PURPOSE: Cataract is the major cause of blindness worldwide yet there is no consensus on its assessment and definition. This study compares age-related cataract prevalence derived from two commonly used methods: clinical assessment using the Lens Opacity Classification System (LOCS III) and photographic grading using the Wisconsin Cataract Grading System (Wisconsin System). METHODS: The Singapore Malay Eye Study is a population-based study of 3,280 Singapore Malays aged 40-80 years. Presence of nuclear, cortical and posterior sub-capsular cataract was assessed clinically during slit-lamp examination using LOCS III, and via slit-lamp and retro-illumination photographic grading using the Wisconsin System. Analyses were conducted to determine agreement in cataract prevalence estimates between the two grading Systems and approaches. RESULTS: Poor agreement was found between severity levels of the two grading scales for all three cataract types. Using currently accepted cut-offs to define nuclear (≥ 4 on both LOCS III and Wisconsin System), cortical (≥ 2 in LOCS III, ≥ 25% in Wisconsin) and PSC (≥ 2 in LOCS III, ≥ 5 % in Wisconsin) cataract, the LOCS III overestimated the prevalence of significant cataract as compared to the Wisconsin System, with nuclear cataract prevalence, 27.5% (LOCS III) versus 17.0% (Wisconsin System), cortical cataract prevalence, 27.9% versus 7.0% and posterior sub-capsular cataract prevalence, 7.8% versus 5.1%. CONCLUSION: The prevalence of cataract in a population varies substantially by measurement methods, with systematically different estimates found using the two most frequent cataract grading systems. This study re-emphasizes the need for global standards to assess and define cataract for epidemiologic and clinical studies.