I am looking to buy a good walk around lens for t3i to finally upgrade from my kit lens. My current preference is Canon 15-85. however I am wondering if I should get Tamron AF 28-75mm f2.8.
I am looking for the general purpose lens which does a little bit of everything: portraits of kids, kids's action, landscape (usually with kids:-)),
I already have canon 50mm 1.4 and love that lens.
I am using the 15-85 lens with my 550D and find the range to be very useful, especially as a walkabout lens. The USM focusing is also great and it should be a useful attribute when shooting children. The variable aperture is the only con compared to the Tamron, as you end up with slow shutter speeds even with the ISO jacked up to 3200 (I've even used 6400 to reduce motion blur). However, for static subjects, the IS typically makes up for the aperture.
The 15mm wide angles also comes in handy in confined spaces and will be a big plus over the Tamron in this regard.
I also have the 50mm f/1.8 that's quite handy in the low light scenarios, but the focusing speed leaves a lot to be desired. I suppose your 50mm 1.4 should be better in that regard.
The bottom line is that the Canon 15-85 lens can replace the kit lens and give you greater versatility along with an image quality and focus performance upgrade. The Tamron however, compliments the kit lens, and due to the missing wide angle range you won't be able to do away with the kit lens. If you are looking to actually replace the kit lens with a fast zoom, then the Tamron 17-50mm suggested by Michael is the best value.
The Tamron 17-50mm f/2.8 XR Di II is probably a better fit for your Rebel T3i than the Tamron 28-75mm f/2.8 XR Di. With the 1.6x crop factor the equivalent field of view (FoV) is for a 27-80mm and 45-120mm lens respectively on a full frame body. The 17-50 is also a newer design that is optimized for cropped sensors. I own one and it is quite a sharp lens for the money.
If you compare ISO 12233 tests at The-Digital-Picture.com, the Tamron 17-50 is sharper than the EF-S 15-85 at the focal lengths they share in common, even when the Tamron is opened up wider than the maximum aperture of the 15-85. Both these lenses are sharper than the Tamron 28-75, which has become a rather dated design that was good enough for film but not quite good enough for pixel peeping in the digital age.
Some people complain that the Tamron's focus motor is noisy and slow. It does make a unique sound while focusing, but I think the sound it makes fools a lot of folks into thinking it is slower than it really is. It focuses about as fast as any Canon lens in the same class that I have used.
The Tamron 17-50mm f/2.8 XR Di II runs about $500, the Canon EF-S 15-85mm f/3.5-5.6 runs about $800. So you are paying about 60% more for about 70% more reach while giving up between 2/3 to 2 stops of aperture and a little sharpness.
With your children indoors, I think the shorter focal lengths between 17-50mm is where you will spend most of your time shooting and the constant f/2.8 aperture of the Tamron will be invaluable in the lower light indoors, allowing you to use lower ISO than the f/3.5-5.6 lens will require. Outdoors the extra reach of the EF-S 15-85mm will come in handy. For shooting static objects in low light, the Image Stabilization (IS) will be useful. A Vibration Control (VC - The Tamron term for IS) version, the Tamron 17-50mm f/2.8 XR Di II VC is also available for about $650. The partial review for the Tamron 17-50mm f/2.8 Di II VC at The-Digital-Picture is disappointing, but at DxO Mark the VC version tested slightly better than the original, non-VC version. The EF-S 15-85 tested a little less sharp at the corners than either Tamron at 35-50mm.
