Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
Radu Tenenbaum
OP Radu Tenenbaum Senior Member • Posts: 2,904
Re: really?

Doug offers an excellent explanation (or speculation?) of body's AF problem, but, as Miro writes, does not address why the lens needs to be adjusted.


Doug Warner wrote:

Miro wrote:

How do you explain my findings, then?
If it was truly closed loop there would be no front/back focusing
and lenses would not require a calibration. It's that simple. There
must be something we don't know.

Front/back focusing happens when the lens mount to AF sensor
distance is different than the mount to imager distance. The AF
sensor siatance is fixed. The imager distance is set by shims, and
fine tuned by firmware offset adjustment (D60 service manual)

The AF calibration uses a reference lens. If a user's lens has to
be "calibrated", I suspect this may be more mechanical, perhaps to
lessen stiction that could restrict the final, fine motion needed
to null in on the focus point. Also, if the USM rings weren't in
perfect contact, or there was dust between them the available
torque might be too low.

 Radu Tenenbaum's gear list:Radu Tenenbaum's gear list
Fujifilm X-E2 Nikon D810 Nikon D500 Nikon AF Nikkor 20mm f/2.8D Nikon AF Nikkor 35mm f/2D +7 more
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