Considering updrading from RX100 to NEX-5N or NEX-6

You mentioned shooting the RX100 at the WA end of the zoom frequently.  At that range, the max aperture is f/1.8.  On the NEX zooms, it's f/3.5 which is two stops slower.  That means that in low light, trying to hold a certain shutter speed, shooting wide angle, you'd be shooting the NEX at a higher ISO (two stops higher).  Some people might quibble over which option yields better results, but the difference will not be worth swapping cameras for.

Toward the tele end, the RX100's max aperture at 75mm equivalent (same as the tele end of the 16-50) is higher (a bit over f/4 if I remember) which means you're setting the ISO on the NEX about 1 stop higher. This should yield a better result, but not better enough IMO to switch).

OTOH, if you also see value in the tilting LCD and (if looking at the NEX-6) EVF, then you're not going to see worse results with the NEX.

Basically, if you upgrade the sensor, but use a slower lens, you're not exploiting the capabilities of the larger sensor.  The same is true with full frame versus APS-C, but there, it's easy to find fast lenses (except at the tele end).

If you'd consider the 35/1.8 then I'd say you can expect better low light shots.  (Or the other f/1.8 lenses).  If you were to go with one of the f/2.8 Sigma's, then you're still not doing all that much better than your f/1.8 wide angle shots.  (If you were to compare the 50/1.8 to the RX100 at 75mm equivalent, then the NEX would kill the RX100 because you get the benefit of the larger sensor *and* a faster lens).

Another consideration is image stabilization.  I don't know if you shoot slow shutter speeds or if you need to keep them high enough to avoid motion blur, but the RX100 reputedly has fairly mediocre IS; NEX has IS in most of its lenses (it's good in the 18-55 and 18-200; haven't tried it in the primes), but the Sigma's don't, and the Fuji X100 does not.

Summary: (my personal point of view, of course) ...

RX100 wins on convenience, NEX wins on tilting LCD and EVF (some models).  For low light, NEX wins with fast primes at wide angle and ekes out a slight win with slower lenses at tele settings.  (It clobbers RX100 with 50/1.8 versus the same zoom setting on the RX100).  RX100 never really "wins" for low light, but should hold its own against kit zooms (again, loses by a bit at tele setting, but not by so much that I'd give up the convenience).

- Dennis

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