The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 18-135mm F3.5-5.6 IS and the Sigma 18-125mm F3.8-5.6 DC OS HSM for their suitability to various photography scenarios:
Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Canon 18-135mm F3.5-5.6 IS is the longer of the two lenses at 96mm. The Sigma 18-125mm F3.8-5.6 DC OS HSM with a length of 89mm is 7mm shorter. Besides being longer, the Canon 18-135mm F3.5-5.6 IS also has a larger diameter of 77mm compared to Sigma 18-125mm F3.8-5.6 DC OS HSM's 74mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Sigma 18-125mm F3.8-5.6 DC OS HSM weighs 490g, 4% (25g) lighter than the Canon 18-135mm F3.5-5.6 IS's weight of 515g.
Lens Mounts
Both Canon 18-135mm F3.5-5.6 IS has the Canon EF-S lens mount whereas the Sigma 18-125mm F3.8-5.6 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.
Canon 18-135mm F3.5-5.6 IS is also available in 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.
Focal Range
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. 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.