The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 100mm f2.8 Macro USM and the Canon 85mm f1.8 USM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is an important deciding factor when comparing two lenses. Canon 100mm f2.8 Macro USM is the longer of the two lenses at 119mm. The Canon 85mm f1.8 USM with a length of 72mm, is 47mm shorter. Besides being longer, the Canon 100mm f2.8 Macro USM also has a larger diameter of 79mm compared to the Canon 85mm f1.8 USM's 75mm diameter.
The 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. Canon 85mm f1.8 USM weighs 425g, 29% (175g) lighter than the Canon 100mm f2.8 Macro USM's weight of 600g.
Lens Mounts
Both the Canon 100mm f2.8 Macro USM and the Canon 85mm f1.8 USM has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon 1D X III, Canon 6D MII and Canon 5D MIV.
Focal Range
Canon 100mm f2.8 Macro USM is a prime lens with fixed focal lenght of 100mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 160mm.
Canon 85mm f1.8 USM is a prime lens with fixed focal lenght of 85mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 136mm.