Poll: AF Accuracy Adjustments

I never checked, and still haven't. Nor am I likely to.

Ditto.

Checked and needed -120 with debug from PK Tether on all of the lenses

Then if all the lenses are the same, perhaps this indicates a fault with the camera?

Thanks John, I have had this suspicion for a while now, but been lazy with finding a good repair shop and dreading dealing with insurance again

Cheers

After reading all the previous posts, I started to worry.

I have only Pentax lenses (and a few home made manual focus).
I have to assume, that Pentax should be good, or smart enough to make all the auto focus lenses to fit the camera bodies they sell with it.
There must be some adjustments in the lenses to make the accurate enough to focus correctly.

However, I can not quite see why it should be the fault of a lens, if the camera does not focus correctly. At some setting of the focus ring, the image of any lens will be sharp and the camera should set the focus to the sharpest part.
For example: If lens a focuses an object with a distance of 3 meter and the focussing ring shows 3 exactly, but another lens shows 3.1 meter, who cares, as long as the camera finds the sharpest spot.

If it does not, then there must be something amiss with the cameras focussing ability and not with the lens

Regards, Horst

Remember that points off centre but nearer to the camera than the 75cm mark will be the same distance from the centre of the lens as the 75cm mark on the central axis - think triangles and Pythagoras theorem.

The height of the ruler above the carpet is relevant too as noted by another poster.

And no, I have not checked any of my lenses but feel that I ought to sometime (responded to poll).

Is it that the debug/menu adjustments have the same effect as positioning a shim between camera mount and lens to compensate for manufacturing tolerances?

Something like that, except you can add a negative thickness shim using the in-camera adjustment, which would of course be impossible in real life without the aid of a file/grinder.

There probably are ways to adjust the lenses themselves if it could be proved that the lens was at fault, but another poster suggests that one lens requires different adjustments depending on which camera it is mounted.

That would suggest to me that the camera focussing mechanism is at fault, hence the availability of the adjustment in the first place.

I've just received a Sigma 70-200mm 1:2.8 EX DG OS HSM lens from those nice people in Watford. It needed -9 AF adjustment to correct for front focussing at 200mm.

To be fair, I made the adjustment under artificial light and with flash. I'll repeat the adjustment tomorrow in daylight.

I nearly wrote sunlight then, I don't want to upset anybody so I changed it to daylight.

It's nothing to do with shimming between the lens and the camera. The adjustment is there to compensate for the differences in distance between the AF sensor and the main sensor.

This explains the light paths ....


http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html

Why not? Only takes a minute