Fisheye sharpness

When I reviewed the Samyang 7.5mm f/3.5 fisheye, it was natural to compare it against the native Lumix G 8mm f/3.5 fisheye lens. Since the Lumix lens costs about 2-3 times the price of the Samyang lens, I would expect that the Lumix lens comes out as the winner.

However, what I found was that the Samyang lens was sharper, even in the extreme corners, and without the in camera CA correction which is done with the Lumix lens when using it on a Panasonic camera.

Some have doubted this result, and I have also been a bit unsure now and then. So I decided to do another test.

This time, I photographed a couple of trees against the bright sky, a useful test for lens sharpness. The focus was set on the centre of the image. With the Lumix lens, I used autofocus, and set a centre spot. With the Samyang lens, I used the 10x focus assist magnification available with the Panasonic GH2 camera. Here are the two images, both at f/3.5:

Samyang 7.5mm f/3.5	Lumix G 8mm f/3.5

To better evaluate the sharpness performance, I have compiled some 100% crops. From the centre of the image:

And from the top right corner:

Based on this study, it is perhaps not correct to say that the Samyang lens is much better. I think the Samyang lens appears to have slightly better sharpness in the corner, but the differences are fairly subtle.

Keep in mind that the Lumix lensis automatically corrected for CA artifacts, though. By looking at the RAW image files, and converting them to JPEG with third party software, we can see how the images were before the software CA corrections. These 100% crops are from the top left corner:

I used the free program UFRaw to convert the RAW images, but any program could have been used here, as long as it allows for doing the conversion without applying CA correction.

Here we see that without the in camera software CA correction, there are some significant red/green fringing artifacts. These are largely removed in the out of camera JPEG image. In the light of this, the Samyang lens's performance is even more impressive, as no in camera CA correction was done.

Conclusion

I wouldn't say that the Samyang 7.5mm f/3.5 fisheye lens is significantly better than the Lumix G 8mm f/3.5 fisheye lens, even if it appears to perform better in this test. So the image quality should not be an important factor when deciding between the two.

You could also consider the fact that they have slightly different projections. The Samyang lens gives images that are slightly less barrel distorted, and may be easier to defish.

The Samyang lens also handles flare better, see a direct comparison between the two in my review. Flare handling is very important for a fisheye lens, since you are quite likely to find the sun or a strong light source inside the image frame, due to the wide field of view.

When focusing manually with the Samyang lens, it is important to keep in mind that you shouldn't blindly trust the focus distance scale. On my lens, it is slightly off, and I reach infinity focus slightly before the infinity mark. This is not uncommon with manual focus lenses. Exact calibration would be very expensive, and the producers usually leave some slack for themselves by allowing the lens to focus beyond infinity.

The autofocus of the Lumix G 8mm f/3.5 fisheye isn't really needed for landscape pictures, as you easily get infinity into focus manually. But when taking closeup images, the autofocus can come rather handy.

Number of aperture blades

Lens specifications are not complete without detailing the number of aperture blades. But what does it mean in practice?

Of course, the shape of the aperture blades is important for the out of focus highlights, the bokeh. I have previously seen that the Lumix 45mm f/2.8 1:1 macro lens has somewhat more rounded aperture when stopped down than the Olympus 45mm f/1.8. When not stopped down, i.e., at the largest aperture, the diaphragm blades move out of the way, and the opening is usually perfectly round.

Adding a higher number of diaphragm blades can make the aperture opening more rounded when stopping down. However, lenses seldom go beyond nine blades, probably for the reasons of cost, complexity, and the risk of having one of them break down.

It turns out that the number of blades is also related to the rendering of flare, strong light sources inside the image frame. I'll look at that later in this article.

Samyang and Lumix fisheye lenses

There are currently two fisheye lenses available for the Micro Four Thirds system, the Samyang 7.5mm f/3.5 and the Lumix G 8mm f/3.5:



Of course, the major difference between these lenses is the price (the Samyang is the cheaper), the image quality (the Samyang is better, in my opinion), and the focus mechanism (only the Lumix lens has autofocus). But in addition: The Samyang has six aperture blades, and the Lumix has seven. These close up pictures show the rear exit pupil at f/5.6 for both lenses. The Lumix G 8mm lens has more rounded aperture blades, but you can still make out seven segments:

Samyang @ f/5.6	Lumix @ f/5.6

Here is a video showing how the aperture blades open up on the Samyang lens:



Affecting the flare

It turns out that the number of aperture blades affect the flare in a fundamental way. It is common to get a star shaped flare when using small apertures. And the number of spikes in the star is the number of diaphragm blades (when that number is even), or twice the number of diaphragm blades (when that number is odd). Hence, using the Samyang should give us stars with six spikes, and using the Lumix should give fourteen. Let's check, by taking pictures at f/22:

Samyang @ f/22	Lumix @ f/22

To see this more clearly, here are enlargements at f/22 and f/11:



It's not so easy to count the number of spikes on the Lumix image to the right, but I think it is still quite clear that it is fourteen.

Conclusion

To my experience, most lenses have an odd number of aperture diaphragm blades, which produces the most spikes on the star shaped flare around bright objects, e.g., the sun. I guess this is because a larger number makes the flare look more blurred, whereas a small number makes the flare more distracting.

A bit of trivia is that Canon has chosen an even number for most of their lenses, while Nikon is going for an odd number. Hence, it is often easy to guess what brand a photographer uses, based on an image where the sun is inside the frame. However, the aperture also needs to be fairly small to see this effect, e.g., f/16 or f/22.  So you might not be able to make this out based on any picture.

Fisheye lenses, different projections?

The Samyang 7.5mm f/3.5 fisheye lens is an interesting addition to the Micro Four Thirds lineup. While most third party Micro Four Thirds lenses so far have been existing manual lens designs given a new mount, this lens is designed for the Micro Four Thirds format from ground up.

How can I tell? A full frame fisheye lens has 180° diagonal field of view. Hence, the lens must match the sensor size exactly. If the lens was designed for APS-C, a larger format, then the corners would not correspond to 180° angle of view. A fisheye lens which does not project to 180° in the corners is pretty much useless. Then it is just a wide angle lens with a lot of geometric distortion.

Compared with the existing Lumix G 8mm f/3.5 fisheye lens, the Samyang lens is a much more traditional design, with a manual focus ring and aperture ring.

Some say the Samyang lens features a different projection type, the Stereographic projection. This is supposed to be less distorted than the Spherical projection traditionally associated with fisheye lenses. Let's look into how their projections differ. Here is a picture taken with both lenses, and also using the Olympus Zuiko Digital 9-18mm Four Thirds wide angle zoom at 9mm:

Samyang 7.5mm	Lumix G 8mm	Olympus 9-18mm @ 9mm

By superimposing both the fisheye images in one image, and doing edge detection, we can see how their projections compare:

And let's look at another example:

Samyang 7.5mm	Lumix G 8mm	Olympus 9-18mm @ 9mm

Superimposing the two gives:

(Click for larger images.)

Conclusion

So, what is the conclusion of all this? First of all, from the second example, we observe that both lenses have pretty much the same diagonal field of view. This experiment does not verify that the diagonal field of view is exactly 180°, as it should be, but at least both lenses have around the same maximum diagonal angle. To be more precise, the Samyang lens appears to render a slightly wider diagonal field of view. This might be due to the shorter focal length, 7.5mm versus 8mm.

Regarding the distortion, we can see that the Samyang lens renders objects which are inside the border of the image a little bit smaller. This means that it distorts the images somewhat less. So the rumor is true: The Samyang lens does give less "fisheye distortion".

But surely, the differences are pretty marginal. You're not likely to notice much difference, unless comparing head to head, as I do in this article. So if you're looking at the Samyang 7.5mm lens to avoid the fisheye distortion, you are going to be disappointed.

One thing to note is that if you plan to convert to rectilinear images in post processing, the Samyang lens has the potential for giving you the widest possible rectilinear images of the two fisheye lenses. This process is called defishing, and you can read about the topic here. It is probably not true that the Samyang lens features stereographic projection. It still has fisheye projection, but with slightly less distortion than the Lumix G 8mm f/3.5 fisheye lens.

Interesting macro images with a fisheye lens

If you want a very wide lens for the Micro Four Thirds system, there are two choices: The Panasonic Lumix G 7-14mm f/4 and the Olympus M.Zuiko 9-18mm f/4-5.6. The Panasonic lens is the widest, and is also the most expensive. Both are considered to be quite good.

But it's easy to forget that there is a third choice, the Panasonic Lumic G 8mm f/3.5 fisheye lens.

When considering which of these wide lenses to buy, I landed on the unconventional choice: The fisheye lens. My reasoning was that it has the widest field of view, and is the most compact. The compactness can be illustrated by this picture:

But I was also fascinated by the very short minimum focus distance. In the specifications, this close focus distance is stated as 0.10m. This doesn't sound very impressive. But keep in mind that the focus distance is measured from the sensor plane. So the distance from the front of the lens is only about a couple of cm, or about one inch.

This close focus distance opens up a number of creative possibilities. Below is one example. I put a LEGO figure very close to the lens. It is actually slightly closer than the minimum focus distance. But to keep it reasonably in focus still, I set a very small aperture, f/18. And I used manual focus to make sure the focus was as close as possible.

The resulting image, when cropped a bit, is this:

I think the distorted perspective due to the very wide field of view makes the image interesting.

A big drawback with this method is that the lens casts shadow on the subject. It is very difficult to achieve a proper lightning, as the field of view is very wide. Putting a light source close, so that it illuminates the clown's face, will usually make the light source visible in the picture. Which is not the intention, clearly.

Let's see what we can do when taking the lens outdoors. Here is a picture of a butterfly:

Unlike the clown picture, this image is completely uncropped. It was taken in the same way, by setting the focus as close as possible, and using a small aperture, f/11, for a very deep depth of field. I used ISO 250 and 1/60 second shutter speed.

To take the picture, I put the front of the lens about 2cm from the butterfly. Of course, this made it rather stressed, and it left the scene shortly after. So the time I had to compose the image was very short.

Looking at the composition, it is clear that it leaves quite a bit to be desired. For example, the butterfly is squarely in the middle of the image, which is usually a bit boring. It is common to put interesting objects about 1/3 from the frame edges, to make the image more exciting. This is usually referred to as the rule of thirds. Some Panasonic cameras can be configured to show the guidelines that correspond to the rule of thirds in the display, to aid in the composition. Here is an example from the Panasonic GH1:

The Panasonic GH1 display, configured to show the rule of thirds guidelines in white. Placing interesting items where these lines meet usually gives an interesting composition. It is common to place the horizon along one of these lines.

Also, it would be fine if the flower shape was repeated more times in the frame.  We can see one uncluttered flower in the right part of the image, but two more would be fine.

Finally the background could be more interesting.  Let's say we had a couple walking down the path behind the butterfly.  That would surely make the image a lot better.

But planning all this is very difficult, since the butterfly will leave the scene after a split second when putting the camera in it's face, literally.  I still think this image illustrates well the creative potential of the lens.

Looking at how a typical macro image of a butterfly looks like, let's consider this one taken with the Panasonic Leica 45mm f/2.8 1:1 macro lens:

This is what a macro picture usually looks like. Even though it was taken with a small aperture, f/7.1, the depth of field is still very thin. So the background is completely out of focus. The other exposure details are: ISO 100, 1/100 second.

The thin depth of focus is an advantage: It means that you don't need to worry about the composition of the background. The background will be blurred out. As long as the bokeh is pleasing, you can put your energy into keeping the subject interesting.

But it can also be a disadvantage. The background could potentially be used to make the image more interesting. Putting an insect into an environment, rather than just picturing it with the background blurred out, can make a very stunning photo.

But a macro lens will virtually always keep the background out of focus, even with a very small aperture. This is due to the relatively long focal length. The macro lens used in the picture above has a focal length of 45mm, which is pretty common for such a lens.

The fisheye lens, on the other hand, has a very short focal length, only 8mm. This gives a deeper depth of focus (DOF), and keeps the background more in focus.

The conclusion here is that the fisheye lens can be used to make very interesting close up images. But as the background is more in focus compared to when using a macro lens, the composition can be very tricky.

Self portrait on a bicycle

The Lumix G 8mm f/3.5 fisheye lens is fun to use, due to it's extremely wide perspective. This type of lenses are commonly used in extreme sports videos, like skateboard and BMX. I don't do this type of things myself, but I figured I could use the lens to record myself bicycling anyway.

To do this, I attached the Panasonic GH2 camera to the front fork of the bicycle. I used a Manfrotto Super Clamp, which is essentially a clamp with a tripod head attachment stud. To be able to attach the camera, I also used a tripod ball head, and I chose the Benro BH0, which is pretty compact. After putting it on my bicycle, it looks like this:

Here's the video I recorded:



I prefocused at about 50cm, and set the camera to Manual Focus (MF) before starting the video recording. This was done to avoid focus hunting during the video. The 8mm fisheye lens focuses very quickly, but still, I wanted to avoid short periods of out of focus footage.

I think the colours look a bit dull in this video. It was a somewhat dull day, but still, I think that the next time, I will increase the colour saturation setting before using this lens for videos.

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.

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.

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.