLR. In short, "yes".

Its a super lens.

Great lens.

It's one of canon gems, you will find it sweet across all apertures, built like a brick ****house.
I would advise on the lenshood, just to reduce flare
I've used one on a 10D, currently on a 1DS MKIi, and it's my favourite lens.

http://lesmclean.photo​blink.com/

So, What is the best way to test if I got a sharp copy?
Thanks for all the answers guys!

You got a tripod and shutter release switch?

Got a manifrotto tripod and can use the camera 10 sec timer!

Gotta get the hood............

If you use it as a "walkaround" it would offer great protection.

Saw a guy playing with a new lens at a concert...............

Had a hood on..............backwa​rds. LOL

Looked like he just got the lens and a XT.

Congrats on the new glass.

I love this lens, its my stay on all the time lens....

So, how do I test if I got a sharp copy?

I also put my hood backwards on Tamron for transportation reasons (My Orion Trekker II barely fits 350D+Tamron in vertical position and I have no place to spare to fit the hood separately; so I put it backwards and it fits, it's in place and always with me )

17-40 is a great lens. I've tried to buy one but it had lotsa dust inside. Looking for another copy now.

I've tested my tamron this way:
a) put an open book on the floor; the book must not have a 'reflective' paper! (sorry, I don't know good english word)
b) put your camera on a tripod facing down the floor, straight at the book (this way you eliminate some of the tripod shake although 17-40 is quite light glass); remember to have exactly 90 degrees between focal axis and book plane - this eliminates any FF/BF errors you might have 'in the body'
c) get some shots (no flash) with focal lengths 17, 25, 40 at apertures f4, f5.6, f8 (so you'll have 3 series of photos). You might try ISOs 100 and 400. You have to have good lighting conditions.
Take a look on them, wheter they are sharp enough for you and there is no obvious blur. I have also cleaned (only with air) the CMOS in the body to be sure I have no dust on it.