But how do the M lenses perform compared to the Sony lenses? I can't understand why anyone would use their M lenses even if they under perform compared to the (most often) cheaper lenses designed specifically for that body - just because they can.
First of all there are very few lenses that would under-perform to such a degree...Native lenses may benefit from in-camera correction, but they also carry extra issues - look no more than the over-corrected NEX-7 in camera profile for the 16-50 OSS. I will say that all of my lenses above 24mm FOV produces breathtaking results on the A7, especially those above 75mm.
Here are my reasons for choosing Leica lenses for the A7, excluding performance:
1. Some focal length/aperture combinations are not available for any system other than Leica, and the "look" is unusal enough to make a difference (50mm f0.95, 21mm f1.4). Zeiss has the superb 15mm f2.8 ZM, which IMO is in a league of its own when it comes to superwides.
2. M lenses are usually physically much smaller than the Sony counterparts and have a smaller front filter size. Extra weight savings when traveling.
3. M lenses have the physical aperture ring which is a great thing to have. The only alternative would be the Fuji XF system, and only for some lenses. I for one enjoy the thought process of separating aperture and shutter settings to lens and body.
4. M lenses have superb build and focus feel, some thing which the Fuji XF lenses
do not offer. The well-built Leica focus mechanism is so much more comfortable to use than plastic manual rings or fly-by-wire
5. AF lenses are no less consumer objects than digital bodies, but M lenses have no such concern. I can be certain that a decade from now all of my M lenses will work properly if maintenanced. The same cannot be said for even the best-built AF lens.
6. I can adapt M lenses to more systems than one, and unify a digital/film kit. With Sony lenses I am stuck with Sony, but with M lenses I can carry one lens system and use, for example, the NEX-7 and my R3M side-by-side. I can also buy into smaller format bodies for the extra reach (EM-5) or change to another mirrorless system with minimum penalty. Should Fuji release a FF X-pro2, I can buy one and use it alongside the A7.
I actually wrote an article titled "Justifying the adapter" a while back when I began working with the NEX-7. The article is in Chinese, but I think my reasoning is quite nicely summed up here
😀