The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-125mm F3.8-5.6 DC OS HSM and the Canon 18-135mm f3.5-5.6 IS 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. Canon 18-135mm f3.5-5.6 IS is the longer of the two lenses at 101mm. The Sigma 18-125mm F3.8-5.6 DC OS HSM with a length of 89mm, is 12mm shorter. Besides being longer, the Canon 18-135mm f3.5-5.6 IS also has a larger diameter of 75mm compared to the Sigma 18-125mm F3.8-5.6 DC OS HSM's 74mm 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. Canon 18-135mm f3.5-5.6 IS weighs 455g, 7% (35g) lighter than the Sigma 18-125mm F3.8-5.6 DC OS HSM's weight of 490g.
Filter Threads
Both the Sigma 18-125mm F3.8-5.6 DC OS HSM and the Canon 18-135mm f3.5-5.6 IS have the same Filter thread size of 67mm.
Below are links to the 67mm filters that we recommend you to consider for these lenses:
Lens Mounts
The Sigma 18-125mm F3.8-5.6 DC OS HSM has the Canon EF lens mount whereas the Canon 18-135mm f3.5-5.6 IS has the Canon EF-S 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 Mount and Canon T8i, Canon 90D and Canon SL3 for the Canon EF-S Mount.
Sigma 18-125mm F3.8-5.6 DC OS HSM is also available in Sigma SA and Nikon F (DX) mounts.
Canon 18-135mm f3.5-5.6 IS is also available in Canon EF-S mount.
Focal Range
Sigma 18-125mm F3.8-5.6 DC OS HSM has a focal range of 18-125mm and
6.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
28.8-200mm when used on a APS-C / DX format camera.
On the other hand, the Canon 18-135mm f3.5-5.6 IS has a focal range of 18-135mm and 7.5X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-216mm when used on a APS-C / DX format camera.