This zoom lens gives you the same "reach" as a 70-200mm zoom lens in a 35mm size DSLR camera. By reach I mean here angle of view. It has the same f/2.8 maximum aperture as the best of competition. What is strikingly different is size and weight. This lens is seriously small compared to those DSLR lenses I am used to shoot with. I have had both Canon and Nikon f/2.8 lenses of this focal length area and they simply start to feel a burden after an hour of shooting and lugging them around. This one doesn´t.
Performance in tests
I did not do any formal test shooting as there are comparative tests done by others. As usual, let´s see what Photozone.de has found out with their Imatest system. Their general description of the lens is in the same lines I would also say. Thinking about resolution, Imatest MTF50 results show you can get excellent performance over the height of image for up to 40 cm high landscape images with Panasonic GX1. Excellent meaning that even the best printers can´t resolve beyond that. Images from any camera having this resolution or higher would show same amout of details in print. At full A1 size you have potential to very good performance over the height of image. I shot with Olympus E-M1 which has the same amount of pixels but no AA filter. With E-M1 one can get even more details.
From optical point of view I did not find any distracting vices. This lens is a solid performer without real weaknesses. I am using words like "distracting" and "real" in the sense that there is nothing to worry about, but on the other hand there really is no other lens to compare with. I actually don´t know yet where to set the limit. Cheaper lenses like Olympus M.Zuiko 40-150mm f/4-5.6 zoom are not in the same class and really can´t be used for any meaningfull comparison.
I only used Olympus E-M1 in-body stabilization and did no comparisons with Panasonic optical stabilization.
One more thing to mention is the excellent lens hood. It keeps the lens covered from oblique light hitting the front lens very well. This is extremely important for keeping micro contrast up.
I shot many kinds of subjects with this lens, which is quite surprising as I thought I would not find use for it. I´ll show here some aviation images which should give an idea of the usable reach for this lens. Click on images to see them bigger.
Note: Blue swastika in white roundel was used as national insignia by Finnish Air Force already since 1918. It has no historic or ideological connection with black swastika (rotated 45 degrees, i.e. standing on its corner) used by nazi Germany during thirties and WW II. This insignia was born when Finnish Air Force´s first ever plane was donated by count von Rosen from Sweden. He painted his good luck symbol on the wings of that plane, and Finns had no reason to change it but went on using it on all their airplanes. After WW II Finnish armed forces has used white-blue-white roundel as national insignia because of what swastika had become to represent for.