The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon RF 100-400mm F5.6-8 and the Canon 75-300mm f4.0-5.6 III 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 RF 100-400mm F5.6-8 is the longer of the two lenses at 165mm. The Canon 75-300mm f4.0-5.6 III USM with a length of 122mm, is 43mm shorter. Besides being longer, the Canon RF 100-400mm F5.6-8 also has a larger diameter of 80mm compared to the Canon 75-300mm f4.0-5.6 III USM'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 USM weighs 480g, 24% (155g) lighter than the Canon RF 100-400mm F5.6-8's weight of 635g.
Filter Threads
The Canon RF 100-400mm F5.6-8 has a filter size of 67mm whereas the Canon 75-300mm f4.0-5.6 III USM 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 67mm filters that we recommend you to consider for the Canon RF 100-400mm F5.6-8:
And these are the 58mm filters that we recommend for the Canon 75-300mm f4.0-5.6 III USM:
Lens Mounts
The Canon RF 100-400mm F5.6-8 has the Canon RF lens mount whereas the Canon 75-300mm f4.0-5.6 III USM has the Canon EF lens mount. Some of the latest released cameras that are compatible with these mounts are Canon R100, Canon R8 and Canon R50 for the Canon RF Mount and Canon 1D X III, Canon 6D MII and Canon 5D MIV for the Canon EF Mount.
Focal Range
Canon RF 100-400mm F5.6-8 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 USM 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.