They are both similar on the specs. except for elements/groups, and image stabilization which seems to be an unnecessary addition. does the Tamron's VC justify its higher price?
The Tamron SP 85mm f/1.8 Di VC was introduced in 2016. The Nikon 85mm f/1.8G was introduced in 2012. Most lenses settle into a lower street price a few months after their introduction than the price they command when first introduced. It's still very early to see where the long term price of the Tamron will settle.
The most obvious difference is the inclusion of Vibration Compensation good for about 3 stops in the Tamron lens and the lack of Vibration Reduction in the Nikon lens. How useful VC would be depends on the actual usage of the lens. Shooting static subjects handheld in low light would make it very significant. Shooting moving subjects in brighter light or shooting from a stabilized position such as a tripod would make it fairly inconsequential.
The Tamron has more optical elements (13) than the Nikon (9).
The Tamron (24 oz.) is twice as heavy and seems to be built more solidly than the Nikon (12 oz.).
The Tamron has 9 aperture blades that are rounded compared to 7 non-rounded blades in the Nikon.
The Tamron has electronic connections for both AF and aperture control. This limits the use of the Tamron lens to newer bodies compatible with Nikon "Type E" lenses. That would include most bodies introduced since around 2007: D3100, D5000, D7000, D300, D500, D600, D700, D800, Df, and D3 as well as any later camera model in each of these cameras' series (e.g. D3200, D3300, D3400, etc.).
It also gives the advantages of an electronically controlled aperture: faster actuation, a potentially greater degree of accuracy without the need to periodically test and adjust the linkage mechanisms, and lack of susceptibility to the mechanical linkage being bent when the lens is attached to the camera. The nikon lens uses the mechanical linkage that has been standard on Nikon cameras for decades.
Most comparisons of optical quality between the two lenses show a clear edge to the Tamron lens at apertures wider than about f/5.6. This would make the Tamron a better choice for many as a portrait lenses where using large apertures for narrow depth of field is often desired. It would make little difference if the two lenses were being considered for usage on a tripod at f/8 or narrower.
The Tamron has two(2) advantages over the Nikon 85/1.8G:
- Vibration Compensation (VC)
- Weather Sealing