However, when using the lens on the Panasonic GH1 camera, I've noticed that the distortion correction is not sufficient. There is still some residual barrel distortion after the in camera correction.
Here are a couple of examples. First, let's look at an example at a close focus distance, close to the minimum focus distance of the lens:
In this example, even when looking at the small image (above, left), it is easy to see that there is still some barrel distortion in the upper, horizontal line. The line is not straight. You can click on the image to view a larger version of it. The corrected image to the left is the JPEG output from the camera.
The right image shows the image as captured by the sensor, without any distortion correction at all. I used the UFRaw RAW processing software. But any software that allows for disabling the distortion correction could be used for this purpose.
And one example with a longer focus distance:
In this latest example with a longer focus distance, it appears that the remaining distortion is not so significant. I added a red, straight guideline to the left in the JPEG image, above left. You can see that there is some barrel distortion still, but not much.
It appears that when using the Lumix G 14mm f/2.5 pancake lens on a Panasonic GH1 camera, there is some residual barrel distortion. This is most apparent at shorter focus distances.
One could speculate why this is so. I don't think it is a firmware issue, since I have the newest firmware available for my camera.
Another speculation is that Panasonic chose to not correct all the barrel distortion, since that would have decreased the diagonal angle of view. The diagonal angle of view for this lens is specified to 75°. Further correction of the barrel distortion would reduce this figure, albeit with a small margin.
The Lumix G 14mm f/2.5 pancake lens uses internal focusing, and it is not uncommon that these designs lead to changes to the distortion properties, and field of view, at closer focus distances.