Because focus doesn't work just " LOOK FOR FOCUS GO YES GOT IT ". There are algorithms and many factors that play a role. For example light conditions. But if you aren't in very bright light, a slower lens (higher minimum f-number) will have a harder time than a fast lens (like f2, f1.4). Secondly, contrast, colours, and flare are important. This can be a problem for film-era lenses with older lens coatings that have not yet been optimized for digital. Another problem can be the lens drive, gears, which might not allow such precise movements that are needed. Older lenses tend to have a longer focus throw, optimized for MF and DoF scales, this means the AF will be slower again.
And some lenses have their own lens motor, which is usually much more silent, but can be a little slower.
Live view uses a different type of AF again, which has other strengths and weaknesses.
AF is a very complex mechanism, the camera doesn't even know the distance of the object, it just tries to lock focus, and it uses various things to speed up the process. Sometimes these things can be fooled. And some lenses are not optimized for modern cameras, which can be problematic.
Think of it this way: Some cars drive more smoothly than others. Why? Because of a hundred reasons, different factors coming together that we ultimately experience just as "smooth drive". And its not just the road surface, even though it is important and most noticeable. Clutch, gears, engine power, torque, wheels, all make a difference. As does the driver.
Usually, you have there factors:
- Poor AF conditions. Sometimes the user doesn't know that AF has its limitations, and then gets made when it fails. This can happen in darkness, aiming at something with no detail/colours, aiming at something that is too near, and so on. This can often lead to hunting, as the camera simply cannot find something to focus on, even though it seems perfectly obvious to the human eye.
- Gear is not optimized for your camera. For example third party lenses might not work best on your camera, because those companies do not have the true K-mount camera specifications (Sigma reverse engineers the mount, btw), they make lenses and then slap on a variety of mounts, and so on. Or old film lenses, made to work well on old cameras and allow some MF, but they might not be best suited on modern digital cameras, since technology keeps being more and more optimized. Optimization means the tolerances are tighter.
- Macro lenses. This doesn't apply only to macro lenses, but it does to most macro lenses. Some lenses have a bigger focus throw, a long area where they have to look for focus. This will automatically mean the camera has to look for focus in a bigger range. And the other part is that some lenses actually can find focus in more than one spot within the same line of sight. A macro lens, for example, can focus on the small fly in front of the lens, or the tree in the background. This can lead to "hunting", as the camera goes between one or another spot. This is not limited to macro lenses, as it can happen with almost any lens, but it happens especially often with macro lenses. This is often the main technical cause for focus hunting
Last edited by Na Horuk; 05-06-2015 at 06:54 AM.