Which Lenses to Buy?

[Kayaker72]

Surely the Canon 15-85mm gives better IQ and functionality than the Canon 17-55mm and the Tamron 17-50mm anyway?

I own and highly recommend the EFS 15-85. However, I have to agree with thekingb and Neuro and wouldn't say that it "gives better IQ and functionality" than the 17-55 or the Tamron 17-50 non-VC (which I understand to be better than the VC version). I have studied the Tamron much less, but in general, I would say that all three of these lenses are used and well liked by different members of the forum. Understand that no lens is perfect, but these are three very good lenses (again, I am more familiar with the Canons) from a cropped sensor body.

As has already been pointed out, if anything, the 17-55 may have slightly better IQ than the 15-85. That said, you will likely only be able to tell that by looking at reviews and even then it is very slight. In real world shooting, the IQ of these lenses is very good to excellent.

Which is "better" in terms of functionality depends the the function. IMO, the case for one of the f/2.8 lenses, especially the EFS 17-55, has already been well made. And I am not posting to disagree. From everything I have read, heard and seen, it is an excellent lens.

But I wanted to chime in with two points:

1) The difference between 15 to 17 mm focal length is noticeable. 15 mm on a crop body is equivalent to ~84 degrees diagonal angle of view (AOV) vs ~77 degrees at 17 mm. That is a 9 percent difference. On the long end, 50 mm has a AOV of ~30 degrees vs ~18 degrees for 85 mm. Also, adjusting 50 mm and 85 mm to full frame equivalents you get 80 mm and 136 mm, which happens to be the classic "portrait" range. That extra range is very useful and, as is common, I do tend to shoot most at the extremes (near 15 mm and near 85 mm) of my EFS 15-85.

2) While f/2.8 will give you 2/3 to 1 2/3 more light in comparison to the 15-85 at similar focal length and exposure settings, it often isn't enough to shoot in low and moderately lit conditions. In lower light I have found f/1.8 to not be enough to "stop action." So, you will likely still need a flash if you are shooting in low to moderate lit conditions.

Getting back to functionality, there are a lot of "functions" within photography and these three lenses will perform very well for most of them. But, the aperture/focal length trade off does favor each lens for certain specific functions. For example, I would recommend one of the f/2.8 lenses to someone that will be taking the time to set up a subject when the thin DOF can really be taken advantage of (say for portrait photography) or for someone that has a family and will want to "stop action" of kids running around the house. For those functions, I think the benefit from the aperture is worth the sacrifice of the focal length range. But then I view the focal length advantage being better for people that want a "general" general purpose lens. For example, I shoot a little of everything and I love being able to leave the EFS 15-85 on my camera for most of it. It is a great walk around lens and covers a wide range with a single lens very well.

Good luck.