Aim: To determine the relationship between accommodative amplitude and central lens thickness/equatorial lens diameter (CLT/ELD) ratio in vertebrates. Methods: Midsagittal sections of lenses from fixed, post mortem eyes from 125 different vertebrate species were photographed. Their CLT/ELD ratios were correlated with independently published measurements of their accommodative amplitudes. Using the non-linear finite element method (FEM), the efficiency of zonular traction (the absolute change in central radius of curvature per unit force [vertical bar Delta CR vertical bar/F]) for model lenses with CLT/ELD ratios from 0.45 to 0.9 was determined. Results: Vertebrates with CLT/ELD ratios <= 0.6 have high accommodative amplitudes. Zonular traction was found to be most efficient for those model lenses having CLT/ELD ratios <= 0.6. Conclusions: Vertebrates with lenses that have CLT/ELD ratios <= 0.6 - i.e. ``long oval'' shapes - have the greatest accommodative amplitudes; e. g. primates, diving birds and diurnal birds of prey. Vertebrates that have oval or spherical shaped lenses, like owls and most mammals, have low accommodative amplitudes. Zonular traction was found to be most efficient when applied to model lenses with CLT/ELD ratios <= 0.6. The implications of these findings on the mechanism of accommodation are discussed.