This paper is aimed at presenting a new virtual camera model which can efficiently model refraction through flat housings in underwater photography. The key idea is to employ a pixel-wise virtual focal length concept to encode the refractive projection inside the flat housing. The radially-symmetric structure of the varifocal length around the normal of the housing surface allows us to encode the refractive projection with a compact representation. We show that this model realizes an efficient forward projection computation and a linear extrinsic calibration in water. Evaluations using synthesized and real data demonstrate the performance quantitatively and qualitatively. © 2013 IEEE.