Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Canon 100-300mm f4.5-5.6 USM is the longer of the two lenses at 122mm. The Canon 90-300mm f4.5-5.6 USM with a length of 115mm is 7mm shorter. Besides being longer, the Canon 100-300mm f4.5-5.6 USM also has a larger diameter of 73mm compared to Canon 90-300mm f4.5-5.6 USM's 71mm 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. Canon 90-300mm f4.5-5.6 USM weighs 420g, 22% (120g) lighter than the Canon 100-300mm f4.5-5.6 USM's weight of 540g.
Below you can find the dimension and weight table of the 'Telephoto Zoom Lenses in Canon EF Mount for size comparison purposes.
Telephoto Zoom Lenses in Canon EF Mount for Size Comparison
Both Canon 100-300mm f4.5-5.6 USM and Canon 90-300mm f4.5-5.6 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.
Canon 100-300mm f4.5-5.6 USM has a focal range of 100-300mm and 3.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 160 - 480mm.
Canon 90-300mm f4.5-5.6 USM has a focal range of 90-300mm and 3.3X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 144 - 480mm.