Background: This study reports how illumination and observation angles affect assessments of the openness of the anterior chamber angle using the van Herick technique. Methods: The angle of the tangent to the corneal surface at the temporal limbus was measured for 50 participants. The effect on anterior chamber depth to corneal thickness (AC:C) ratio of varying the illumination angle away from the perpendicular to this tangent was measured. The effect of varying observation angle was also evaluated. An optical model was constructed to simulate the parameters likely to affect measurement of the AC:C ratio. Results: The average angle of the tangent to the corneal surface at the temporal limbus was 43.1 ± 5.4° (SD). With illumination perpendicular to the limbal-corneal surface, varying the observation angle caused clinically unimportant (less than 0.1 ratio difference) but statistically significant changes in AC:C ratios (F = 7.235, p < 0.0001; repeated measures ANOVA). With observation 60° away from illumination, varying the illumination angle away from the perpendicular to the limbal-corneal surface caused clinically important and statistically significant changes in AC:C ratios (F = 75.981, p < 0.0001; repeated measures ANOVA). The optical model confirmed the experimental results and predicts that the conclusions are applicable outside our study population. Conclusions: Illumination should be within 10° either side of the perpendicular to the corneal surface at the limbus for measurements of the AC:C ratio. The observation angle away from the illumination direction is not critical and 60° is appropriate, but smaller angles can be used.