jaddottart Loc: Florida

Hi Guys,
Auto-Focus camera's sometimes hunt to focus on a particular subject.The book that came with the camera[Sony] explains minimally saying a low contrast subject will cause this.Ablue sky and a white wall,but, what causes it to hunt continuously?
Thanks Joe.

banjonut Loc: Southern Michigan

The white wall and blue sky will indeed cause the autofocus system in your camera to hunt for focus. There is nothing to lock onto. There needs to be an object on one of the focus points to obtain focus. Otherwise, the lens will keep racking back and forth. If there is one tiny bird for instance, and lots of blue sky, you will play heck getting the bird in focus unless you can get him on one of the focus points. Good luck.

RWR Loc: La Mesa, CA

As your book says, lack of contrast.

RWR Loc: La Mesa, CA

Perhaps this will be of more help:

http://dictionary.reference.com/browse/contrast

ggttc Loc: TN

All of today's cameras have a series of focus points. And there are two types of those... Regular and crosshatch. The regular focus points look for contrast vertically (in most cases)

Crosshatch focal points will look for contrast both vertically and horizontally. But the bottom line is it all depends on contrast... If there is no contrast in the picture the camera won't focus no matter how many focus points you have.

Low light inhibits contrast.

jaddottart Loc: Florida

Gentleman,

I thank each and every one of you,its clearer now,thanks again.

Joe.

SharpShooter Loc: NorCal

1, Joe, firstly, the crosshatch points that gg refers to are actually cross-points. Yes they do sense contrast both in the verticle and horizontal configurations and would be shaped, simply put, as a cross(+) as opposed to a straight horizontal or verticle line(- or |). Some better cameras also have what are called dual-cross focus points. These points have a cross with an additional cross going diagonally along with the cross. Imagine they are shaped like an asterisk( *) and are the most sensitive of all the points. But usually there are only a few duals clustered around the center of a focus screen.
2, With Canons, all(most) lenses of f2.8 or faster will allow a camera to use the sensitive dual-cross points. Slower lenses will not. The slower the lens gets, the less and less focus points that can be activated, thus the more prevalent a len's tendancy to want to hunt. Beyond f8, cameras will not focus at all. Focus is a function of the camera, not the lens.
So faster more expensive lenses allow a camera to lock focus with less hunting, apart from generally being sharper. One reason why sport pros tend to use fast, expensive lenses.
3, lastly, you can use your on-camera flash to pulse and create more contrast where normally there may be very little because of flat or poor light to help keep the lens from hunting and lock focus.
The better off-camera flashes also have a built-in infrared beam that can be activated to send out a cross-hatch pattern that may not be visible to the eye but can be seen by the cameras focus system, on say a pure white wall and enable it to lock focus and hunt less. The beams of course don't go very far, maybe 20-30 feet max.
Hope this clarifies a bit more.
The moral to this story...., if you always spend $2000+ on lenses, they will hunt less!!! :lol: :lol:
SS