Side Mirror Field of View
This is the area of road the driver can see in the exterior mirror, and it depends on the position, angle and surface shape of the mirror. When towing a caravan or trailer the vehicle becomes longer at the rear, so the area covered by standard mirrors is no longer sufficient and the view has to be extended sideways.
A flat mirror preserves the perception of true size and distance but shows a narrow area. A convex surface widens the field of view; in return objects appear smaller and further away than they are. On most vehicles these two properties are combined by making part of the mirror convex.
When towing a caravan the problem is geometric: the length added behind the vehicle makes the trailer follow a different path in turns, and the existing mirror is not enough to show the side face of the trailer. That is why extending the view sideways while towing is the basis of safe lane changes and reversing manoeuvres.
Setting the mirrors correctly matters at least as much as the hardware: in the mirror your own vehicle body should appear only as a narrow strip, with the remaining area given to the road. In bad weather the mirror surface must stay clear; AXI's CM1405 fully automatic mirror set has been developed to provide a clear view against mist and icing with its mirror heating module.
Context in which this term is used: Guide to automatic RV levelling