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by Russell McMahon

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I recently bought an NEX-6, the 24mm F1.8 (SEL24F18Z) lens, and the black 18-200mm F3.5-6.3 (SEL18200LE) lens. I love the 24mm lens, but I'm having a what I believe one would call a "focus hunting" problem with the SEL18200LE. Basically, if it's in AF-C mode, it nearly always focus hunts. The severity of what I would call a "pulsating" image in the viewfinder depends on the aperture and focal length, but varies from slight to quite pronounced, far beyond the point of distraction. It doesn't matter whether the camera is handheld or on a stable surface, whether I'm using it inside or outside, or whether it's pointed at a distinct small object or at a large surface like a wall -- it does it in all those conditions.

I've updated my NEX-6's firmware to version 1.01, and the camera reports the 18-200mm lens as being at version 02, which as far as I know is current.

I've called Sony technical support about this. They didn't have an SEL18200LE immediately available to try, only the (silver) SEL18200, and said they were seeing the same thing with it, so their immediate response was that it's "normal behavior" -- that the camera is "anticipating scene changes" (their words).

So...

Am I correct in my assumption that focus hunting, with a distractingly pulsating viewfinder, is most definitely not "normal behavior", whether for cameras in general, for the NEX-6, or for the SEL18200LE specifically, and that I shouldn't be satisfied with this?

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One variable you didn't mention in your question was the overall brightness of the scene. Does your camera and lens still focus hunt in bright sunlight? Or only in reduced lighting levels? –  Michael Clark Apr 16 '13 at 23:26
    
The lens exhibits this behavior in both indoor and outdoor environments, from incandescent lights to indirect sunlight to direct sunlight. –  Frank B. Apr 16 '13 at 23:38

3 Answers 3

up vote 5 down vote accepted

AF-C mode is a continuous focus mode. That is, it will not "lock" focus and hold it until the picture is taken. It is best used for when you are tracking a moving subject such as an athlete moving across a field. It will constantly check the focus as the scene changes and adjust to those changes.

The reason you see this behavior even in static scenes is that the NEX-6 uses a contrast detection AF system. Unlike a phase detection system that measures how far and in which direction the lens is out of focus and then moves the lens the (hopefully) correct amount, contrast detection focuses by moving until finding the focus distance that provides the most contrast in a scene. Even when the lens is focused on a static object in AF-C mode, the camera is asking itself, "Is there another focus distance that will provide even more contrast?" I would imagine that the firmware specifies how long to hold that point when it finds it before trying again when in AF-C mode.

In less than ideal light your NEX-6 with a narrow aperture lens will tend to hunt when focus on a subject has been lost. One reason you've noticed it with your 18-200mm f/3.5-6.3 lens is that the smaller aperture allows less light through the lens to be used by the AF system than your 24mm f/1.8, especially at the longer focal lengths of the lens. This is generally one of the weaknesses of high ratio zoom lenses like an 18-200mm. Most higher end zooms only have a zoom ratio of 3x-4x to allow for wider, constant apertures.

If you want the lens to "lock" onto your static subject once it has achieved focus, use Single Shot AF instead.

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I understand that AF-C is a continuous focus mode. But it shouldn't be hunting for focus in a static environment with a static subject, right? Further, note that this doesn't happen at all with the 24mm F1.8 lens, so it's not an inherent characteristic of AF-C on the NEX-6. –  Frank B. Apr 16 '13 at 23:40
1  
The 24mm f/1.8 allows 4x as much light through the lens as the 18-200 @ f/3.5 and well over 10x as much light as the 18-200 @ f/6.3. This will have a significant effect on AF speed and accuracy on just about any platform, including high end professional DSLR bodies. The firmware for the two lenses probably comes into play here as well. A 24mm prime lens tends to be designed for and used with less dynamic scenes than lenses with longer focal lengths which are more often used to photograph action, so the designers probably took this into account. –  Michael Clark Apr 16 '13 at 23:49
    
Points all well taken and appreciated. I have a friend with an NEX-6 and an SEL18200, and I'm going to ask him to test whether his lens exhibits the same behavior in the same environments. If so, then, pending additional information, I'll probably assume this is inherent to the type of lens and leave it at that. –  Frank B. Apr 17 '13 at 3:18

With a contrast based autofocus, the only way the camera can know that it is in focus is to move out of focus and look for what point has the maximum sharpness. Since they are trying to make sure that the best focus is still achieved they are constantly moving in and out of focus to ensure it is still the best sharpness. Why they didn't store the maximum sharpness and only adjust when it dropped below that I'm not sure, but that appears to be what is happening.

I would expect some amount of focus hunting issues with any contrast based AF(my cellphone camera does the same thing), but it does sound like your camera is overdoing it for a dedicated camera body. The answer is still the same though, tell it not to continuously focus by using a shot focus instead of a continuous focus.

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Yes, it's a reasonable point; I can take the NEX-6 out of AF-C mode and use it in single-shot AF mode. In that case, it still does one cycle of hunting -- that is, it quickly focuses, then slightly defocuses, then refocuses. As Michael points out above, this happens at smaller apertures with less light available, but still, is it normal for lenses similar to mine? I ask because the NEX-6 and its lenses are the first interchangeable lens system I've bought in the digital era (decades ago I had a Nikon FG but hadn't used it in nearly 30 years). –  Frank B. Apr 17 '13 at 16:02
    
@FrankB. - Any contrast based AF is going to have to focus hunt. They may be able to tweak the algorithm to make it less obvious or may simply freeze the image on screen while doing it, but behind the scenes, the only way for a contrast based system to verify it's focus is to hunt for it. There is an alternate technique called phase detection which can do it without having to hunt, but that's not particularly common when working with a CMOS sensor directly. (Phased detection normally occurs on a separate chip, but a few hybrid sensors do exist.) –  AJ Henderson Apr 17 '13 at 16:04
1  
AJ, the NEX-6 has a hybrid PDAF system that operates at apertures of F6.3 and larger, at least with certain lenses, and when those lenses' firmware is up to date (along with that of the NEX-6). I need to do more experimenting to determine if the focus hunting issues are confined to apertures smaller than F6.3, which, if so, would indicate that it's a contrast detection issue. At least I think I have that right. –  Frank B. Apr 17 '13 at 18:47
    
@FrankB. Yeah, focus hunting when phase detection is in use would be rather odd, the behavior seems like it is in contrast detection mode. Good luck by the way. –  AJ Henderson Apr 17 '13 at 20:00
    
Belatedly following up on this, I discovered two things upon doing further research: 1) The focus hunting was indeed confined to apertures smaller than F6.3, in other words, apertures at which PDAF does not operate. 2) The seriousness of the issue varied with the specific lens instance. The first SEL18200SE I owned had a fairly severe problem -- at one point it was focus hunting like crazy in a brightly lit outdoor scene without a cloud in the sky. I exchanged it for a new copy of the same lens and found the problem much reduced. Of course I verified both had the same firmware. –  Frank B. May 27 '13 at 23:50

Yes it is normal. I have a NEX too with a 18-200 (Tamron) and depending on the light it takes time, sometimes it doesn't even focus properly.

Unfortunately you have to deal with it, as is a limitation of the Contrast AF. Phase detection will not work for static subjects.

One reason I didn't update my NEX-5 is because autofocus didn't seem to improve that much yet.

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Did you mean to say, "AF-C will not work for static subjects."? –  Michael Clark Apr 18 '13 at 9:05

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