If you have never heard of a DKL lens mount, don't worry; most people have not either. In truth, there aren't that many DKL lenses to begin with, and most of them are small aperture lenses with maximum aperture of f2.8 or f4. The only exception is the Schneider-Kreuznach 50mm f1.9 Xenon, with a maximum aperture of f1.9. There are many variations of this lens, and most popular in Exakta mount. There might be a M42 mount of this lens as well, but it would be very rare.
Colours & Bokeh -- NEX-5N & Schneider-Kreuznach Xenon 50mm f1.9 DKL. Click for larger
One other rather disappointing aspect of the DKL (and probably same for other mounts) is the pretty long minimum focus distance. Even for most of the 45mm to 50mm lenses, it's something like one meter (three feet), and a 135mm f4 is 2.5 meters. Again, the only exception, as far as I know, for the 50mm focal length, is this Schneider-Kreuznach 50mm f1.9 Xenon, with a 0.6 meter minimum focus distance. This is the reason why the 50mm f1.9 Xenon is the most popular and sought after lens for DKL. Of course, the great optical performance of the lens helps too.
Big Cat -- NEX-5N & Schneider-Kreuznach Xenon 50mm f1.9 DKL. Click for larger
Yesterday I received a dedicated DKL to NEX adapter which supports aperture of f1.8 and I can now finally use the 50mm f1.9 with full aperture control. Personally, I think a lens shows its best personalities at maximum aperture, especially its bokeh, and I love the bokeh of the 50mm f1.9 Xenon. It's delicious!
Roses -- NEX-5N & Schneider-Kreuznach Xenon 50mm f1.9 DKL. Click for larger
The 50mm f1.9 Xenon is indeed quite a nice lens. The build quality is exceptional and the chrome finish is exquisite and beautiful. After decades, it, like all my other Schneider DKL lenses, still looks stunning. Most of the lens is made from brass and though small, it has a good heft and feels dense. Even wide open, this lens is uniformly sharp across the frame on the NEX-5N. I am sure the true corner is not as good in full frame, but it's excellent on the NEX-5N. Fringing is actually quite well controlled, as is flare. Stopping down to f2.8, sharpness and contrast improve quite a bit.
Do I like this lens? Quite a bit. Be sure to look for the 0.6m minimum focus distance (MFD) version, as I think there might be versions exists with longer MFD.
Back lit leaves -- NEX-5N & Schneider-Kreuznach Xenon 50mm f1.9 DKL.