Published in DAP in my Sony NEX-7 review is a page detailing the diffraction cutoff aperture for peak sharpness and contrast. The results might come as a bit of a shock for those not versed in such things, at least for those who shoot top-grade glass.
This is not an effect specific to the NEX-7 or any lens; it applies to all cameras and all lenses. See Making Sharp Images for detailed coverage of diffraction.
Which image below would you prefer? The only difference is the aperture (left/right halvs shown).
Read my report to understand which apertures must be avoided (with all lenses) on the NEX-7, and why well-corrected f/1.4 and f/2 lenses should be on your short list for the NEX-7.