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 Canon 15-85mm f3.5-5.6 IS USM 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 Canon 15-85mm f3.5-5.6 IS USM with a length of 88mm, is 8mm shorter. On the other hand, the Canon 15-85mm f3.5-5.6 IS USM has a larger diameter of 82mm compared to the Canon 18-135mm f3.5-5.6 IS STM's 77mm 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 95g (16%) lighter than the Canon 15-85mm f3.5-5.6 IS USM which has a weight of 575g.
Filter Threads
The Canon 18-135mm f3.5-5.6 IS STM has a filter size of 67mm whereas the Canon 15-85mm f3.5-5.6 IS USM has a 72mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 67mm filters that we recommend you to consider for the Canon 18-135mm f3.5-5.6 IS STM:
And these are the 72mm filters that we recommend for the Canon 15-85mm f3.5-5.6 IS USM:
Lens Mounts
Both the Canon 18-135mm f3.5-5.6 IS STM and the Canon 15-85mm f3.5-5.6 IS USM has the same Canon EF-S lens mount. Some of the latest released cameras that are compatible with the Canon EF-S lenses are Canon T8i, Canon 90D and Canon SL3.
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 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.