However, after some use, I see some examples where the background blur is a bit distracting. The bokeh some times exhibit a bit of ringing, which means that it doesn't blur the background as effectively as one could wish for.
I decided to test the bokeh with some out of focus highlights. To do this, I photographed the same subject using three lenses, the Panasonic Lumix G 100-300mm f/4-5.6, Nikkor 200mm f/4 AIS, and Panasonic Lumix G 45-200mm f/4-5.6. The three lenses are shown below:
The Nikkor 200mm f/4 AIS lens in the centre is shown including the adapter needed to connect it to a Micro Four Thirds camera.
The images were taken at dusk, with the camera on a sturdy tripod, and focused on the tree about 2 meters from the camera. I used ISO 160, the base ISO for the Panasonic GH2 camera. As I took the images late dusk, the lightning changed quickly, and they may have different exposure.
Here are the images, click to enlarge:
Lumix G 45-200mm @ 200mm f/5.6
Nikkor 200mm f/4
Lumix G 100-300mm @ 200mm f/4.9
I have made some enlargements as well. These are 100% crops from the centre of the images.
Based on this study, I cannot see any problems with the bokeh of these lenses. They all look just fine. One problem with the Nikkor 200mm f/4 AIS lens, though, is that the aperture diaphragm blades are not rounded. Hence, when stopping down, the lens gives nine-sided out of focus discs, they don't have a rounded edge:
When it comes to the sharpness, the crops above are not suited for comparing the sharpness across the lenses. The Lumix lenses support autofocus, of course, and after viewing the images on PC, I noticed that the camera has prioritized to get the edge of the tree in focus. When focusing the Nikkor 200mm lens manually, I focused on the centre of the branch. So they are not comparable. However, after looking at the images, my conclusion is the same as before: The Panasonic Lumix G 100-300mm f/4-5.6 is significantly sharper than the Panasonic Lumix G 45-200mm f/4-5.6 at 200mm. The Nikkor 200mm f/4 lens appears to sit between them, in terms of sharpness.
The Panasonic Lumix G 100-300mm f/4-5.6 is my only lens capable of doing 300mm focal length. So I don't have anything to compare with. But here are a couple of images taken at 300mm f/5.6 and f/8:
Again, the bokeh looks just fine, no perceived problems here.
While I believe the Panasonic Lumix G 100-300mm f/4-5.6 is prone to giving slightly distracting bokeh in certain cases, I could not reveal any in this simple test.
We see clearly that the Nikkor 200mm f/4 AIS lens has non-circular out of focus rendering when stopped down, due to non-rounded aperture diaphragm blades. This is not so good for the resulting pictures.
I think that this simple test illustrates another example that legacy lenses are not perfect for use on modern cameras. While the lens is reasonably good in terms of sharpness, the primitive aperture makes it problematic when stopped down. In a previous study, I saw that the Pentax FA50 f/1.4 normal lens exhibits quite distracting bokeh between f/1.4 and f/2. The whole point of getting a fast legacy normal lens is to use it pretty much wide open, and that example shows that it can be non-optimal for such use.
So take care if you buy older manual lenses for use on Micro Four Thirds: Do some pre purchase research to make sure the lens fits your needs.