One of the features of mirror-less cameras is the small distance between the lens mount flange and the sensor. That small flange to sensor distance opens up the possibility to use adaptors to mount older manual lens on the camera. If you can afford them, you can mount Leica M-mount lenses on a NEX, these lenses have the advantage of being designed for a mirror-less film camera which itself had a short mount to film distance. I don't have that kind of money to spend at the moment!
A few years ago my father in law died, and his old camera gear was passed on to me. I was looking through my store cupboard recently and found his Pentax Spotmatic complete with a Zeiss Jena Flektogon f2.4 lens. I did a bit of research and found that this lens has quite a following and fetches quite high prices on eBay so I decided to buy a Pentax M42 to Sony E-Mount adaptor.
The results are pretty good. There are a few camera settings which are important for this configuration. The first thing you have to do is set the 'Release w/o lens' option. Neither the Flektogon or the adaptor have any electrical connection to the camera so the NEX doesn't even know that there is a lens attached. The other setting is the 'Peaking level' which can be set off, low, mid, or high. I recommend setting peaking to low. Peaking low sets the focus sensitivity to its most precise, high shows areas which are definitely going soft as being in focus. There is another 'feature' of the NEX viewfinder which makes manual focusing much easier. The Sony electronic view finder (EVF) suffers from a shimmering effect (I believe caused by aliasing) which some complain about, but here's the thing, you only see the simmering when the subject is in focus. So, you have two things to go on when manually focusing, the focus peaking and the alias shimmer, this combination makes manual focus easy.
One of the great advantages of the EVF is that the image has the same brightness whatever the aperture. This does mean that care is needed. To obtain the most precise focus the lens should be set to f2.4 and then stopped down to take the picture. There are two ways to achieve that, one is to leave the camera on manual and change the aperture ring between f2.4 and the desired shooting aperture, the other is to use the auto/manual switch on the lens, when on auto the lens will always be at f2.4 (unless your adaptor holds the auto pin on the back of the lens in), flick it to manual to get your chosen shooting aperture. My post 'Why no depth of field preview button' talks about how the NEX 6 EVF shows the effect of your chosen aperture.