I was told that Nikon will officially license third-party autofocus lenses for Z-mount only if they complement Nikon’s current lens lineup, rather than compete with the existing Nikkor Z lenses.
You can already see that strategy with the current third-party Z lenses from Tamron: the Nikon Z 28-75mm lens by Tamron is not the same as the Nikkor 24-70 f/2.8, 24-70 f/4, etc. The Tamron 70-300mm f/4.5-6.3 Di III RXD lens for Nikon Z is also a unique focal length zoom in the lineup.
This is not the case with manual focus lenses – Nikon can authorize the same focal lengths as already existing Nikkor Z models – for example, most of the Voigtlander Z lenses have the same Nikkor focal length equivalent.
You can expect the same scenario with the upcoming Sigma lenses for Z-mount.
