MTF testing for the assessment of spectacle lens image quality and the relationship to visual acuity

Optical quality assessment of spectacle lenses has changed very little over the past 50 years, usually consisting of simple power and astigmatism measurements at a few points on the lens. In the past, this was satisfactory since lenses were either single-vision or bifocal where the image quality varies little over the lens. Recently, more complicated lens forms, such as progressive addition lenses (PALs), have gained in popularity. The old testing methods are not adequate to assess the optical quality of such lenses. In a PAL, the power changes continuously from the upper to the lower part of the lens, leading to zones with large amounts of aberration. A means of objectively and accurately measuring the image quality across the aperture of such lenses was required.; The modulation transfer function (MTF) has been used for some time in the precision optics field as a reliable measure of optical system image quality. MTF testing was selected as the basis for a fully-automated, lens measurement instrument. The Spectacle Lens Image Mapper (SLIM) measures and maps the power, astigmatism, prism, MTF, and a new quantity named the maximum predicted visual acuity (MPVA) over the aperture of a spectacle lens. The MTF is obtained thru the Fourier transform of the point spread function produced by the test lens using a high-resolution camera.; Since the MTF is not well understood in the ophthalmic community, a quantitative relationship between the MTF and wearer visual acuity (VA) was desirable. Thus, the MPVA, a measure of the VA reduction that would be experienced by a wearer looking through a given point on a lens, was developed. A clinical study of VA was conducted on four PALs in order to provide validation of the MPVA. Over 80% of the MPVAs were within 2 lines of the measured VA on a standard eye chart and two-thirds were within 1 line, with a number of avenues for improvement already identified.; Many studies using the SLIM have been completed on commercially available PALs. The wide variation in lens quality seen in these studies confirms the effectiveness of and the need for the SLIM.