The Panasonic 20mm 1.7 pancake lens has entered the market with the start of the micro-43 system in 2009. The lens is very compact and light, without decreasing the optical performances.
Practice shots taken with the 12 MP Panasonic DF1 and the Panasonic 20mm f/1.7 ASPH LUMIX G20 mm, f/4.5, 1/500 s.
Construction and Autofocus
Despite its low weight and compact dimensions, this lens is solidly built and nicely finished. The drive of the autofocus takes place with a “drive by wire” system, making you feel no mechanical resistance when manually focusing. The focus ring also remains running when you have reached 20 cm or infinity. That takes some getting used to. But automatic focusing of the Panasonic 20 mm 1.7 on a Panasonic GH2 happens accurately and quickly. The lens is very compact; there is no room for image stabilization. But given the focal length and brightness of this lens, image stabilization will rarely be missed.
Vignetting
Only at full aperture, this lens clearly suffers from vignetting, as shown in the upper right corner of the recording below. Because this is one of the few micro-43 lenses with brightness under f/2, this lens will be used at full aperture relatively often. Fortunately, vignetting can be corrected well by software afterwards.
Panasonic 20 mm f/1.7 ASPH LUMIX G, f/2,8, 1/100 s
Distortion
The distortion of this lens is automatically corrected for both the RAW files (with most RAW converters) and jpg files. The end result is very good.
Bokeh
This lens offers a beautiful background blur/Bokeh at maximum aperture, as the above image cropping (taken at f/1.7) shows. But given the short focal length, you must be careful that you diaphragm as little as possible, otherwise you lose that beautiful vague Bokeh again. The right image was taken from the same point, but now with aperture 11. The red berries nearly one meter away from the green leaf in the foreground do not disappear into the background blur anymore now, but are displayed recognizably.
Flare
When testing the Panasonic 20 mm 1.7, we came across flare incidentally in the lab under extreme conditions (as shown at the left). Not in practice shots.
If you are planning to use this lens for night photography with many streetlights, you can occasionally encounter flare. But usually the – according to Panasonic – multi-coated lens elements to avoid ghosting and flare do their job properly and you will not suffer from flare.
Resolution
The resolution is tested by putting the sensor ratio of the test camera ( Panasonic GH2) on 2:3, making the measurement results directly comparable to the measurement results of lenses on a camera with an APS-C or full-frame sensor. If you use the camera in the standard 4:3 ratio, the number of line width per picture height is even higher.
The resolution of this lens in the center is very high, appears from both our practice recordings as our measurements. The best performance (with an image ratio of 2:3) was obtained at aperture 2.8 with 2000 line width per picture height (LW/PH) in the center, 1500 LW/PH at the edges and 1250 LW/PH in the farthest corners. 
From the measurements, it appears that the resolution in the extreme corners is always lagging behind the resolution of the center. In practice, it is not noticeable, because the resolution in the distal corners remains well over 1000 LW/PH to aperture 4. The Panasonic 20 mm 1.7, coupled with a Panasonic GH2, is between the very best lenses for Micro-43, APS-C and full-frame together in terms of resolution.
What is striking is the difference in resolution at the center between a shot taken at aperture 16 and aperture 2.8. Due to diffraction, the image taken with aperture 16 is much less sharp.
Chromatic aberration
According to Panasonic, the use of two aspherical lenses ensures that chromatic aberration is minimized. Moreover, both the RAW files (with most RAW converters) and jpg files are automatically corrected for chromatic aberration.
In our practice shots, we encountered no chromatic aberration with the Panasonic 20 mm 1.7 and the measured values for chromatic aberration are very low too.
Conclusion Panasonic 20mm f/1.7 ASPH LUMIX G review
Pros
- Good optical performances for resolution, chromatic aberration and distortion
- Bright
- Nice Bokeh
- Compact construction and low weight
- Solidly built and nice finishing
Cons
- Vignetting at full aperture
- no mechanical resistance when manually focusing
The Panasonic 20 mm 1.7 is a nice lens without real drawbacks. The optical performances are very good. The Panasonic 20 mm 1.7, coupled with a Panasonic GH2, nestles in terms of resolution between the best lenses for Micro-43, APS-C and full-frame together. From the measurements, it appears that the resolution in the extreme corners always lags behind the resolution at the center. In practice, it is not noticeable. The vignetting at full aperture can be corrected well by software, when it is necessary.
