Unless you must have only have the smallest, lightest lenses, the 12-40 f/2.8 is the way to go and may be all you need on your m43 camera. This lens is a little heavy but it's sharper than the 12-45 f/4. Witth image stabilization in both lens and body, f/2.8 isn't a bad maxiumum aperture. Optically, the newer mk II is the same as the original although I have to admit the mk II I bought is just slightly sharper than my old v.1 lens. Probably just the years of wear and tear on the original.