Panasonic 20mm f/1.7 Auto Focus

Did you read this post of mine (the second in the thread)

http://www.youtube.com/watch?v=i34FBAFEBUk#t=4m26s

or this one by mfj197

http://forums.dpreview.com/forums/post/50857995

If you had done so, you would already know that the difference between the AF speed of the 20 and that of other lenses is indeed significant if the lens elements have to do a lot of travel. mfj197 found that it took the 20/1.7 0.8 to 0.85 s to go from infinity to minium focus distance as compared to 0.35 to 0.4 s for the 45/1.8. I found that it took the 20/1.7 about 0.5 s to go from infinity to about half a meter as compared to 0.4 s for the 12/2 and 14/2.5. I also found that when refocusing on a target at the same distance as the previous one, the 20 was no slower than the 12/2 and 14/2.5: about 0.35 s in all three cases.

Now consider the practical relevance of these tests. How often do you need to AF, and in particular AF quickly, starting at a focus distance of 0.2 m and going all the way to infinity or the reverse. For me personally, that never happens. If I need to AF quickly, I would always be prefocused at roughly (and I stress that word) the right distance. And roughly here would typically be somewhere between one meter and infinity, where the AF speed of the 20 will be practically the same as when refocusing on a target at precisely the same distance as the previous one.

Now let's consider the relevance of that video. The first AF target is pretty much at minimum focus distance (0.2 m) and the second about half a meter away. How much is that in terms of the distance the lens elements have to travel? Well, it so happens that it accounts for no less than about two thirds of the total travel the lens elements have to do in order to go from minimum focus (0.2 m) and all the way to infinity. In short, the video tells us that if you frequently need to AF very quickly at distances between 0.2 and 0.5 meters, you may be better off with a lens other than the 20. For the rest of us, the video is of no practical interest whatsoever.