The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Samyang 50mm F1.2 AS UMC CS and the Rokinon 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. Rokinon 85mm F1.8 is the longer of the two lenses at 81mm. The Samyang 50mm F1.2 AS UMC CS with a length of 74mm, is 7mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 68mm.
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. Rokinon 85mm F1.8 weighs 344g, 8% (31g) lighter than the Samyang 50mm F1.2 AS UMC CS's weight of 375g.
Below you can find the dimension and weight table of the Telephoto Prime Lenses in Fujifilm X Mount for size comparison purposes.
Telephoto Prime Lenses in Fujifilm X Mount for Size Comparison
Lens Mounts
Both the Samyang 50mm F1.2 AS UMC CS and the Rokinon 85mm F1.8 has the same Fujifilm X lens mount. Some of the latest released cameras that are compatible with the Fujifilm X lenses are Fujifilm X-T5, Fujifilm X-H2 and Fujifilm X-H2S.
Samyang 50mm F1.2 AS UMC CS is also available in Canon EF-M , Micro Four Thirds and Sony E mounts.
Rokinon 85mm F1.8 is also available in Canon EF-M and Sony E mounts.
Focal Range
Samyang 50mm F1.2 AS UMC CS is a prime lens with fixed focal lenght of 50mm which has an effective (full-frame 35mm equivalent) focal range of 75mm when used on a APS-C / DX format camera. Rokinon 85mm F1.8 is a prime lens with fixed focal lenght of 85mm which has an effective (full-frame 35mm equivalent) focal range of 127.5mm when used on a APS-C / DX format camera.