On both lenses, I am using 46mm to 37mm step down rings as lens hoods. If you go down the same route, you will also need a 37mm front lens cap.
Beyond the size difference, the 25mm lens features internal focusing, while the 20mm lens has an old style focus mechanism, where the whole lens array moves back and forth. The internal focus achieves faster focus, and makes less noise. The Lumix 20mm lens on the other hand is known to focus slowly, and for making more noise.
In this article, I aim to see if the difference in focus mechanisms make the Lumix Leica DG 25mm f/1.4 better suited for continuous autofocus during video.
Ideally, I would have liked to put the lenses on the same camera model, to compare their focus performance accurately. However, since I don't have two identical cameras, I used the GH2 and GH3 I have. Since I know the Lumix 20mm lens has a disadvantage, I paired it with the newer of the cameras:
The cameras are mounted on a Mini Desmond Dual Bracket, good for, e.g., 3D photography with two cameras. I set the 25mm lens to f/1.6 for better similarity with the 20mm lens. Both cameras were recording video at 1080p, 25fps. Here are some results:
First of all, let me say that I was surprised by the results. Even with the faster focus of the Leica 25mm f/1.4, the GH3 with the 20mm lens focuses faster.
What's going on here, is that the GH3 is a revolution in terms of video autofocus. I have previously demonstrated how much better than GH3 is, compared with the GH2. While the GH2 needs to jog the focus back and forth to confirm the focus, the GH3 hits spot on much faster. It works almost as if it had PDAF technology, when it comes to video autofocus. So this video serves better to demonstrate the superior video autofocus of the GH3, more than compare the two lenses.
Another thing to note, is that the Leica 25mm f/1.4 suffers from focus breathing. This is very common for lenses with internal focusing, and means that the field of view changes when the focus distance changes. You can see this in the video: The objects in the image frame change size when changing the focus from near to far, hence the term focus breathing.
Focus breathing is not a problem for still image photography. Neither for video, as long as you don't change focus while recording. But if you change the focus while making a video, using some focus pulling technique, focus breathing can be quite distracting. It's not a good feature for a video lens.
You'll also hear that the GH3 with the 20mm lens makes more noise focusing, just as expected.
I did not achieve my aim with the demonstration video, to determine how much better the Leica 25mm f/1.4 is for video autofocus, compared with the Lumix 20mm f/1.7 lens. On the other hand, there are other interesting things to learn.
One thing to note, is that the Leica 25mm f/1.4 suffers from focus breathing, making it less than optimal for video use, given that you plan to use it for focus pulling.
If you are thinking about replacing your Lumix G 20mm f/1.7 with the Leica 25mm f/1.4 for better video AF performance, you may consider to replace the camera instead.
The Panasonic GH3 improves upon the preceding cameras to such a degree that, even with the Lumix G 20mm f/1.7, widely considered the slowest focusing lens in the Micro Four Thirds lens lineup, video autofocus becomes very smooth and fast.
So if you have a pre GH3 camera, just getting the Leica 25mm f/1.4 isn't necessarily going to give you much better video focusing. Older cameras require to jog the focus back and forth to confirm the focus, which looks bad on the Leica 25mm due to the focus breathing. I haven't tried newer camera models like the Panasonic G6 or Panasonic GX7, but I guess that they are able to focus better, just like the GH3.
I plan to look more closely at this in my upcoming review of the Lumix Leica DG 25mm f/1.4. In the mean time, you can read my bokeh comparison, in which the 25mm does not come out as a winner, unlike what many may have guessed.
Other manufacturers are adding on sensor PDAF technology, one of the big hypes currently. So far, the Panasonic strategy has been to rather improve the image processing. And it appears to work: The Panasonic GH3 behaves almost like it had PDAF in these examples. Of course, my simplified example does not have any moving subjects. With moving subjects, PDAF may be needed for the best autofocus at the current technology level.