The diminutive 20mm F1.7 isn't the smallest lens currently on the market (the Olympus M. Zuiko Digital 17mm F2.8 Pancake shaves a couple of millimeters off in each dimension), but it's unusually fast for such a compact design, letting in a stop and a half more light than the above-mentioned 17mm (or Olympus's Four Thirds-mount Zuiko Digital 25mm F2.8). Construction is reassuringly solid, with high quality finish and a metal mount. The weight is just 100g.
Design-wise there's nothing much to see - the manual focus ring occupies a large proportion of the lens barrel, and the entire optical unit moves back and forwards to focus, with just 3mm travel between extremes. Notably there's no bayonet mount for a hood, presumably to help keep size to a minimum.
'Focus-by-wire' manual focus
The most unusual feature of this lens's operation is the focus-by-wire manual focus system, which drives the focusing group indirectly via the lens's autofocus motor. As a consequence, the feel of the manual focus ring never changes, regardless of whether the camera is set to auto or manual focus, or the focus has reached the limits of its travel, and this lack of tactile feedback can be a little disconcerting in some situations. However, this system does have some advantages; it allows gearing of the focus action, such that turning the manual focus ring quickly makes very fast initial adjustments, but then turning it slowly allows very fine subsequent tweaking. This essentially mimics the effect of having an extremely long focus travel, and consequently allows (in principle at least) a high level of manual focus accuracy.
One oddity we did notice is that, while Panasonic's G-series cameras hold the lens's aperture wide open while you're viewing the scene in record mode, the E-P1 has a habit of stopping down, presumably to regulate the amount of light reaching the sensor. This obviously increases the depth of field in the live view image, which can potentially lead to inaccurate manual focus when shooting at large apertures (exacerbated by a false impression that manual focus is easily obtained). Fortunately this behavior can be countered by assigning depth of field preview to the function button, and using it to force the camera to open the lens up to maximum aperture for manual focus (just hope you don't need to set a custom white balance, which also requires the function button, at the same time).
On the camera
The lens pairs most naturally with the Lumix DMC-GF1 (see our preview of that camera for many more images), but it works well on all Micro Four Thirds bodies. It makes a compact and portable package in combination with the Olympus E-P1, although there's something of a clash of styles in this case (the E-P1 is better matched stylistically by the Olympus 17mm F2.8 Pancake, and the 20mm F1.7 by the GF1). The 20mm also makes for a relatively small and discreet package when paired with Panasonic's SLR-like G1 (above right) and GH1.
Compared to a Nikon AF-S Nikkor 35mm F1.8G DX
Here we see the size advantage of the Micro Four Thirds system in a nutshell - the tiny 20mm mounted on a GF1 is far smaller than any conventional DSLR system with similar low-light capability, for example the Nikon 35mm F1.8 G DX on a D3000 as seen here. One will slip into a large-ish pocket for an evening out, the other most certainly won't (in fact the GF1 + 20mm combination takes up less space than the D3000 body alone).
Autofocus is driven by a micro motor in the lens body, which works very well. It's not as fast as the Panasonic 14-45mm or 14-140mm zooms, but it's not terribly slow either (obviously this depends to some extent on the specific body used, with the E-P1 feeling just a little bit slower than the G1 or GF1). The focus motor is very quiet, and unlikely to intrude on any occasion. The 20mm F1.7 is also capable of continuous autofocus in movie mode.
Lens body elements
Reported aperture vs focal length
The lens allows apertures from F1.7 to F16 to be selected.