The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Rokinon 85mm F1.8 and the Samyang 300mm f6.3 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 300mm f6.3 with a length of 74mm, is 7mm shorter. Besides being longer, the Rokinon 85mm F1.8 also has a larger diameter of 68mm compared to the Samyang 300mm f6.3's 65mm 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. Samyang 300mm f6.3 weighs 318g, 7% (26g) lighter than the Rokinon 85mm F1.8's weight of 344g.
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 Rokinon 85mm F1.8 and the Samyang 300mm f6.3 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.
Rokinon 85mm F1.8 is also available in Canon EF-M and Sony E mounts.
Samyang 300mm f6.3 is also available in Canon EF-M , Micro Four Thirds and Sony E mounts.
Focal Range
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. Samyang 300mm f6.3 is a prime lens with fixed focal lenght of 300mm which has an effective (full-frame 35mm equivalent) focal range of 450mm when used on a APS-C / DX format camera.