Saturday, 20 October 2012

Autofocus noise comparison

Traditionally, lens focus mechanisms have been rather simple, just a cam which pushes the lens elements back and forth as the focus ring turns. Moving the lens assembly away from the film/sensor gives you a shorter focus distance.

In the Micro Four Thirds family of lenses, this mechanism is rather unusual. Only the two first pancake prime lenses, the Olympus 17mm f/2.8 and the Lumix G 20mm f/1.7 feature this type of mechanism, in which all the lens elements move during focus.

Most other lenses feature internal focus, meaning that only one or some of the lens elements inside the assembly move during focus. This has many advantages: It is usually more quiet, the lens can be more solid since there are no moving elements on the outside, and it is usually less noisy.

On the negative side, this type of focus mechanism can give you a change of effective focal length as the focus moves. This is not much of a problem for still photography, but if you are focusing during video recording, it can lead to what is called "focus breathing", as the image zooms in and out a bit as the focus jogs back and forth.

Also, internal focus can lead to changing geometric distortion characteristics as the focus changes. For the Lumix G 14mm f/2.5 pancake lens, I have seen that it gives a bit of barrel distortion at close focus.

In this article, I will look at the noise characteristics. When operating the focus, there is some noise, and it can vary between lenses. To test this, I mounted the lenses to the Panasonic GH1 in AF-C mode. When half pressing the shutter release button in this mode, the camera makes the focus jog back and forth continuously. This is a good mode for testing the focus noise.

To measure the noise, I put a smartphone next to the lens, with the microphone 1.5 cm from the lens barrel (about 1/2 inch).

Here is a video demonstration of the noise test:

LensNoise reading
Lumix G 8mm f/3.5 fisheye67 dB
Lumix G 14mm f/2.5 Pancake69 dB
Lumix G 20mm f/1.7 Pancake75 dB
Sigma 30mm f/2.8 EX DN70 dB
Olympus M.ZD 45mm f/1.870 dB
Panasonic Leica Lumix DG Macro-Elmarit 45mm f/2.8 1:1 Macro70 dB
Lumix G HD 14-140mm f/4-5.866 dB
Lumix G 14-42mm f/3.5-5.668 dB
Lumix X PZ 14-42mm f/3.5-5.666 dB
Lumix G 45-200mm f/4-5.666 dB
Lumix X PZ 45-175mm f/4-5.666 dB


As expected, the Lumix G 20mm f/1.7 pancake lens has the most noisy autofocus, with the old fashioned focus mechanism. It is also the slowest to focus in my experience.

As for the zoom lenses, they are all pretty similar in terms of autofocus noise. The Lumix G HD 14-140mm f/4-5.8 was marketed as a video optimized lens with very silent autofocus, but the study shows that the other zoom lenses are pretty much comparable in this respect. In terms of autofocus speed, it is also comparable with the rest. So to me, it seems that the video optimization of this lens is to a large degree a matter of marketing only.

You'll notice that the prime lenses are generally more noisy than the zoom lenses. I would guess that this is due to two reasons mostly: The prime lenses generally have larger apertures, meaning that they have fairly large lens elements that take more effort to move during the focus. Also, the larger aperture means that they must be more precise in terms of focus: With a smaller maximum aperture, the zoom lenses can allow themselves to be more sloppy in terms of where to set the focus, as the depth of field is larger. For a large aperture prime lens, the focus must be more exact.

With the exception of the Olympus 17mm f/2.8 pancake lens and the Lumix G 20mm f/1.7 pancake lens, most Micro Four Thirds lenses are pretty much noiseless, and you should not worry about the focus noise.

No comments:

Post a Comment