However, it has a flaw: The focus mechanism is the old style which moves the whole lens assembly back and forth. This is slow and noisy. Modern cameras like the Panasonic GH3 overcome the slowness quite well, making the lens more usable even for autofocus during video. It still fair to say that it is a slow focusing lens.
Leica Summilux 25mm f/1.4 and Lumix G 20mm f/1.7
The Leica Summilux 25mm f/1.4 is a more recent lens with a promise to fix this: It has the more modern internal focus design. This means that only one lens group moves inside the lens when focusing. This is much faster, and generates less noise. Plus, it makes the lens more solid, as it has no moving parts on the outside.
But, can you believe it, there is a snag anyway. The internal focus design has a drawback: It causes the focus breathing effect. This means that the focal length changes as the focus distance changes.
Effectively, this means that the field of view changes ever so much as the focus distance changes. I have illustrated this in the illustration below. I focused on various places on the fence, illustrated by the blue plus sign. I set f/16 to achieve a deep depth of focus, so that most of the picture is in focus, even with the changing focus point:
As you can see there is certainly more focus breathing with the Leica 25mm lens.
Here is another example illustrating this. I'm using the Leica 25mm f/1.4 lens on the Panasonic GH2. I chose the GH2 deliberately, since it does not focus as efficiently as the GH3. It usually jogs the focus back and forth a bit more to confirm the focus correctness. This creates more focus breathing issues:
In the first example, I am focusing on a model car in the foreground (about one feet away), and infinity. In the second example, I am using the 10mm macro extension ring from Meike/Neewer to be able to focus very closely.
As you can see, the internal focus mechanism of the Leica 25mm lens comes at a price: You also get significantly more focus breathing artefacts.
So, is this a problem? If you are using the lens solely for photos, this is no problem at all.
It is only a problem if you intend to use the lens for video recording, and changing the focus during video recording, called focus pulling. Only then is it a problem. The name "focus breathing" comes from the video use: When changing the focus, objects in the frame appear to change size, to be breathing.
For serious video use, you may want a manual focus lens, like the Voigtländer Nokton 25mm f/0.95. The well dampened focus ring makes it easy to focus smoothly. It has the classic focus mechanism with a minimal focus breathing.
There are two autofocus Micro Four Thirds lenses that don't have internal focusing: The Lumix G 20mm f/1.7 and the Olympus 17mm f/2.8. They have no focus breathing issues. Then again, they focus slower and with more noise.