Ok, whats the difference??

Sorry, but Im SO uneducated on lenses! Ive decided to get Tamrons 17-50mm f2.8 lens, and Ive found some great deals. My question is: Whats the difference between the
SP AF 17-50mm f2.8 XR Di II and the Tamron AF 17-50mm f2.8 XR Di II LD ZL? Anyone know what the ZL means? I can get a perfect one for less than $300, and the best I can find on the XR is $415. Thank you for the advice.

where did you find this? links?

ZL stands for Zoom Lock .

http://www.tamron-usa.com/lenses/technol​ogy.asp#ZL

Weird...The "regular" 17-50 does have a zoom lock, but no ZL in the name on the box, lens or Tamron's site. Searched a bit and some sites do add ZL to the already too long name of the non-VC lens.
So this and the SP AF 17-50mm f2.8 XR Di II should be the same thing. No difference.

Thank you! The guy says hes only taken a dozen shots with it or so, and his new body has the larger sensor, so it doesnt work well. Again, thanks for the input.