The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 50mm f1.8 II and the Canon 40mm f2.8 STM for their suitability to various photography scenarios:
Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Canon 50mm f1.8 II is the longer of the two lenses at 41mm. The Canon 40mm f2.8 STM with a length of 22mm is 19mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 68mm.
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.
Below you can find the dimension and weight table of the 'Standard Prime Lenses in Canon EF Mount for size comparison purposes.
Standard Prime Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both Canon 50mm f1.8 II and Canon 40mm f2.8 STM 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 50mm f1.8 II is a prime lens with fixed focal lenght of 50mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 80mm.
Canon 40mm f2.8 STM is a prime lens with fixed focal lenght of 40mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 64mm.