I recently bought a used, 5-year-old Leica SL. I didn't buy any new lenses at the time, as I wasn't sure I'd even like the camera. Turns out I liked the camera very much, so I ordered a Sigma 24-70 f2.8 zoom. I figured the zoom would cover my bases but I also bought the Leica M-to-L adapter so I could use my Leica M lenses.
The M lenses work flawlessly on the SL, and are even easier to focus on it, given the super bright EVF and focus peaking. M lenses are wonderful, but they are manual focus only. I was using the Sigma zoom a lot and falling for the convenience of auto-focus. This got me thinking about prime lenses for the SL. I prefer primes in almost all cases and so the research began in earnest.
The Sigma zoom is fine, but I really wanted a Leica native SL lens. And I wanted a prime. But what focal length should I get? Since I can only afford one lens (by "afford" I mean sell off most of my other gear to pay for it), I opted for the APO-Summicron-SL 35mm f/2 ASPH. I can never decide which length I prefer, 50mm or 35mm, but I went with the 35 because I feel it's just a tad more flexible.
I must admit that hearing Peter Karbe, Leica's designer of the M and SL lens lines, say that the APO-Summicron-SL 35mm is his favorite Leica lens and that it might be the best lens Leica has ever made helped push me over the edge. You can listen to Peter go into glorious technical detail about the SL lenses for like an hour and a half in this video.
The lens arrived a few days ago and so far all I can say is that my first impressions are that it has easily exceeded my high expectations. It's beautiful, and the construction and feel of it are just wonderful.
The SL lenses are significantly larger than the M lenses I'm used to. Here is a photo showing the size difference between the tiny, jewel-like Summicron-M 35mm f/2 ASPH and this new 35mm f/2 SL.
A nice feature of the SL lenses is close focusing distance. The SL lenses are more useful close up. For example the SL can focus down to 27 cm while the Summicron-M can only get to within 70 cm. That matters a lot more often than one might think.
What about image quality? Good question. I don't know much yet, as I've only been plinking around the house. What I do know is that the few images I've taken have looked fantastic. Most decent modern lenses make great images, but I have convinced myself that the files coming out of the SL with the APO-Summicron are somehow even better. To my eyes, they are noticably, meaningfully better, and that's the only criteria I need.
Is the APO-Summicron-SL 35mm ASPH worth $5,000? Of course not. No one needs a lens like this. I certainly don't. I mostly take photos of my dog or selfies or family snapshots, so there's no point trying to justify the cost other than to say that it might be the best lens I've ever used. It feels fantastic. It looks fantastic, and, combined with the SL, it makes fantastic images and I'm very happy with it.