Gf3 - best lens for indoor shots with fast moving toddler?

Started Oct 18, 2012 | Questions thread
Anders W Forum Pro • Posts: 21,466
Lots of talk about AF times but what about some measurements

Lots of talk in this thread, as usual when it comes to the 20/1.7 about its allegedly slow focus. Few, however, appear to have taken the trouble to actually measure things objectively and systematically.

More than a year ago, Pekka Potka measured the AF speed of the E-P3 versus the E-P2. He found a considerable difference between the two bodies but hardly any difference between the native MFT lenses he tried although his test included the allegedly slowest (the 20/1.7) as well as one of the allegedly fastest (12/2).

During the spring of this year, I repeated his test using similar methods and came to similar conclusions with regard to the lenses tried: very little difference. A couple of weeks ago, however, after still more talk on the forum about the slow AF speed of the 20, I decided to measure things still more precisely. I set up my camera (E-M5) on a tripod about half a meter from my computer screen. I then used my iPhone to shoot a video of the AF process as the camera focused on the screen. One can then, as I did, look at the video frame by frame in order to determine how many video frames the AF process takes from start to finish. Once you know the video frame rate (29 fps in this case), you can then calculate the AF time in seconds as well if you want.

I conducted this test for four different lenses, two of which old and allegedly slow (because they lack internal focus), i.e., the 17/2.8 and the 20/1.7, and two new and allegedly fast, i.e., the 12/2 and the 14/2.5. For each lens I ran two different tests. First, I checked how long it would take for the lens to simply refocus on the target (the computer screen) when that target was already in focus. This gives the minimum AF time. Second, I checked how long it would take for the lens to go all the way from infinity focus to the very small focus distance (about 0.5 meters) of my setup. This gives a value close to the maximum AF time.

The results were as follows:

On the first test (time to refocus on the same target), there was no significant difference at all between the lenses. All managed to complete the test in about 0.35 seconds. On the second test, there was a difference but a rather small one: For the two older lenses, the process took about 0.5 seconds. For the two newer ones, it took about 0.4 seconds.

Of course, for any photographer with a bit of foresight, the first of the two tests is much closer to reality than the second. You would hardly encounter many scenarios where it is of practical importance to be able to move swiftly from infinity focus down to half a meter or vice versa. In most cases, the new focus distance is close to the previous one.

Note that these results were obtained in spite of the fact that the older lenses both have longer FLs than the newer ones, and therefore require that the lens elements travel a physically longer distance. In reality, this means that the test I conducted is slightly biased against the two older lenses.

By way of summary, as long as we are talking about ordinary static AF, the test shows that there is hardly any difference between the AF speed of the older and the newer lenses. For continuous AF and, especially, for video, the smoother and more silent AF action of the newer lenses may nevertheless be preferable.

 Anders W's gear list:Anders W's gear list
Panasonic Lumix DMC-G1 Olympus OM-D E-M5 Olympus E-M1 Panasonic Lumix G Vario 14-45mm F3.5-5.6 ASPH OIS Panasonic Lumix G Vario 7-14mm F4 ASPH +28 more
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