Pupil size has role in image quality

February 15, 2011

Results of a study applying image quality metrics already in use by the photography industry provide better understanding of the imaging properties of existing presbyopia-correcting IOLs.

Dr. Pepose highlighted the findings from the study that evaluated one accommodating and five multifocal IOLs available in the United States that showed pupil size had a profound effect on image quality attained with each lens.

"The autofocus cameras used every day rely on an algorithm to determine where the image is in best focus," explained Dr. Pepose, director, Pepose Vision Institute, and professor of clinical ophthalmology, Washington University School of Medicine, St. Louis, MO. "In this study, we adapted and applied that methodology to evaluate image quality of presbyopia-correcting IOLs while mimicking different pupil apertures and viewing points.

The IOLs investigated included three models of the apodized diffractive multifocal IOL-the spherical AcrySof ReSTOR +4.0, the aspheric AcrySof ReSTOR +4.0, and the aspheric AcrySof ReSTOR +3.0 (all Alcon Laboratories); the multizonal refractive ReZoom and the acrylic diffractive Tecnis multifocal IOLs (Abbott Medical Optics); and the Crystalens HD accommodating IOL (Bausch + Lomb).

Testing involved use of an anatomically and optically accurate model eye (average corneal power +43 D, spherical aberration +0.27 µm over a 6-mm aperture) and evaluated image quality at four pupil diameters-2, 3, 4, and 5 mm-and at object vergence points ranging from –1 to +4 D in 0.25-D steps. For each combination of IOL model, pupil diameter, and object vergence setting, a USAF target was projected through the model eye and the image was captured digitally. Then the image sharpness of each digital image was objectively scored using a 2-D numerical gradient function.

Pupil aperture

Summarizing the results by pupil aperture, Dr. Pepose said that at the smallest pupil aperture (2 mm), distance image sharpness was best with the Crystalens HD and ReZoom multifocal IOL and poorest with the Tecnis and ReSTOR +3.0. The ReSTOR +4.0 IOLs offered the best near image sharpness at this pupil size.

"The results favoring the Crystalens HD and ReZoom IOLs at distance are expected considering the Crystalens HD is a monofocal IOL with a broad depth of field and the ReZoom is a zonal multifocal lens that has a distant dominant central zone," Dr. Pepose said.