The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sony FE 400mm F2.8 GM OSS and the Sony FE 85mm F1.8 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. Sony FE 400mm F2.8 GM OSS is the longer of the two lenses at 359mm. The Sony FE 85mm F1.8 with a length of 82mm, is 277mm shorter. Besides being longer, the Sony FE 400mm F2.8 GM OSS also has a larger diameter of 158mm compared to the Sony FE 85mm F1.8's 78mm diameter.
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. Sony FE 85mm F1.8 weighs 371g, 87% (2524g) lighter than the Sony FE 400mm F2.8 GM OSS's weight of 2895g.
Below you can find the dimension and weight table of the Telephoto Prime Lenses in Sony E Mount for size comparison purposes.
Telephoto Prime Lenses in Sony E Mount for Size Comparison
Lens Mounts
Both the Sony FE 400mm F2.8 GM OSS and the Sony FE 85mm F1.8 has the same Sony E lens mount. Some of the latest released cameras that are compatible with the Sony E lenses are Sony ZV-E1, Sony A7R V and Sony FX30.
Focal Range
Sony FE 400mm F2.8 GM OSS is a prime lens with fixed focal lenght of 400mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 600mm.
Sony FE 85mm F1.8 is a prime lens with fixed focal lenght of 85mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 127.5mm.