I love my E-M1 and my collection of primes. I have the Samyang 7.5, P20, O45, The Bokina, Canon fd 100 f2, and one zoom, the magical canon fd 80-200 f4 L. On a whim, I snagged the Olympus 12-40 from the refurbished page and its going back. It is as sharp as the P20 and the output is identical, during the day time. If all you want is a great lens with good range to walk around town all day and shoot in the sunshine, the Olympus 12-40 is terrific. But it is not a night time lens. Focus is very slow at night. On the E-M1 my copy hunts for focus worse than the P20 on the E-M5. Colors and contrast at night are not as good as during the day. For me, my primes cover the focal ranges I need. I thought the convenience of a fantastic zoom would suffice. But I work during day when I travel on business and do a lot of shooting at night. The zoom is going back.