A year ago, I never believe an AF camera can have focus problem. With what I
had understand how the AF works, it should never happen as long as the AF
sensing system is physically calibrated properly. But it do happen and all
the manufacturers admitted the problem and the newer camera bodies all come
with lens calibration table that users can test and setup his own data for
their lenses.
Even with the user table I doubt it can solve all the problem, my friend had
problem with his D200 and 70-200/2.8 VR, he sent the camera to N*kon service
center to ask for adjustment. The technician asked him if he would like to
calibrate for the short end or the long end! I think even the focusing
distance can make a different, if you cal. the lens at infinity the close
focus may not accurate too.
BTW, I found the error seems light source related, it further complicated
the issue.
On the other hand, the E-520 does not shown any focus problem with liveview
focusing but just too slow for any moving objects.
C.H.Ling
----- Original Message -----
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
> Autofocus problems are referred to as "front focus" or "back focus". It
> simply means that, when the camera thinks it has achieved proper focus,
> it's actually focused either in front of or behind the intended focus
> point. For example, in making a portrait, you will focus on the eyes
> but might discover that the best focus is actually on the ears. It may
> focus that way consistently or it may vary with the lens. Sometimes,
> correcting the problem may require calibration of camera and lens
> together.
>
> I don't understand why it can vary with the lens but I have often
> wondered whether "coasting time" is a problem. That is, when the focus
> motor is commanded to stop moving it doesn't stop instantly but instead
> maybe continues turning for another millisecond or two or three. That
> would mean the camera has to anticipate the approach of best focus and
> shut off the motor before best focus is actually achieved. This is
> purely uninformed speculation on my part. I have no idea what really
> causes focusing errors.
>
> Chuck Norcutt
>
>
>
--
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