Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Zeiss Milvus 15mm F2.8 is the longer of the two lenses at 100mm. The Nikon Fisheye-16mm f2.8D with a length of 57mm is 43mm shorter. Besides being longer, the Zeiss Milvus 15mm F2.8 also has a larger diameter of 102mm compared to Nikon Fisheye-16mm f2.8D's 63mm diameter.
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. Nikon Fisheye-16mm f2.8D weighs 290g, 226% (657g) lighter than the Zeiss Milvus 15mm F2.8's weight of 947g.
Lens Mounts
Both Zeiss Milvus 15mm F2.8 and Nikon Fisheye-16mm f2.8D has the same Nikon F (FX) lens mount. Some of the latest released cameras that are compatible with the Nikon F (FX) lenses are Nikon D6, Nikon D780 and Nikon D3500.
Zeiss Milvus 15mm F2.8 is also available in Canon EF mount.
Focal Range
Zeiss Milvus 15mm F2.8 is a prime lens with fixed focal lenght of 15mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 22.5mm.
Nikon Fisheye-16mm f2.8D is a prime lens with fixed focal lenght of 16mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 24mm.