The principal function of live preview on Canon DSLRs from the 20Da onwards has been to allow, via LCD viewing, rapid acquisition of consistent and predictable exposure selection before taking a photograph via its 'exposure simulation' (ExpSim) enabled mode, or exposure priority display. This mode replaces the shortcomings of traditional estimation involved in the through-the-lens (TTL) metering techniques of the older framing type live view DSLRs with autogain/framing only, DSLRs without live view, as well as the shortcomings of optical viewfinders when ambient lighting becomes too low.