Michael is right, choose the focal plane so it is the same as the PCB surface. That's the ideal setting. It will physically cancel out any tilt errors in the nozzle Z axes.
OpenPnP will hold the part above the camera, so its underside
aligns with the focal plane, i.e. it adds the part height
to the Z.
> Or can openPNP work with something out of focus too and this really does not matter?
Yes it can, if you enable nozzle tip calibration. Then
any tilt errors in the nozzle Z axes are compensated by the
software. Whether the software compensation is any worse
than using the same Z, I don't actually know.
Generally, it is more important to make absolutely
sure the homing fiducial is at PCB Z. I also strongly
recommend to have the primary calibration fiducial at PCB Z.
https://github.com/openpnp/openpnp/wiki/Vision-Solutions#calibration-rig
In addition, it is favorable to have the feeders also roughly at
PCB Z. One could argue that the part underside should
align with PCB Z, because that is what we will be relevant when
doing bottom vision and when the part is placed on its pads.
However, don't panic if that is not the case. It is only relevant
for tall parts and tall parts usually have clunky contacts, so
placement need not be super accurate (self-alignment in reflow).
_Mark
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