I purchased a Nikon Z6II and a Nikkor lens about a month ago. I just learned that Nikon will not be permitting Z lenses to be made by third-party vendors (i.e. tamron, sigma, et al). I know Nikon makes great glass, but they are expensive, as everyone knows. So now I'm faced with keeping the Nikon or returning it for what was my close 2nd choice, a Sony a7iii. As some one here on UHH mentioned to me, there are a great many 3rd party Sony lenses. While I realize I could purchase the Nikon Mount Adapter FTZ II, it is $250.00 on top of the cost of whatever lenses I might buy. You may also sacrifice some lens functions with the when using other lenses with the adapter. I don't have a big budget for extra glass. I shoot pretty much everything, regardless of its nature (i.e. landscape, portrait, fast moving subjects and so on). I'm pressed for time so I need you input ASAP. Please, please no lectures, just advice. Many, many thanks.

To be clear, there are significant lens flange-to-sensor distance differences between dSLR lenses and mirrorless lenses. The shorter flange distance on mirrorless designs, due to elimination of the mirror box, enables better optical designs, especially for wide angle lenses and short zooms. Lenses specifically designed for mirrorless bodies are likely to perform better than their older dSLR versions, both because of the shorter flange-to-sensor distance, and due to improvements in computer-aided design, lens coatings, and lens manufacturing processes.

All THAT said, dSLR lenses that are correctly adapted to mirrorless bodies should perform as well OR BETTER than they did on dSLRs. Most people notice an improvement in autofocus performance over what they had on their older dSLR bodies. Optical performance should be the same, although better sensors in the mirrorless bodies may eke out a bit more image quality from an older lens.

When mounting some older lenses on very high megapixel count cameras, you reach a point where the lens cannot resolve what the sensor can record. That is where newer, better, designed-for-mirrorless lenses excel. At 24MP, you probably won't have issues with older dSLR lenses. At 60+ MP, you probably want the latest, greatest glass you can find. OR, you need to examine your reasons for buying a 60 MP camera, to be sure they are sound! (Bias-ply tires don't work well on a high end sports car, do they?)

The title of this thread is a bit misleading as Sigma is not producing lenses for Canon or Nikon mirrorless cameras--neither Canon nor Nikon are allowing Sigma to do so at this point. Thus the use of Sigma lenses on Canon or Nikon mirrorless bodies involves the use of adapters, which is less than ideal.

Sigma produces L-mount lenses, and of course they make lenses for the Sony E-mount. Early on, Sigma adapted Art lenses designed for DSLRs to the Sony E-mount. More recently, they have developed a line of lenses specifically for the Sony e-mount, designated as DG DN. These are quite good, but so far the only significant contribution to mirrorless systems.