Green Hex focus point changing

Hi,

Can anyone please confirm whether or not there is a difference in the actual focus point at which the green confirmation hex lights up, depending upon the direction from which you approach the focus point. ie from the minimum focus distance to green hex confirmation and then from the infinity side back to green hex confirmation point.

I ran a series of test shot today to try and find an acceptable AF correction setting for my K200D using the Debug AF adjustment and became more and more confused by the inconsistency of the results of the adjustments and came to the conclusion the the direction of focus was the cause.

A test run of ten photos auto focusing from infinity back to the focus point and the confirmation hex all showed a common focus point on a 45deg chart, The same exercise auto focusing from the minimum focus distance out to the focus point also all showed a common focus point, but the focus point had moved forward by about 15mm.

I was using my FA 100mm f2.8 Macro lens at about 1500mm from the target, tripod and 2sec mirror lockup, no SR.

Please enlighten me.

Greg

Absolutely nothing on this earth is 100% precise and there could well be a difference in focus point when the lens is travelling from opposite directions. I would try several readings from both directions and take the average as the correct setting.

Focus extend to the rear so when you go from the rear to the front it will faster say it's in focus then when you go from the front to the rear.

Does that make sense or do you want me to explain it, you know what you are asking then right

This might be an interesting read if so then please tell me because I haven't read it yet.
http://toothwalker.org/optics/dof.html

Thanks John that's what I'm aiming for.

Stefan that makes sense.

As I see it to get a consistent focus point I must develop the habit of always focusing and refocusing from the same direction, ie say from near to far as there is less DOF.

That is a complicated explanation you linked to, do you really want me to understand it

Thanks for the comments.

Greg

Yeah at least it would give the conformation roughly the same every time.
Also when something is father away it might give you the conformation faster