Most modern Canon EF lenses have the ability to send distance data to the camera. For example, if you re currently focussed on an object 4 metres from the camera then the lens would send that approximate distance data to the camera body. Canon have built and sold lenses since 1990 with this data, but it wasn t until 2004, with the advent of E-TTL II flash metering, that Canon really put it to good use. E-TTL II has the ability to factor distance data into flash metering calculations under certain conditions. This distance data can improve the reliability of flash metering when it s available and appropriate. For more information on this topic please consult my EOS flash photography article, which has a section on E-TTL II and a list of Canon EF lenses capable of returning distance data.