Focusing Speed and Light

There may be an obvious/well known answer to this question, but I haven't been able to find it.

I have noticed with my new used Sigma 70-200 f2.8 that at different times it moves the focusing mechanism at different speeds. For instance, when I "try" to focus it on a contrast-less surface, the velocity at which the focus indicator ring turns is very slow compared to when it will actually be able to find a focus.

Also, I've noticed that if I'm on the far end from where it should be (let's say currently focused at infinity, the object is 6 feet from me), if i turn the manual focus dial towards infinity while initially focusing, it will fly across to the other end, at nearly twice the speed it would move as if I just left it alone.

I was curious as to how this speed difference is determined. Is this related to the amount of light available? Does it move faster when it has "some idea" as to where it needs to be? Am I crazy and imagining this?

OK, I will be the first to say I don't know this for sure... but here are my thoughts. You are no dreaming, I have seen AF slow down in less than ideal situations

AF is heavily reliant on contrast in order to function at it's best, so in situations where contrast is low (low light is one example) you would probably experience 'hunting' for focus.

But (and this is where I go into theory) it is possible that newer focusing systems are able to detect this and slow themselves down so they don't miss the point of focus as the mechanism was travelling too quickly. This would certainly make sense...

Almost all control systems (such as AF control) utilize negative feedback to reduce hunting and to prevent severe oscillations in the control process. By negative feedback, I mean that the control signal that creates the motion gets smaller as the the difference between the desired setting and the actual setting become closer. Since AF works off contrast differences, if there is a small difference in contrast, the control signal is relatively small and the lens motion will be slow (dampened). Hunting is minimized by having some offset (degree of accuracy) built into the system, so that when the lens reaches an acceptible degree of sharpness, no control signal is generated. Some overshoot may occur when initially focusing the lens, but this should be quickly dampened.

Mark

Nice info there, makes sense and explains what I was sort of thinking, thanks