Pana 20mm - Difference btw 1.7 and 1.8?
The 20mm is pretty much sharp across the board until you get to the smaller apertures where maybe diffraction kicks in. It is just a teeny tiny bit sharper at f/2.8 than wide open. It actually has less chromatic aberration wide open. So there is not much advantage if at all in stopping it down. See http://www.photozone.de/olympus--four-thirds-lens-tests/464-pana_20_17?start=1