Lens to Camera calibration - Q

[nigel_b]

I don't understand why a lens has to be "matched" to a camera? Surely the autofocus tells the lens to move until the edge its looking at is sharp, then tells it to stop?

I can see there might be a mismatch between the plane of the autofocus sensor and the plane of the actual sensor but that would not be lens dependent?

Can anyone explain this please, and if it is something to do with the lens how can you calibrate more than one lens on one camera and are you adjusting the lens or the camera?

[slimrbp]

I found this in a review on the luminous Landscape:

"Almost all cameras and lenses are mass produced. Any mass produced product, even very expensive ones like DSLRs and high-end lenses, is designed and then manufactured within certain allowed tolerances; those tolerances being set by the manufacturers themselves. The reason for this is that perfection is impossible (this is real-world manufacturing not Philosophy 101) and so unless you're NASA with a celestial budget you have to live with the fact that the distance between the lens elements and the lens flange – the sensor and the focal plane, the autofocus mirror assembly and the focus sensor, and the myriad of other critical tolerances are not all perfect."

So from that I am thinking that when the camera assumes it has focussed the image onto the sensor it only gets it right if the sensor is where should be in relation to the lens. So it could be a combination of the lens/camera.

On my 7D there is a lens micro adjustment feature and it adjusts the camera rather than the lens, you can store profiles for up to 20 different lenses.

[nigel_b]

Thanks for the reply.

I think what I'm saying is that the autofocus adjusts the lens until it achieves best focus. Then the mirror lifts and the image is projected on the sensor.

Where are the autofocus zones, on the main sensor or the same distance away as the main sensor up near the view finder, and is it these two distances which are sometimes a little out?