The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 15-85mm f3.5-5.6 IS USM and the Sigma 17-70mm F2.8-4 DC OS HSM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Sigma 17-70mm F2.8-4 DC OS HSM is the longer of the two lenses at 89mm. The Canon 15-85mm f3.5-5.6 IS USM with a length of 88mm, is 1mm shorter. On the other hand, the Canon 15-85mm f3.5-5.6 IS USM has a larger diameter of 82mm compared to the Sigma 17-70mm F2.8-4 DC OS HSM's 79mm diameter.
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Sigma 17-70mm F2.8-4 DC OS HSM weighs 520g, 9% (55g) lighter than the Canon 15-85mm f3.5-5.6 IS USM's weight of 575g.
Filter Threads
Both the Canon 15-85mm f3.5-5.6 IS USM and the Sigma 17-70mm F2.8-4 DC OS HSM have the same Filter thread size of 72mm.
Below are links to the 72mm filters that we recommend you to consider for these lenses:
Lens Mounts
The Canon 15-85mm f3.5-5.6 IS USM has the Canon EF-S lens mount whereas the Sigma 17-70mm F2.8-4 DC OS HSM has the Canon EF lens mount. Some of the latest released cameras that are compatible with these mounts are Canon T8i, Canon 90D and Canon SL3 for the Canon EF-S Mount and Canon T8i, Canon 90D and Canon SL3 for the Canon EF Mount.
Sigma 17-70mm F2.8-4 DC OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Canon 15-85mm f3.5-5.6 IS USM has a focal range of 15-85mm and
5.7X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
24-136mm when used on a APS-C / DX format camera.
On the other hand, the Sigma 17-70mm F2.8-4 DC OS HSM has a focal range of 17-70mm and 4.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27.2-112mm when used on a APS-C / DX format camera.