First, I would suggest reading the review on this site for both lenses if you haven't already.


When comparing the Tamron 17-50 f/2.8 to the 17-40 f/4, there are several important points:


Advantages of the 17-40 f/4:
  • Build quality.
  • Brand name.
  • Quiet focus.
  • Large, smooth focus ring.
  • Full time manual focus.
  • Weather sealed (filter required).
  • Does not extend while zooming.
  • Compatible with full frame.



Advantages of the Tamron 17-50 f/2.8:
  • Double the aperture, which has several important factors:
  • f/2.8 allows twice the shutter speed (usable 1/60 vs unusable 1/30)
  • f/2.8 allows half the ISO (usable ISO 3200 vs unusable ISO 6400).
  • f/2.8 allows more control over depth of field and background blur.
  • f/2.8 allows activation of more autofocus sensors (on 450D?).
  • More zoom range on the long end.
  • More resolution and contrast at all focal lengths.
  • 55% less money



If you're wondering how the Tamron can be so much better at aperture, resolution, and cost, the reason is this: it's a normal zoom. A normal zoom is very different from an ultra wide zoom, which is what the 17-40 is.


I recommend the Tamron.