I did a similar article for Nikon DSLRs, so now let's do the same thing for mirrorless: You got a new mirrorless camera kit for Christmas, congratulations. You've got a little money left over and you want to make the one upgrade that has the most bang for the buck. That's generally going to be a better lens than the kit offers.
For each mount, here are my suggestions:
- Fujifilm Kit — Relax, the kit lenses are all fine. I was actually surprised to find even the X-A1's kit lens, the 16-50mm f/3.5-5.6, is not a slouch. I suspect that the thing that would stand out most optically for Fujifilm users who got the kit lens is to get one of the f/1.4 optics (23mm or 35mm).
- m4/3 Kit — If we're talking one of the lower end models with a variable aperture lens, generally the best upgrade is the same as I mentioned with consumer DSLRs: add a prime lens to your collection, in particular the Panasonic 20mm f/1.7 if you're trying to stay small and on a budget. The Olympus 12mm, 45mm, and 75mm are also lenses that are going to show you what your m4/3 camera can really do, especially that last one, which may be the best prime lens we've seen from a major manufacturer yet for for m4/3.
- Nikon 1 Kit — Not a lot of choices for an optical upgrade, though I'd point out that if you didn't get the 30-110mm f/3.8-5.6 with your kit, you might want to start there, as it's a very good lens (and optically better than the 10-30mm kit lens). The other two logical choices are the 6.7-13mm f/3.5-5.6 and the 18.5mm f/1.8. I'd pick the 18.5mm first and foremost, because you need all the aperture you can get on the small sensor Nikon 1 cameras, and used well, you can actually get a bit of usable depth of field isolation with that lens that you won't be getting with the kit zoom.
- Samsung Kit — The 16mm f/2.4 would be my choice here, though every one of the Samsung primes (16, 20, 30, 45, 60) are excellent. The 16mm (and 20mm or 30mm) makes for a very compact system that shows off what the Samsung 20mp sensor can do.
- Sony NEX (APS) kit — Sony's E-mount lenses have been a little on the underwealming side. But primes to the rescue: the 35mm and 50mm f/1.8 really do start to show off what the Sony APS sensor can do, and they're nice and compact, as well, which fits the (old) NEX scheme to a T.
- Sony A7 or A7r — Real simple answer: the 35mm f/2.8. If you have one of these cameras and don't have that lens, you don't know what your camera can do yet. The 28-70mm kit zoom doesn't begin to approach the 35mm.