Image quality in wearers of a centre distance concentric design bifocal contact lens.

The optical performance of eyes wearing bifocal concentric contact lens was studied using the double-pass technique. Retinal image quality was measured for four subjects wearing CIBA Bisoft contact lenses presenting the central zone for correcting distance vision. Lenses with two different central optic zone diameter (COZD), 3.2 and 3.8 mm, were studied and the influence of pupil diameter and viewing distance were analysed. Results show that the best optical performance is obtained for far vision conditions when no lens is worn even if the pupil coverage by the COZD is complete. For near vision conditions, the optical performance when the lens is worn is, in general, better than when no lens is worn. When the lens is worn the best optical performance corresponds to a pupil diameter of 3 mm and far vision conditions. For this pupil diameter, variations in the situations analysed can be explained by changes in the percentage of pupil coverage corresponding to the far or the near vision zone of the lens. For a pupil diameter of 5 mm, the retinal image quality is more similar in all situations studied and pupil coverage alone cannot explain the results obtained and the influence of other parameters related to the design or contact lens fitting characteristics must be considered.