In TV‐based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiencyg2of the camera lens. It is shown that the appropriate expression isg2=(16n2)−1×[F(1+1/m)]−2, wherenis the refractive index of the scintillator,Fis theF‐number of the lens andmis the optical magnification. This expression yields estimates forg2that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.