The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 100-400mm f4.5-5.6L IS 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 a crucial factor to consider when comparing two lenses. Canon 100-400mm f4.5-5.6L IS USM is the longer of the two lenses at 189mm. The Canon 75-300mm f4.0-5.6 III with a length of 122mm, is 67mm shorter. Besides being longer, the Canon 100-400mm f4.5-5.6L IS USM also has a larger diameter of 92mm compared to the Canon 75-300mm f4.0-5.6 III's 71mm 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 75-300mm f4.0-5.6 III weighs 480g, 65% (900g) lighter than the Canon 100-400mm f4.5-5.6L IS USM's weight of 1380g.
Filter Threads
The Canon 100-400mm f4.5-5.6L IS USM has a filter size of 77mm whereas the Canon 75-300mm f4.0-5.6 III has a 58mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 77mm filters that we recommend you to consider for the Canon 100-400mm f4.5-5.6L IS USM:
And these are the 58mm filters that we recommend for the Canon 75-300mm f4.0-5.6 III:
Lens Mounts
Both the Canon 100-400mm f4.5-5.6L IS 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 100-400mm f4.5-5.6L IS USM has a focal range of 100-400mm 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
160 - 640mm.
On the other hand, the 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.