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.
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.