The Olympus M.Zuiko ED 12-45mm f/4 PRO offers very high performance in a very small package. In terms of sharpness, it doesn't show any major weakness across the zoom range. If anything, the borders/corners could do with a bit more bite at the 12mm. Micro-Four-Thirds is auto-correcting most of the other technical image aspects. Thus there's just a hint of barrel distortion at the wide and the vignetting is quite moderate. When looking under the hood, the distortions are extreme at the 12mm setting, though. Lateral CAs are low and nothing to worry about. The quality of the bokeh is generally Ok within the limited capabilities of the slow max aperture at least.

Despite being tiny, the lens remains true to its "PRO" designation. It features a tightly assembled, weather-sealed, metal body and smooth control rings. The AF is both quick and noiseless.

The Olympus M.Zuiko ED 12-45mm f/4 PRO is certainly not a lens for everyone. The rather slow max aperture was subject to quite some debates at the time the lens was announced. While it is a limiting factor for some use-cases, you have to keep in mind that Olympus had different design objectives here. If you want more speed, just go for the ED 12-40mm f/2.8 PRO. However, if you want maximum portability, the ED 12-45mm f/4 PRO has its merits - especially when combined with one of the smaller Olympus cameras like the E-M5 III. Especially outdoor enthusiasts may appreciate such a package.