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 STM and the Sigma 18-125mm F3.8-5.6 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. Canon 18-135mm f3.5-5.6 IS STM 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 STM also has a larger diameter of 77mm 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 STM weighs 480g, which means it is 10g (2%) lighter than the Sigma 18-125mm F3.8-5.6 DC OS HSM which has a weight of 490g.
Filter Threads
Both the Canon 18-135mm f3.5-5.6 IS STM and the Sigma 18-125mm F3.8-5.6 DC OS HSM 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 Canon 18-135mm f3.5-5.6 IS STM 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.
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 STM 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.
On the other hand, the 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.