The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 90-300mm f4.5-5.6 USM and the Canon 75-300mm f4.0-5.6 III 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 75-300mm f4.0-5.6 III 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. On the other hand, there is no difference between the max diameters of these lenses, both coming at 71mm.
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 90-300mm f4.5-5.6 USM weighs 420g, which means it is 60g (12%) lighter than the Canon 75-300mm f4.0-5.6 III which has a weight of 480g.
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
Lens Mounts
Both the Canon 90-300mm f4.5-5.6 USM and the Canon 75-300mm f4.0-5.6 III 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 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 a 35mm (FF) equivalent of 144 - 480mm.
Canon 75-300mm f4.0-5.6 III has a focal range of 75-300mm and 4.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 120 - 480mm.