whāroa arotahi
1. (noun) focal length.
Ko te whāroa o te arotahi, koia te tawhiti mai i te pū o te arotahi ki te haukotinga hihi, ki tua atu o te arotahi (RTA 2014:80). / The length of the focus is the distance from the centre of the lens to the place where the rays are intercepted, beyond the lens.