I recently upgraded to full frame and got myself a 5D iii. When I shoot in landscape mode the auto focus works great, but once I tilt the camera into portrait mode the AF starts to struggle. When there is low contrast it is unable to focus properly. But when in landscape mode I can get the same spot perfectly in focus in a split second. Note that this only happens when using the 61 point Auto selection, the other AF modes works perfectly fine even when the camera is tilted. What have I missed here?