I recently made the switch to Olympus and purchased an OM-D E-M1X along with three M.Zuiko PRO f/2.8 zoom lenses. I bounced back and forth between the PRO 12-40 mm f/2.8 and the PRO 12.-100 f/4. I eventually went with the 12-40 f/2.8 to gain the additional stop of light. It was a great decision. This is a fantastic lens... sharp... fast focusing... and versatile. The extra stop of light comes in very handy. When compared to the 12-100 mm f/4... I only gave up 1/2 stop of IBIS performance which is no issue for me.

The hidden advantage of the M.Zuiko 12-40mm f/2.8 lens is its completely acceptable infrared (IR) capability, especially when combined with a suitable Olympus camera body. Actual performance using a customary IR filter shows no "hotspot," which is a problem actually with other M.Zuiko lenses, and no substantial loss of contrast or ghosting, which also is often encountered. Unfortunately, the usual IR lens database (often cited online) is erroneous concerning several Olympus lenses and also fails to list this particular lens. Two online tutorials both demonstrate the IR capability of the M.Zuiko 12-40mm f/2.8 lens but do not point out that not all M.Zuiko lenses are IR-capable.