For mirrorless cameras you are correct. As long as the plane of the sensor is perpendicular, there is some slop. If the sensor gets too far away though you migh

For mirrorless cameras you are correct. As long as the plane of the sensor is perpendicular, there is some slop. If the sensor gets too far away though you might loose infinity focus. For DSLRs the distance through the backmirror to the AF sensor must be exactly identical to the distance of the image sensor, because the AF sensor focuses on the wrong distance otherwise. DSLRs generally have an adjustment for this (often per-lens-ID). (Worth pointing out that DSLRs are calibrated at the factory. I suspect they go for a flange distance that is slightly under the spec, because this direction is easy to remove by calibration. Note how the distributions in the article match up rather well with this expectation). These issues are one of the main reasons why lenses don't have hard infinity stops any more. The other is that it's expensive to do, and the third is that it is less necessary with autofocus. If the optical axis is not normal to the sensor(s) however you have all sorts of problems focusing across the frame.