Canon 15-85 IS USM vs Canon 17-85 vs Sigma 18-125

 

When I bought my first Canon camera, the first lens I got was the Canon 17-85 IS. I have used it for more than one year on the old 350D; I liked its fast autofocus, its versatility and the image quality was quite good, even though it has a pretty noticeable chromatic aberration and distortion. In 2009, Canon has announced the 17-85 replacement: the new 15-85 is a bit wider and it promises better image quality. Does it maintain the promise? I have tested it on my Canon 7D, together with the Sigma 18-125 OS, a lens that offers similar features at a much lower price.
 
 

Specifications

 
 

Built quality, image stabilization and autofocus

The old Canon 17-85 had a quite good build quality, and the Canon 15-85 is a bit improved, it has a slightly more solid, professional feel. My Canon 24-105 L IS USM feels more solid, but in practice, this feeling does not matter much: last year the aperture of my 24-105 got stuck without any reason, and it cost me 250 Euros to repair, and this year the lens was broken again, this time for my fault (I dropped it holy cow!). Actually, the lighter weight of the 15-85 may even help to reduce the damage in case of droppings. Regarding the Sigma, the built quality is not bad, but it is not as good as the Canons.  
 
There is no difference between the AF of the new Canon 15-85 and old 17-85: both have very fast autofocus, with the silent ultrasonic motor and full time manual focus. The Sigma is much worse: in spite of the "HSM" label, it has not the true ultrasonic AF motor; its AF is clearly slower, noisier and it lacks of full time manual focus.  
 
In terms of image stabilization, again, there are no differences between the two versions of the Canon, while the IS of the Sigma 18-125 feels a little less effective. If you plan to use often the lens without tripod, the stabilization helps a lot!

 

 
 

Image quality comparison

I have tested the lenses on my Canon 7D (18 megapixel, APS-C). The lenses were mounted on tripod; I have used mirror lock up and self timer. The following images are 100% crop from the unprocessed RAW file. I have tested the lenses both wide open and at f/8.

At the shortest focal length (15mm for the Canon 15-85, 17mm for the 17-85 and 18mm for the 18-125), the new Canon 15-85 is clearly the winner. In the center it is about on par with the old 17-85, and even the Sigma is not too bad. In the corners, instead, the 15-85 is sharper than the old 17-85 and it has less chromatic aberration. The corners of the Sigma are a disaster.

At 35mm, the new 15-85 is again the winner. It is a little sharper than the 17-85 in the center and a lot sharper in the corners. The Sigma 18-125 is good in the center, while the corner are pretty poor, even though they are not as bad as at 18mm.

At 85mm in the center the 15-85 is the best, followed by the 17-85 and the Sigma 18-125. In the corners, instead, the old 17-85 shows a little less chromatic aberration and more detail then the new 15-85; the Sigma has much softer corners than both.
 
 

Distortion and angle of view

Even though there is not a night and day difference in angle of view, the 15-85 has some advantage in comparison with the 17-85 and 18-85. Other than that, the new 15-85 show a bit less distortion than the 17-85 and Sigma 18-125.
 
 

Conclusions

The new Canon 15-85 is a great zoom; it improves both image and built quality in comparison with its predecessor. Is it worth the price? Personally, I think it is a bit overpriced...$ 720 is a lot for a 3.5-5.6 zoom, in spite of the good quality. That said, I expect the price to come down with time, and overall I like this lens. If you have a limited budget and you want something similar, don't forget the old Canon 17-85 - it is much cheaper and it comes close to the quality of 15-85!  
 
I don't recommend, instead, the Sigma 18-125, due to its poor image quality and its slow autofocus. The Canon 17-85 costs just $100 more, and it is much better in every respect!