I bought an RF 16mm. Here are my observations following some testing:
- Sample variation can be significant. I shot jpgs with two RF 16mm's in the shop and compared the jpgs on my laptop. One was noticeably sharper than the other across the frame at f/2.8 and f/4. I looked more closely when I got home using a split screen on my monitor and this confirmed what I'd seen on my laptop in the store. There did not appear to be any inaccurate focusing in the softer lens. I experienced the same comparing two RF 50mm f/1.8 lenses.
- Raw files converted in DPP and imported as tiffs into Lightroom are significantly sharper than in-camera jpgs imported into Lightroom. For critical subjects its worth processing in DPP.
- I shot some test charts on the side of my house. There is a noticeable improvement stopping down to f/4. F/5.6 is still a little sharper.
- I compared the RF 16mm against my fully manual Samyang EF 14mm f/2.8. The Samyang is noticeably sharper and has noticeably less colour fringing than the RF lens. The difference is (not surprisingly) greatest in the corners. The RF 16mm stopped down to f/4 is still not as good as the Samyang wide open.
- I tried to manage my expectations, but I had hoped that the RF 16mm at f/4 would match the Samyang at f/2.8. That is clearly not the case. It remains to be seen if the RF16mm is usable as an astrophotography lens. I might be able to test this tonight as we will have clear skies, but an almost full moon. I am particularly interested in coma.
- As you will already know, the RF lens is much, much smaller and lighter than the Samyang, especially with adapter. It also focuses much closer. See the photo of an isopod on our beach.
- So overall, I am sure that the RF 16mm will get a lot more use than the Samyang 14mm. It is a perfect complement to my RF 24-105 f/4L, especially when I go hiking. But it looks like I will be keeping the Samyang for astro photography.