Using protective filters on lenses

A topic of much debate is whether or not to use clear glass filters on the front of lenses, for protection. Some would argue that putting on a clear filter protects the lens against objects touching it, and against water, dust, and so on. If something hits the front of the lens, they would say, it breaks the filter and not the lens itself.

Others argue that the front lens element typically is strong, and does not need extra protection. Some would even say that adding a filter may be negative, that it breaks more easily, and that the glass shards could add more to the damage of the front lens element.

Another argument is that an extra glass filter deteriorates the optical properties of the lens.

Now, I wouldn't want to drop my lens on the pavement to examine if using a filter helps protect it or not. Besides, as all lens accidents are different, one drop would not prove much anyway.

But to look at the resulting images when using a filter, as opposed to not using a filter, is possible. To examine the effect of using a clear glass protective filter, I took the same picture with and without a filter.

Example @ 200mm

Here is an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 200mm f/5.6.

without filter	with filter

This is an image with fairly low contrast, and on first glance, the two images are pretty similar. Let's look at an enlargement:



So is there any difference? Not really, I would say. Perhaps one could say that there is a tad bit more contrast in the left image, without the filter.

Example @ 14mm

Another example taken using the Panasonic GH2 with the Lumix G 14mm f/2.5 lens at f/5.6.

without filter	with filter

In this case, we barely need to look at an enlargement to see the differences. Clearly, there is more flare when using a filter. The right image also has some odd "phantom lights" in the frame, from light reflections between the filter and the lens. Here is one example in the centre of the frame:



Beyond the flare, it's hard to argue that the right image is worse in terms of sharpness, for example.

Example @ 45mm

Finally an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 45mm f/4.

without filter	with filter

In this case, it is very clear that the presence of the filter deteriorates the image. Not only is there much more flare, there is also a severe reduction in the contrast and sharpness across the whole frame.

Here is an enlargement which demonstrates this:



Conclusion

In low contrast situations, it is hard to find much evidence of negative impact from using a filter. However, when there is more contrast, e.g., a strong light source in front of the camera, the presence of a filter generates significantly more flare, and potentially also less contrast.

People who are proponents of using clear protective filters may still not agree with me. They could argue that if I had used higher quality multi coated filters, I would not have gotten these negative impacts. And perhaps they are right.

Since I don't believe in using filters for protection myself, I haven't invested in the most expensive filters. So it could be that higher quality filters would have reduced the deterioration.

Still, I believe it is fair to say that adding a filter will generate some deterioration of the image quality. But if using a protective filter gives you more peace of mind when using your expensive lens, perhaps that is an acceptable cost to you.

Personally, I prefer using hoods for basic protection of the front lens element.

Appendix

The filters used were: "Kenko Digital Filter UV" (52mm) for the Lumix G 45-200mm f/4-5.6 lens, and "Green Digital Filter UV" (46mm) for the Lumix G 14mm f/2.5 lens. Both filters are littered with terms like "High Quality" and "Premium Quality", but I would take that with a grain of salt.

Bokeh comparison @ 14mm

The Lumix G 14mm f/2.5 pancake and Lumix G 14-42mm f/3.5-5.6 are both rather new lenses. What they also have in common, is that they have received a mixed reception online.

The pancake lens has some disappointed over lack of sharpness, and not being as fast as the Lumix G 20mm f/1.7. While the comment about not being as fast is correct, I have found the 14mm to be very good optically.

The new kit zoom lens is said to be worse than the lens it replaces, the Lumix G 14-45mm f/3.5-5.6. While it is probably true that the old kit lens was better, I have found the new one to be good, too.

In this article, I have taken a look at how the lenses render out of focus parts of the image, the bokeh. I did this by taking the same image with both lenses. The lenses were focused on the checkered pattern in the lower right part of the image, which is about 30cm from the camera. (click for larger images):

Lumix G 14mm @ f/2.5	Lumix G 14-42mm @ 14mm f/3.5

The pictures were taken with ISO160, base ISO for the Panasonic GH2.

To better evaluate the images, here are 100% crops from the left, centre and top parts of the images, respectively. The images were taken with apertures ranging from f/2.5 to f/8:

(Click for larger images.)

Due to diffraction, one should avoid using apertures smaller than f/8, i.e., avoid larger aperture numbers. This is especially true if you are going to be studying enlargements from the images. If you plan on publishing the images in web size only, go ahead and use apertures as small as f/22. The diffraction will not be a problem if you are going to downscale the images that much.

Conclusion

I think this test does not reveal any bokeh problems with any of the lenses. Perhaps the test could have been made more challenging by having some strong highlights in the background.

The patterned textile in the first compilation image is in focus. We can see that it very sharp already from the smallest aperture. This appears to verify my initial claim that both lenses are in fact quite good, despite their rather bland reception.

The Lumix G 14mm f/2.5 pancake lens appears to be sharpest. This was to be expected, I think, since it is not a zoom, and hence has fewer optical compromises. Further, it also lacks an OIS lens group, and has a simpler construction with only six lens elements. The 14-42mm zoom lens has twice as many lens elements. Generally, more lens elements can lead to worse optical performance, for example reduced contrast. However, modern quality zoom lenses can retain superior optical performance with 15-20 lens elements.

TTL flash metering and flash delay

Flash metering has come a long way the recent decades. TTL flash metering for SLR cameras was first introduced by Olympus in the mid 1970's. TTL refers to Through The Lens. The camera measures the amount of light coming onto the film through the lens during the exposure, and cuts off the flash as the exposure is sufficient.

Film based SLR cameras

For film based SLR cameras, this is usually implemented by having a flash light meter in front of the film plane. The amount of light reflected off the film from the flash is metered, and the flash is turned off when there has been a sufficient amount of light for the desired exposure. See the illustration below.

Film based SLR camera with lens

In this illustration, the mirror is raised for exposing the film.

This generally worked well, at least as long as the subject was not too dark or too light, in which case you needed to manually adjust the flash exposure.

Digital SLR cameras (DSLR)

With digital cameras, this does not work well, since the imaging sensor, replacing the film, is not reflective enough. To overcome this problem, most DSLR cameras fire a pre-flash before raising the mirror, and then fire the main flash after exposing the sensor.

The pre-flash is used to determine the amount of flash needed for the exposure. With this method, the TTL flash meter is no longer needed, the camera's ordinary light meter is used. See the illustration.

DLR camera with lens

There are some DSLRs that still measure the amount of light reflected off the sensor chip, and avoid the pre-flash. The Fujifilm S1 and S3 does this.

Mirrorless cameras

As you know, Micro Four Thirds is a mirrorless camera system. The camera has no light sensor anymore. The imaging sensor is the light sensor. So to find the correct flash exposure, a pre-flash is triggered while the sensor is exposed. Then the camera must make the sensor ready for a second exposure, and fire off the flash with the correct amount of light. This typically takes a bit more time than with a DSLR. The DSLR used the separate light meter for the pre-flash, and could expose the main imaging sensor only once.

Here's a basic illustration of a mirrorless camera with lens. It is much simpler, since there is no mirror, pentaprism, or light meter.

Mirrorless camera with lens

Flash and pre-flash timings

To examine the pre-flash and main flash timings, I have video recorded operating the Panasonic Lumix GH1 and GH2 cameras. I also measured the Pentax K10D, which is an older DSLR from 2006. I turned off autofocus, to measure the flash delay only, and not also the autofocus delay.

I used 50fps when recording, which gives an accuracy of approximately 0.02s.

	GH1	GH2	K10D
Time to pre-flash	0.16s	0.12s	0.08s
Time to main-flash	0.12s	0.14s	0.12s
Total flash delay	0.28s	0.26s	0.20s

The first timing is the delay from pressing the shutter until the pre-flash is fired. The second is the delay from the pre-flash until the main-flash. The third figure is the sum of the two first: The total delay from pressing the shutter until the main flash is fired.

You can see the recordings here. I uploaded them as 25p videos, so they are not as good for verifying the actual timings.



Conclusion

One could say that the GH2 improves slightly on the GH1 in terms of flash delay. However, the difference, 0.02s is not significant with my way of measuring. So we can only say that they are comparable.

When comparing with the older Pentax K10D, we see that the GH1 and GH2 perform almost as good. The difference between 0.20s and 0.26s is not very large. Probably, the autofocus speed is more important to the average user, and we have seen that both the GH1 and GH2 perform very well in terms of autofocus.

One way to avoid the TTL pre-flash, is to use a flash in Auto mode.

Now, we should not conclude that the extra flash delay with Micro Four Thirds cameras is exclusively a bad thing.  As opposed to DSLRs, which have a limited number of light metering sensors, the Micro Four Thirds cameras essentially take one full picture to determine the correct exposure.  This means that the camera has at least 12 megapixels of information available.  It probably doesn't use all of this information.  But what it can potentially do, is to use the information about where in the frame faces are, to enhance the exposure.  Also, the camera knows which areas are in focus, and can make sure that these areas are properly exposed.  The extra information the camera has can be put to good use to give you a better exposure.

Self portrait with a fisheye lens

Most of what I write in this blog is non-personal. While the technical analysis is generally based on fairly objective observations, it is supplemented with subjective comments. But it's still far from personal.

With this post, I take a turn towards the more personal side with a self portrait:

It was taken with the Lumix G 8mm f/3.5 fisheye lens. It has an amazingly short minimum focus distance of 0.1 meter. This is measured from the sensor, which means that the distance from the front lens element is around 2cm, or approximately one inch. The field of view is still very wide, so it cannot be called a macro lens. Here is one example where I used the lens to photograph a LEGO figure up close.

At this distance from my face, the perspective gets very distorted. You can see this in the self portrait, since my nose and eye looks enormous compared to the rest of the face. Of course, the fisheye lens is far from what you would call a portrait lens.

To get as much as possible in focus, I set the aperture to f/9. There was still enough light to get a shutter speed of 1/20 second at ISO320.

Composing the image was fairly easy. I could just flip out the LCD screen on the Panasonic Lumix GH2 and see how the resulting image would be, while holding the lens towards my eye. I used autofocus, which is very fast with this lens, even at this close distance.

Example video capture, GH2+Lumix-Leica 45mm f/2.8 macro

Here is an example video capture using the Panasonic Lumix GH2 camera with the Panasonic Leica Lumix DG Macro-Elmarit 45mm f/2.8 1:1 macro lens.

The camera was handheld, and I left the aperture at f/2.8, shutter speed around 1/100 second. I used the 720p mode, which, since I have the European camera model, gives me 50fps. It was downmixed to 25fps using Openshot video editing software.



I left the autofocus on, using the face detection focus mode. Initially, the camera chose to focus on the toy men on the train station platform, which suited me fine.

As the train enters the frame, the focus starts wandering off, which doesn't really help in this case. However, the focus returns to the toy figures afterwards, which is desirable. In this case, it would probably have been better to turn off autofocus during the video capture, since it doesn't help much at all.

Other lenses, like the Lumix G 14-42mm f/3.5-5.6 would probably have focused faster. However, with a maximum aperture of f/5.6 at 42mm, it would not have given the nice blurred effect of items out of focus. On the other hand, a large part of the frame is in fact out of focus in this video. So perhaps f/5.6, giving more of the subject in focus, would have been a better choice here anyway.

Chromatic Aberration and lens correction

Chromatic Aberration (CA) is a type of lens distortion. It is caused by light of different colours being refracted differently by the glass lens elements.

This is one reason why lenses often consist of pairs of lenses grouped together: The two lens elements in the pair are made of different glass types, and have different optical properties. The aim of the construction is to neutralize the effect of chromatic aberrations, so that all visible colours focus in the same place.

Here is an example image illustrating CA. The image was taken with a Panasonic GH1, using the Olympus Zuiko 9-18mm f/4-5.6 wide angle zoom lens at 9mm f/4:

An enlargement of the lower left corner reveals the CA artifacts:

You'll see that there is red and green fringing off the high contrast areas, where the white and black paint meet. These artifacts are typically seen in the corner of the frame, while the centre of the image is generally free from them.

In the extreme corner, the artifacts take up about 2-3 pixels, which is a moderate effect.

CA artifacts are generally found in the corner, especially when using wide angle lenses.  The lack of CA artifacts indicate a high quality lens design.

Automatic lens correction

Panasonic Lumix G lenses are automatically corrected for CA artifacts by the camera. In the JPEG output files, the camera has adjusted the images with the intention to remove these distortions.

When using Panasonic Micro Four Thirds lenses on Olympus cameras, the CA corrections are not done with current camera models. Future cameras from Olympus may employ the same technology as Panasonic uses, and adjust for these effects.

We can still see the original exposures by opening the RAW image files in a converter which allows for not implementing the CA adjustments. One such RAW converter is UFRAW. By using this converter, we can compare the original exposure with the out of camera JPEG output, to see what kind of adjustments were done.

I am fully aware that there are RAW converters which will do the CA corrections as well.  So this is not a test of the RAW converter, but rather a look at the image before the CA correction, to see what kind of CA artifacts the lenses generate.

I have done these comparisons for three lenses. The images were taken with the Panasonic GH1 at ISO 100:

Lumix G 8mm f/3.5 fisheye

Here is the full image, take with a wide open aperture at f/3.5:

And the upper right corner at f/3.5 and f/5.6:

In this example, we can see that the fisheye lens is very sharp in the corner, even wide open at f/3.5.

In the extreme corner, there is some red and green fringing where white meets black.  Perhaps around 2-3 pixels of colour artifacts.  They are corrected well in the JPEG output, though.

Lumix G 14mm f/2.5 pancake

Here is the full image, take with a wide open aperture at f/2.5:

And the lower right corner at f/2.5 and f/5.6:

We see here that when using the lens wide open, there is some vignetting in the corner, and also, the sharpness is not optimal. This is quite common for any lens, really, and I wouldn't say it is a problem.

Looking at the CAs, it looks like there is about 1-2 pixels of red and green fringing in high contrast areas in the original RAW image. In the adjusted JPEG image, there is still some purple fringing.

Lumix G 20mm f/1.7 pancake

Here is the full image, wide open at f/1.7:

And the lower right corner at f/1.7 and f/5.6:

My comments here are mostly the same as for the Lumix G 14mm lens: There is vignetting, and there is dullness in the corner at f/1.7. Again, this is not uncommon, and especially so for a low light lens with a large aperture.

Before conversion to JPEG, there is some small amount of green and red fringing. In the converted JPEG image, the green fringing is gone, but there is still some purple fringing.

Olympus Zuiko Digital 50mm f/2 1:2 macro (Four Thirds lens)

This lens is rather well known for it's CA artifacts. Here is a video showing the lens being used on a Panasonic GH1 camera (with an adapter). When focusing manually close to the minimum focus distance, you can see clearly black text on white background is black only when in perfect focus. Focusing a bit longer gives a green outline. And focusing closer gives a red outline. Doubleclick on the video to get a larger view, up to 720p is possible.



Here is a 100% view of a photo taken using the Olympus 50mm f/2 macro at 1:2 magnification, and f/2 aperture. As you see, text which is beyond the focus point has green fringing, while text nearer has red fringing.

The Panasonic-Leica 45mm f/2.8 macro lens for Micro Four Thirds does not exhibit these kinds of artifacts.

Conclusions

When using Panasonic lenses on Panasonic cameras, some CA artifacts are corrected automatically. However, there are still some purple fringing left in the corners.  It is generally restricted to around one pixel width, which is not much.

Even before the CA correction, the CA artifacts are very moderate on the Lumix G 20mm f/1.7 and the Lumix G 14mm f/2.5 pancake lenses. The Lumix G 8mm f/3.5 fisheye lens has somewhat more CA effects in the extreme corner, but it is well corrected by software.

All in all, I think that CA artifacts are nothing to worry about with these lenses. Even when using the lenses on Olympus cameras, without in camera CA correction, this should not bother you much.

Comparison: Lumix 14mm vs Lumix 20mm pancake lenses

The Lumix G 14mm f/2.5 and Lumix G 20mm f/1.7 pancake lenses are similar, yet still very different. They are of course similar since they share a pancake characteristic: They are both very compact. They are shown below, with "home made" lens hoods:

(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.

It is an interesting fact that you have exactly the same field of view with the pair of inexpensive pancake lenses from the Nikon Z system: Nikon Z 40mm f/2 and Nikon Z 28mm f/2.8. As these are full frame lenses, there is no crop factor, so the M4/3 20mm lens corresponds exactly to the Nikon Z 40mm lens. And the same goes with M4/3 14mm versus 28mm on full frame.

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.

Sharpness

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.

Sharpness Conclusion

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.

Chromatic aberrations

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.

Bokeh

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. See another bokeh comparison here, which has the same conclusion.

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 favourite 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.

Aperture range

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 using 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.

Video use

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.

Compared with the Sigma 19mm f/2.8 EX DN

In 2012, Sigma released their first Micro Four Thirds lenses, the Sigma 19mm f/2.8 EX DN and the Sigma 30mm f/2.8 EX DN. The 19mm lens is quite similar with the Lumix G 20mm f/1.7 pancake lens, so it makes sense to compare them.

See my main comparison article here. A brief summary: The Sigma lens focuses much quicker, and more silently. It also has a short startup delay, just like the Sigma 30mm lens, and it rattles when not in use. The rattle is no problem, it can just be a bit annoying.

The Sigma 19mm lens has the most pleasing bokeh. It is also cheaper.

In terms of image quality, I think it is clear that the Lumix G 20mm f/1.7 lens is the better. The 20mm lens also has a larger maximum aperture, and a smaller size.

I think that reasons for buying the Sigma 19mm lens over the Lumix G 20mm lens could be to save money, and to get better autofocus performance, especially during video recording.

After just a year, Sigma discontinued the Sigma 19mm f/2.8 EX DN lens, but introduced a new version at the same time. The new version has a different exterior design, but other than that employs the same optical layout, and, hence, the same image quality. The new lens retails for a bit more than the old one did before being discontinued. I guess that Sigma thinks the new metal exterior appears more desirable, and allows them to charge a premium price:



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:


Conclusion

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.

Appendix

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:

Wide angle: 8mm vs 9mm

The Micro Four Thirds format is blessed with a number of compatible very wide angle lenses. There are:

Lumix G 8mm f/3.5 Fisheye

Lumix G 7-14mm f/4

Olympus M.Zuiko 9-18mm f/4-5.6

The Olympus Zuiko 9-18mm f/4-5.6 Four Thirds lens can also be used on Micro Four Thirds cameras, given that you have the appropriate adapter, e.g., Panasonic DMW-MA1, Olympus MMF1 or Olympus MMF2. These are all functionally similar. This lens will autofocus on Micro Four Thirds cameras, but the focus can be a bit slow.

In this article, I am comparing the first and the last on this list. Here's a picture of them both:

The Olympus Zuiko 9-18mm f/4-5.6 (left) is shown without the appropriate adapter. Mounting the adapter will add 18.67mm length, since that is the difference between the register distance of the two formats.

The Panasonic Lumix G 8mm f/3.5 fisheye is a truly compact lens. Olympus has a similar lens on their 2011 roadmap, and it remains to see how compact it will become.

Field of view

At 9mm focal length, the Olympus lens has a diagonal field of view of 100º. The 8mm fisheye lens, on the other hand, has a diagonal field of view of a whopping 180º! How can one mm difference in focal length make up such a massive difference in field of view?

The answer is that the projection is different in the two lenses. Projection in this case refers to the mapping of the real world objects in three dimensional space, down to the image sensor and two dimensions.

Most photographic lenses feature a rectilinear projection. This is what we have become used to. A rectilinear lens will produce an image where straight lines in the real world object are straight also in the resulting image.

Fisheye lenses are fundamentally different. With a fisheye lens, only straight lines the pass through the image centre are straight. All other lines will be bent. There is a significant amount of barrel distortion.

Within fisheye lenses, there can also be variations. Circular fisheye lenses will give a disc of exposure. A 180º view in all directions is mapped into a single disc, and the rest of the sensor frame is left black.

Full frame fisheyes are perhaps more common. They feature a 180º only in the diagonal, and otherwise fill out the entire sensor area. The Lumix G 8mm f/3.5 is a full frame fisheye lens.  To most users, these are more useful, as they give a rectangular image, as we are used to.

Example images

To further illustrate the difference between a rectilinear wide angle lens and a fisheye lens, let's look at an example. The images below were taken at base ISO, and on a tripod.

Olympus 9-18mm @ 9mm f/4	Lumix G 8mm Fisheye @ f/3.5
Olympus 9-18mm @ 9mm f/8	Lumix G 8mm Fisheye @ f/8

As you can see, the fisheye image is wider, and also features significant barrel distortion. Straight lines in the real objects are bent in the depiction.

This is not really the right type of image to evaluate the vignetting, but it seems that the 8mm fisheye lens vignettes a bit more at f/3.5. However, with such a wide angle of view, it is unlikely that you have the same tone across the field anyway, so I cannot see that vignetting will be a significant issue with this lens.

Note that the light source, the setting sun, is in the middle of the frame.  Both lenses handle this fairly well. There is not a big amount of flare or lack of contrast caused by the light source in the centre of the frame.

Sharpness

To evaluate the sharpness, let's look at 100% crops from various parts of the image. These images have not been sharpened. Click for a larger version.

Here are crops from the centre of the frame:

We see quite clearly that the Lumix 8mm Fisheye is the sharpest lens, straight from wide open at f/3.5.

Corners and Chromatic Aberration

From these border crops, we see basically the same thing. We can see some softness in the 8mm Fisheye image at f/3.5, but it sharpens up well at f/5.6.

Also, we see some Chromatic Aberration lens distortion artifacts in the Olympus images.  There's the red and green fringing off high contrast areas.  These artifacts typically appear near the borders, and become more significant the further away from the image centre you get.  This can be corrected pretty well by software, so it's not a big issue.

Panasonic lenses are automatically corrected for Chromatic Aberration (CA) distortion during the in-camera image processing, when using Panasonic cameras. So the JPEG out of camera images I have used could have been corrected for these effects, which may be why we don't see any CA in the Fisheye images.

To examine the effects of the automatic CA correction in the 8mm fisheye lens, let's look at one example. This picture of the Morris Jumel Mansion was taken with the GH1 and the Lumix G 8mm f/3.5 Fisheye:

Here are 100% crops from the extreme top right corner, and from the middle right border:

We see that the corrected image still shows some colour fringing artifacts in the extreme corner, but they are mostly gone in the border crop. From the rest of the frame, i.e., not the extreme borders, you will be hard pressed to find any CA artifacts in the out of camera JPEG image.

From the RAW images which have not been corrected for CA distortion, we see that there are some CA artifacts. These fringes are about 1-2 pixels wide in the extreme corner, which is not very significant. For comparison, the colour fringes are about 2-3 pixels wide in the images from the Olympus Zuiko 9-18mm f/4-5.6 wide zoom lens.

It's also fair to comment that the image from the Lumix 8mm fisheye is remarkably sharp in the extreme corner. Keep in mind that the corner is at a 90º angle from the optical axis.

Defishing

It is possible to convert the fisheye image to a normal image. This process is usually refered to as defishing the image.

Many different programs allow this kind of transformations. I have used a program called Hugin for this purpose.

Below is a comparison of the original images taken with the rectilinear lens at 9mm, the fisheye, and, in the bottom row, the fisheye image converted to rectilinear.

Olympus 9-18mm @ 9mm f/8	Lumix G 8mm Fisheye @ f/8
Fisheye image converted to rectilinear	Fisheye image converted to rectilinear and cropped

The original fisheye image can be stretched to look fairly similar to the rectilinear image. But it has an even wider field of view. I would say the difference in field of view is significant.

This defishing process is hardly optimal, though. The corners have been stretched, and hence lack some resolution compared with the centre of the image. Also, it is difficult to frame the image correctly if you intend to defish it later. But having the option to defish the image makes the fisheye lens more useful.

Here is another example of defishing. The original image is from the Apple Center in New York, Manhattan:

After defishing, it looks like this. There is still a bit of barrel distortion, which could have been removed with some tweaking of the parameters. You can see that the image is less sharp in the corners, due to the stretching needed in the defishing process.

Another note is that the aspect ratio changes when defishing the image. The original fisheye images were taken with a 4:3 aspect ratio, while the defished image has a much wider aspect ratio, closer to 16:9.

On first inspection, this might look like a mistake. However, it does actually make sense. A fisheye lens creates an image where the field of view is not constant across the frame. What I mean is that the field of view is more compressed in the corners than in the centre. Hence, the ratio of horizontal to vertical field of view becomes larger than that of the original image.

When using the 4:3 aspect ratio, the output image of course has a 4:3 ratio in terms of pixels.  However, due to the compressed field of view in the corners, the horizontal to vertical field of view ratio is 124º:92º. 

Conclusion

In concluding, it is clear that the Lumix G 8mm f/3.5 Fisheye lens is better optically than the Olympus 9-18mm zoom. The Lumix 8mm fisheye appears to vignette a bit with wide open aperture, but I can't see that being a big problem.

On the other hand, the 9-18mm zoom is more versatile. In the longer end, it gives a pretty normal field of view, and can be used for general photography. In the wide end, it is an extreme wide angle lens. It can be used to make stunning and interesting wide angle images, as well as pictures with a more normal perspective.

The fisheye lens is an exotic lens, and is not always easy to use. When you nail an interesting image with the fisheye lens, it can be very rewarding. But many pictures end up looking just hideous, or like clichés. It is a lens with a required taste. Given the high price, I would not recommend buying it unless you know what you are doing.

Finally, the 8mm fisheye lens focuses much faster and more silently on Micro Four Thirds cameras.

Of course, nobody with a Micro Four Thirds camera should buy the Olympus Zuiko 9-18mm f/4-5.6 Four Thirds lens, since it requires and adapter, and is much larger than the M.Zuiko Micro Four Thirds version of the lens.  From what I have read, the M4/3 version has comparable optical qualities, and focuses much faster.  It is also collapsible, and much lighter and more compact.