Roel,
The following is my personal opinion, and it is no more than that, so please bear that in mind. However, it is an opinion based on actual long term experience, not based on ideas I picked up from reading and believing internet reviews or any other kind of received wisdom. I am not parroting anything, in other words. I owned and used the Leica SL for several years as my main digital body, and replaced it with a Nikon Z7 a year and a half ago.
At the time, I was in no way unhappy with the SL. I liked the ergonomics, and thought the button layout was genius, and was happy with the results, though was always a bit ambivalent about the capabilities of the SL sensor after coming from a Nikon D810 and having used a D850. The SL resolution was fine for my needs, it was the color depth and DR that I found to be a bit lacking in real world use. (To put that in context: It’s a great sensor in historical terms, just perhaps already a generation behind the day it was released, as Leica sensors have tended to be for years now. Sorry. It’s not an issue of resolution. Nor is it in any way a “bad” sensor, it just is what it is.)
Something to keep in mind while trying to decide if my experience is relevant to what yours might be is that I tend to gravitate towards using adapted manual focus lenses on mirrorless bodies simply because there are some rendering traits that I can get from specific lenses which are of value to me, and autofocus isn’t something I particularly need most of the time. I had the normal range “kit lens” zoom for the SL, and it’s a superb lens, but was a little big, even for me, and I am not one who is bothered by bodies or lenses which others seem to find either too big or too heavy.
The main reason I switched to the Z7 from the SL was that, although my preference is using manual focus adapted lenses, the cost of the native SL mount AF Leica glass seemed prohibitive in terms of building out an entire kit, especially to someone who would mostly only be using them when he needed auto focus.
Plus, some of the Nikon glass released in the recent past has been truly outstanding in every way ( the 28/1.4 E, and the 105/1.4E for example, as lenses whose characteristics were, for me, not only as good as anything Leica produces, but better. For me.) So my thinking was, at the time, that it would make more sense for me to go back to the Nikon platform and see how that worked, especially if the theoretical optical advantage of the Z mount over the F mount actually made a practical difference.
As it turned out, the Z7 has been a notably better camera for me to use than the SL was, in every way, even not counting the resolution “advantage” at all. The ergonomics, yes, though that is a personal preference. The sensor is just better in terms of what you can massage out of a RAW image, as compared to the SL, in terms of color, noise, and dynamic range. The resolution is incidental to all that, for my purposes. If one is only shooting jpgs, it is probably a wash.
One of the talking points in favor of the SL for Leica glass is the incorporation of in body profiles for Leica lenses. I used a lot of Leica R glass on the SL as well as some M rangefinder glass. Bottom line is that I now get better results with the R glass on the Z7 than I ever did with the SL. Every major RAW processing engine has lens profiles or you can create them yourself to suit. Whether the lens profile is applied in body or by your RAW processor is irrelevant. Wide angle M glass is going to be (somewhat) better on the SL than on the Nikon Z (see below) but that’s due to the way Leica has managed the microlenses re the sensor, it’s not due to the fact the SL has in body profiles. M glass 50mm and greater is going to be fine on the Z body, 35mm and wider will be better on the SL in terms of peripheral sharpness, but nothing else. Your 24mm, 28mm, and 35mm would be better on the SL in terms of corner sharpness due to ray angle considerations than they would be on the Z body. How significant that difference in corner sharpness is to you is a matter of personal discretion, when considered in tandem with the fact that there might be advantages of the Z sensor in terms of color, dynamic range, and resolution. There are pros and cons. If you are interested, you can find plenty of examples of shots made with e.g 21mm Super Elmar-M on Z bodies that look quite nice, even if lacking that last little bit of peripheral sharpness, and those are worth looking at if interested in separating theoretical considerations from actual obtainable results. Whether the issues with corner sharpness using wide angle M lenses on a Nikon Z body end up being noticeable enough to matter to you is, again, a personal decision, and should be weighed against other considerations. It wasn’t enough to keep me in the SL or tempt me to return.
Time will tell, but judging from the MTF results Nikon has achieved with the 58mm/ 0.95, the 50mm/1.8, and the 70-200/2.8, my guess is that the upcoming Nikon 20mm/1.8 will be, at least in terms of sharpness, corner and otherwise, sharper than anything coming from Leica, or at least as good. If I want an ultimate wide angle lens, thats where I would tend to look for a solution, and use wide M lenses elsewhere, though, actually, the imperfections in corner sharpness of wider M lenses on Z bodies is minor enough to be irrelevant to me, personally. If I were doing commercial product photography where that mattered more, or mattered at all, I would feel differently.
Rendering is another matter entirely. And, I have been personally ambivalent about the overall rendering from some of the Native Z lenses. Love the 85/1.8 and what I have seen so far from the 70-200/2.8, not completely convinced by the rendering of the 50/1.8, though that is completely subjective (and the 50/1.8 is an Otus level lens in terms of sharpness and corrections, and really cheap. Not cheap for a nifty fifty, but cheap for what it is.) Am looking forward to the 50/1.2 this year as likely providing a different take on what a lens might do.
