bobn2
Forum Pro
Cheap adapters can lose you image quality my failing to mount the lens parallel to the sensor, or by having sloppy mounts which don't locate properly. However, spend enough for a good one, you should be OK.
As others have said, unless you gat a 'smart' adapter, you'll lose AF and aperture actuation. This is a big problem if your lenses don't have aperture rings (that is they are G type lenses or later. In the F mount aperture actuation is mechanical, so the adapter needs a motor in it to drive the aperture. I would guess that there could be a problem with accuracy of aperture selection if the adapter isn't carefully manufactured, another reason why a satisfactory adapter is likely to be expensive.
So far as I can see, there are two autofocus capable adapters from F mount to Sony FE mount, one from Fotodiox and one from a venture called MonsterAdapter (yes, really). The Nikon F mount is a tricky candidate for adaptation to a mirrorless camera. Nikon made a good adapter for their Z system, but they had the advantage of all the inside knowledge on how both protocols worked. An independent vendor reverse engineering things might have a more difficult time, although Sony does licence the FE mount protocols, at least in base form.
I haven't used either of the above adapters, but my general experience with these cross-brand independent adapters is that they range from unusable to reasonably useful in limited circumstances. Unusable might range from failure to work at all with a large selection of lenses, focus hunting and unreliable focus or triggering camera fault modes. Reasonably useful tends to mean decent, if a bit slow performance in single-shot AF modes across quite a range of lenses. I've never had one that can be relied on with every lens, nor one that works well enough to be usable for continuous autofocus or subject tracking.
I would expect that any Nikon F lens used with an adapter on an A9 would not be any good at all for any kind of sports photography, so given that is what you want to do, I'd forget that idea. If you were a landscaper or portraitist, yes, you could likely get some use. I think you have two options. The first is to sell your Nikon lenses and use the cash to start to build a Sony lens system. their lenses have lately been excellent, so there is no loss in function there, though obviously some loss of cash.
The other option is to be a little patient and wait for Nikon to nail mirrorless AF. I'm pretty confident that they will get there. Already firmware updates have produced big improvements, and you can expect the forthcoming Z9 to have as good AF as anything on the market - that will be Nikon's number one goal, and I expect them to achieve it. Whilst that's likely to be outside your price range, you'd expect them to spread that type of AF performance to models lower down the range quickly. Nikon is in a position where it has no option to perform, and that's a big incentive to get it right.
