Most people who had a Micro Four Thirds camera by this time probably already owned a kit lens which covers the 20mm focal length, e.g., the Lumix 14-140mm, Lumix 14-45mm, or Olympus 14-42mm. So why add another lens in the same range?
There are several reasons why a lot of people like this lens. The focal length corresponds to 40mm on an old style 35mm film based camera, which can be considered as a slightly wide normal focal length. While there is nothing magical about this "normal" focal length, it is considered to give images that look normal to a human observer, in terms of perspective. It sits between wide angle and tele lenses, and while different photographers have different tastes, it remains a useful focal length for a lot of situations.
Another reason for the popularity of this lens is the speed. The lens features a large maximum aperture, f/1.7, the largest aperture so far for a micro four thirds lens. This gives two advantages: A large aperture means that you can take pictures in less available light, e.g., indoors without a flash, or at a concert where a flash would disturb the artist.
A large aperture also gives a narrow depth of field. When you focus on an object at a certain distance, the depth of field is the range of distances in front of and beyond this focus point that are also in focus. When taking a portrait, it is common to use a narrow depth of field, to make the background go out of focus, making the subject stand out more. This subject isolation technique may be impossible to achieve using a kit lens with a maximum aperture of, say, f/4. Hence, adding a fast prime to your lens collection may be a neccesity, if you want to take traditional portrait pictures.
The lens is also loved for it's pancake characteristic: It is very small, as can be seen by the comparison with the Lumix G HD 14-140mm kit lens here. It delivers the compact size promised by the Micro Four Thirds system.
A large aperture comes at the expense of sharpness. At maximum aperture, this lens is remarkably sharp in the center, but soft in the corners. This is not unusual for a fast prime lens like this. Stopping the lens down to f/2.8 improves the corner sharpness, but not by an impressive amount. To get the best sharpness across the whole frame, you will need to stop down to f/4 or f/5.6. In real life, this is not a big issue, since you're not likely to need sharp corners when you're photographing at full aperture.
I have compared the 20mm lens with the more recent Lumix G 14mm f/2.5 pancake lens. My study shows that both lenses are very capable. The 14mm lens might be slightly less sharp in the corners. But for most uses, neither lens will disappoint.
I have made a sharpness comparison with the Sigma 30mm f/1.4 lens, which is priced similarly. In my experiment, the Lumix 20mm lens is by far the sharpest. On the other hand, I used different cameras in the experiment, and the Pentax K10D, on which the Sigma 30mm lens was mounted, has an older generation sensor.
I have made a study of the chromatic aberration (CA) artifacts of this lens, and some other prime lenses. It shows that there are some quite small red/green fringing artifacts in the corners of the frame, but it is effectively corrected by software.
All the example images on this page are from the out of camera JPEG images, which have been adjusted for CA artifacts. Panasonic Lumix G cameras automatically apply this correction when it produces JPEG images, and it is also done by some RAW conversion programs.
Even before this correction, the artifacts are not very annoying. So users of Olympus cameras, which to date do not apply this correction, should not find this a big problem.
This is not an "HD" designated lens, and the autofocus speed is not stellar. For normal use, this should not be any issue at all. But for action photography, you may experience that the focus is not fast enough. The autofocus is more noisy than the Lumix G HD 14-140mm lens, but it should not be any problem.
According to the specifications of the lens, it is not supposed to autofocus while recording movies. I've found this to be untrue; on the Panasonic GH1 it does actually autofocus continuously while recording movies, however quite slowly. It takes a couple of seconds to regain focus if you move it around.
Here is an example video capture using the Lumix 20mm f/1.7 pancake lens. Notice how the lens focuses in and out now and then. This is the camera's way of verifying that the image is in focus, I guess.
The focus mechanism is the traditional one, in which the whole lens assembly moves back and forth. This has a positive side, that the focal length remains fairly constant during change of focus, and it avoids the "focus breathing" effect during video recording. On the other hand, a moving lens assembly is slower, requires more power from the batteries to operate, and is less solid and water resistant compared with rear focus or internal focus. The newer 14mm f/2.5 pancake lens features internal focus.
Not a portrait lens
You would not normally classify this as a portrait lens. If you move close enough to get a head-and-shoulders portrait with this lens, you're so close that you're going to get distortions of the perspective. The can be noted by objects close looking too large, typically the nose or the chin. To get naturally looking portraits, you need a longer focal length, so that you can take a picture from distance.
On the other hand, you can of course take a step back and take the picture at a larger distance, for an environmental portrait. That can solve the distortion problems. Keeping a distance of at least 1 meter (3 feet) from your subject will solve the distortion problems when photographing people.
Portrait lenses for traditional 35mm film cameras are typically 85mm f/1.4-1.8. This would correspond to a focal length of 43mm on Micro Four Thirds. At the moment, the Panasonic Leica DG Macro-Elmarit 45mm f/2.8 1:1 Macro Mega O.I.S. is the closest to a portrait lens in the Micro Four Thirds lineup. However, with a maximum aperture of f/2.8, it is not really fast enough to be considered a genuine portrait lens. The Four Thirds Olumpus 50mm f/2 is closer, but it requires an adapter, and does not auto focus on Panasonic Micro Four Thirds cameras.
Traditionally, lack of distortion in one factor which is commonly associated with quality lenses. The Lumix 20mm pancake lens is special in this way, since it features a fair amount of barrel distortion. However, this distortion is automatically corrected in post processing inside the camera. So most users will never notice.
Employing post process distortion correction has the potential of keeping the lens smaller, less complicated, and less expensive. At the same time, the designers can concentrate on having the optics correct for issues which can not be corrected in post processing. I think this system is a very good idea, since it gives the user better lenses at smaller sizes. So far, we don't really see better prices for the optics, though.
I have investigated the bokeh of the lens, and found that it has somewhat hard edges for out of focus highlights, but otherwise the bokeh is pleasant and even, without ringing.
Here is another bokeh study, where I compare the Lumix 20mm lens with the sigma 30mm f/2.8 and Olympus 45mm f/1.8.
Filters and hoods
The filter thread diameter is 46mm. No hood is supplied with the lens, which is quite unusual for Four Thirds lenses. Personally, I would have liked to see a hood for this lens, as the hood not only protects against stray light, but also protects the front lens element from objects touching it accidentally. To find a hood for this lens, you could try searching the internet for a 46mm screw-in hood. Here is an example hood that can be got on ebay for about US$10.
Originally intended for Leica Summicron, this hood is not optimal for the 40mm field of view of the Lumix G 20mm, but it does some job of keeping out stray light, and it does protect the front glass element against foreign objects. There is no extra vignetting due to using this hood.
My preferred solution is to use a 46-37mm step down ring as a hood for this lens. It looks like this:
You'll also need a new front lens cap with a 37mm diameter. Both can be ordered from various auction sites for less than US$10 in total.
Be warned that you cannot use a step-up ring on this lens. When powering up or down, the lens will retract the filter thread a little bit into the chassis of the lens. A step up ring is wider than the thread, and hence will jam the focus mechanism when attached to the lens. This could potentially damage the focus motor. A step-down ring could also be problematic, since they usually have a wider collar which could jam in the same way. I have had the lens jam when using a step-up ring, so be careful. Mounting a stand off extension ring between the lens and the step-up ring should be ok.
This lens does not feature any image stabilization. When used with Panasonic Micro Four Thirds bodies, you do not get any optical image stabilization at all. Some would say that with a fairly wide angle (40mm in 35mm camera equivalent), and with a fast aperture, the need for image stabilization is minor. However, there are many cases where the available light is not enough for a handheld image, and OIS would have made a difference.
Using the lens on an Olympus body with sensor based image stabilization, e.g., E-P1, E-P2, or E-PL1, will give you working image stabilization with this lens.
Another drawback of the lens not having any image stabilization built in, is that it also lacks an orientation sensor. This means that a Panasonic camera cannot autorotate the image based on the orientation when shooting. Again, using an Olympus body will fix this problem.
New version of the 20mm lens
In the summer 2013, this lens was discontinued, and a new version of the lens, Lumix G 20mm f/1.7 II (H-HS020A) was released. The new version has the same basic specifications, and has the same optical design. The exterior design is new, though, with a black or silver metal finish.
As the new lens has the same optical design, it still has the old style focus assembly which moves all the lenses back and forth. Reports indicate that the focus speed is the same as the first one, i.e., not very impressive. Even with the new design of the lens, the autofocus is still the slowest among the Micro Four Thirds lenses.
So the only reason to buy the new version of the lens would be if you prefer the new design to the old one.
The new designs of the Panasonic Lumix G 20mm f/1.7 II:
Compared with the Lumix 14mm f/2.5 pancake
It is natural to compare the lens with the other slim pancake from Panasonic: The Lumix G 14mm f/2.5. In the picture below, in which I am using a step down ring as a simple hood on the 20mm, it is very clear that the newer 14mm lens is significantly smaller. However, note that the 14mm lens has the slimmer rear lens cap mounted, which makes it even smaller.
The lens designs are rather different. The 20mm lens features 7 lens elements in 5 groups (2 aspherical), while the 14mm lens has 6 elements in 5 groups (3 aspherical).
The two lenses are quite different when it comes to autofocus, as well. The Lumix 14mm lens uses a different focus mechanism compared with the Lumix 20mm. Whereas the Lumix 20mm lens uses a traditional focus method, in which the whole lens assembly moves back and forth, the newer Lumix 14mm lens has internal focusing.
The advantages with internal focusing are apparent: The elements that need to be moved are smaller, hence, they can be moved faster, more quietly, and using less energy. Also, the front of the lens does not move at all, which makes the lens more rigid, and less prone to water contamination.
In use, it is immediately apparent that the Lumix 14mm has the fastest autofocus. My examination reveals that it is about twice as fast. Still, it is not as fast as the HD rated Lumix G HD 14-140mm superzoom lens.
In terms of field of view, the lenses are rather different. The 14mm is a wide angle lens, while the 20mm lens can be considered a slightly short normal lens.
So which lens should you choose? The Lumix G 20mm f/1.7 lens is the true low light lens, with a significantly larger aperture. It is a good lens for indoor photos of people, without using flash.
On the other hand, it can be a bit narrow for photography indoors, if you want to include a group of people. If the space is limited, you may want to use the 14mm lens to cover a sufficient field of view.
A fun fact is that the lens is smaller than the Nikon F adapter alone, as shown in this comparison image.
A low light concert movie using the Lumix G 20mm f/1.7 pancake lens on a GH2:
More information about the video parameters used.
The following video was recorded outdoors using the Lumix 20mm f/1.7 lens on a GH1. You'll see that the camera loses focus now and then, which is a bit annoying. Both the lens and the camera have had firmware updates since this video was recorded, and the autofocus performance during video has improved.
More information about the video parameters used.