(From the left: 20mm and 14mm.)
Still, there are many issues which make them different. The Lumix G 14mm f/2.5 has internal focusing, which makes the autofocus very fast and silent.
The Lumix G 20mm f/1.7, on the other hand, has a more traditional focus assembly, which moves all the lens elements back and forth. This is not as fast, and does generate some noise. This is not an issue in daylight, since both are very fast in sufficient light. But in low light, the 20mm lens can take in excess of one second to focus, which can be annoying.
And low light brings us to another area where the lenses are different: The 20mm lens is a true low light lens, with a maximum aperture of f/1.7. The 14mm, still gains about one stop of light gathering compared with the kit lens, but it is not at all a low light lens.
And, of course, the field of view is quite different: The Lumix G 14mm f/2.5 is a wide angle lens, and the Lumix G 20mm f/1.7 is more of a "normal" lens, which can be used for, e.g., environmental portraits in low light without a flash.
In terms of pricing, they tend to sell for approximately the same amount. The 14mm lens was included as a kit lens for the Panasonix GF3 camera, and these kits did not sell well. They were often split, and the lens sold separately off auction sites, which lowered the perceived value of the lens for some time.
What about the sharpness? The general opinion is that the 20mm lens is very sharp in the centre, even wide open at f/1.7. It does require stopping down to at least f/2.8 to get sharp corners, thought. When it comes to the 14mm lens, people generally say that it is not as sharp as the 20mm lens.
I prefer to find out for myself, so I made a field study. I put the Panasonic Lumix GH2 camera on a tripod, set it at base ISO (160), and used 2 second shutter delay to avoid camera shake.
I took the same picture using both lenses, at different apertures. I used the out of camera JPEG images. The shutter speed was always 1/100 second or faster. Below are the full images, scaled down and resharpened. Both images were taken at max aperture.
Lumix G 14mm f/2.5
Lumix G 20mm f/1.7
To better compare the sharpness, I have cut out 100% crops from the images. These crops were not sharpened. Here are some comparisons. These are from the centre of the images:
And here are crops from the top left corner:
Click to see larger versions of the images.
First of all, we can conclude that the centre images are boring: They are virtually the same from max aperture down to f/5.6. They show that there is little to gain by stopping down the aperture when it comes to the centre resolution. Perhaps we can say that the Lumix 20mm lens is a tad bit softer at f/1.7 than f/2. But the difference is very subtle.
In the corner, though, there are more issues to comment. The Lumix 14mm lens does sharpen up a bit when stopping down, and appears to reach an optimal aperture around f/4. Stopping down further to f/5.6 does not appear to give better performance.
The Lumix 20mm lens appears to sharpen up quicker when stopping down. f/2.8 appears to give a sufficient sharpness, bit there is a tad bit of improvement also when going to f/4.
What about comparing between the two lenses? In the centre, I would say they are equally sharp. There is little to complain about in terms of sharpness at any of the aperture values.
In the corner, though, it seems that the 20mm lens is a bit sharper overall. Also, the 14mm lens has some purple fringing artifacts around the branches, which cannot be found in the 20mm corner images.
When it comes to vignetting, the 14mm lens again appears to have somewhat more vignetting wide open. You must close down to around f/3.5 to lose the vignetting, but it is not a huge problem even at larger apertures.
Optically, it seems that the 20mm lens still has an edge over the 14mm lens, especially in the corners. However, unless you are very critical, and make huge enlargements, I don't think any of the lenses will disappoint in terms of optical performance.
When using these lenses on Panasonic Lumix G cameras, the JPEG output images are automatically corrected for some Chromatic Aberration artifacts, like red/green fringing around high contrast areas, especially in the corners of the frame.
As at the current date, Olympus cameras do not employ this CA adjustment.
Based on my examination of the CA artifacts, these lenses do not generate a significant amount of them anyway. So even without the in camera CA correction, CA artifacts are not a significant problem.
Some purple fringing are left after the in camera processing, as we can see in these images as well.
Geometric distortion correction
Perfect rectilinear projection is one of the traditional quality indicators of lenses. If the lens gives a pincushion or barrel distortion, then that is commonly interpreted as a sign that the lens design is bad.
Both of these lenses give a significant barrel distortion without any post processing. Bear in mind, though, that this processing is done automatically in the camera, so that the JPEG images come out looking rectilinear. And when using RAW, most RAW converters will apply the geometric distortion correction seamlessly. So the user might very well never notice that the lenses feature significant distortion.
Using some third party RAW converters, it is possible to look at the images prior to the distortion correction. This reveals the true nature of the distortion properties of the lens. Below are images of a tiled wall. The black lines show how the image looked after the automatic in camera correction, while the red lines illustrate how the camera sensor actually saw the scene:
Lumix G 14mm f/2.5
Lumix G 20mm f/1.7
Read more about this study here. The 14mm lens has somewhat more barrel distortion. To get a rectilinear image, the 14mm lens requires a correction of -16% in the "Lens Distortion" filter in The Gimp, while the 20mm lens requires -11%.
What this means, is that the 20mm lens wastes less pixels in the corners of the image frame, and, potentially, can give slightly better corner sharpness. On the other hand, this effect is rather subtle, and for any real life application, I'd say you can basically ignore it.
Further, the 14mm lens does not correct enough for distortion at close focus distances. Hence, while you get good, rectilinear images at moderate to far focus, you'll get some small amount of barrel distortion at close focus. Again, this is not a problem for most real life usages, but it may be worth to note that this is not a lens for close focus reproduction of art, for example. The 20mm lens, on the other hand, is well corrected for all focus distances. I would guess that this difference is due to the internal focus of the 14mm lens, which is known to produce different geometric distortion properties at different focus lengths.
The lenses are designed to require post processing for a reason: Lens design is a matter of balancing various optical properties against each others. With this choice, the lens designers can focus improving the artifact that cannot be corrected in post processing, while leaving the geometric distortion to be adjusted in post. This can, potentially, lead to smaller lenses with better quality.
I have made a comparison of the out of focus highlights rendering for both lenses. The study shows that neither lens has a "perfect" bokeh. They exhibit various problems, for example non-circular out of focus highlight discs, ringing, dirty and uneven bokeh.
However, you must focus quite close in order for these problems to show. When photographing people, you will normally want to keep a distance of one meter or more to avoid perspective distortion, and the bokeh should not be a problem with this distance.
Field of view
Obviously, the 14mm lens has a wider field of view than the 20mm lens. The 14mm lens is a wide angle lens, while the 20mm lens is what people would normally call a "normal" lens. Normal lenses have a focal length which correspond roughly to the diameter of the sensor. The Four Thirds sensor diagonal measures 21.6mm, so the 20mm lens is in fact a slightly wide normal lens.
Based on the field of view difference, which is quite significant, which lens would you want to buy? Experienced photographers will probably not ponder long about this. They are already well aware of the concepts "wide angle" and "normal lens", and know their preferences. What about the rest of us?
If you have used the kit zoom lens for some time, you could take a look at your favorite photos and see what focal length they were taken with. Did you typically use the wide end of the zoom lens? Or the longer end? The answer here might determine your focal length preference.
There is a philosophy which goes like this: You can always get closer to an object, but you cannot always get further away from it. So to be able to photograph what you want, choose the widest lens. In this case, this philosophy dictates that you choose the Lumix G 14mm f/2.5 lens over the Lumix G 20mm f/1.7 lens, since the former is wider.
However, it doesn't take much thinking to see that the premises are not always right. Let's say you want to photograph people. Then, you should not get closer to them than around 1 meter. Going closer will give you perspective distortion, which can make the photo unflattering.
Hence, if you intend to photograph a person, and want to have their face as the main part of the image, you will want to choose the longer lens. At a 1 meter distance, their face will be just a small spot in the frame with the 14mm wide angle lens. Even the 20mm lens is not long enough to be a portrait lens, but it is still the better choice. For a portrait headshot, you will generally want a focal length of around 40mm or higher. But the 20mm lens can be used to take an environmental portrait.
On the other hand, if you intend to photograph a group of people, you will want to choose the wide angle lens. You cannot always back up more, so the widest lens is best to cover a group of people.
We have already discussed the different maximum apeture. The 20mm f/1.7 lens has the larger maximum aperture, obviously. However, the 20mm lens also has the larger minimum aperture. Here are the ranges.
14mm: f/2.5 - f/22
20mm: f/1.7 - f/16
The smaller possible minimum aperture for the 14mm lens is an advantage when shooting video. Generally, one would not want to have too fast shutter speed when recording a video.
For motion pictures, a 180° shutter is commonly used. This means that the shutter is open half the time. If you have 30 frames per second, this means that the shutter speed should be 1/60 second.
When recording a video outdoors on a sunny day, you may need to close down the aperture a lot to achieve 1/60 second shutter speed. In that case, the f/22 option comes handy with the 14mm lens. Otherwise, you may need to use an ND filter to get the right shutter speed.
Common knowledge says that you should avoid using small apertures, due to diffraction. Diffraction is known to blur the image at pixel level when usigng very small apertures. However, when shooting video, the resolution used is smaller, so I don't think diffraction is any problem. In fact, one could say that diffraction acts as an extra anti aliasing filter, which could actually improve the image quality during video capture.
Both lenses work perfectly fine with video. However, the quick and virtually noiseless autofocus of the Lumix G 14mm f/2.5 pancake lens makes it preferable for general video use.
In low light situations, you could find that when using the Lumix G 20mm f/1.7 you can lose focus for some seconds when there is movement in the scene. This is not so likely to happen with the 14mm lens, in my experience.
Apart from the autofocus differences, the choice between the two lenses largely comes down to the same issues whether you intend to use them for video or photo: The field of view and the maximum aperture. So the considerations in the rest of the article apply just as well for video use.
Here are some example videos.
Low light video with some action using the Lumix G 14mm f/2.5 pancake lens on a GH2:
More information about the video parameters used in the movie above. You'll notice that the audio quality is poor in the video. However, this is due to the sound system, which clips the sound at high levels.
This video showing the ice breaking up in Stockholm was recorded using a Lumix G 14mm f/2.5 pancake lens on a GH2, at 1080p, 24fps:
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.
It seems to me that the Lumix G 20mm f/1.7 is valuable as a sharp, low light lens, while the main benefits of the Lumix G 14mm f/2.5 are the very compact size and the fast autofocus. Both lenses are optically very good, but the 14mm lens, lacking the true low light capability, is not as interesting. From my perspective, anyway.
This note is written six months later: After using the Lumix G 14mm f/2.5 lens a lot, I have come to like it more and more. Now, I use it more than the Lumix G 20mm f/1.7 lens.
The reasons for liking it more are the same as I have written above: Fast and silent autofocus, small size, very good optical qualities. It's also very good for video, due to the AF performance and silence. Besides, the field of view is generally quite useful when photographing and videographing people.
To make the comparison of the images easier, I have applied auto levels to each row. That way, the exposures are more comparable.
The centre of the images:
The corner of the images: