Light field rendering (LFR) is an active research area in computer vision and computer graphics. LFR plays a crucial role in free viewpoint video systems (FVV). Several rendering algorithms have been suggested for LFR. However, comparative evaluation of these methods is often limited to subjective assessment of the output. To overcome this problem, this paper presents a geometric measurement, Effective Sampling Density of the scene, referred to as effective sampling for brevity, for objective comparison and evaluation of LFR algorithms. We have derived the effective sampling for the well-known LFR methods. Both theoretical study and numerical simulation have shown that the proposed effective sampling is an effective indicator of the performance for LFR methods.