Modulation Transfer Function
As IOLs are developed to provide vision over many distances, a quantitative measure of optical performance is required. One particularly useful measure is the modulation transfer function (MTF). The MTF is the Fourier transform of the point spread function, a parameter often used when discussing wavefront aberrations. In the context of intraocular lenses, the MTF describes contrast degradation of a sinusoidal pattern as it passes through the optical system as well as the cutoff special frequency beyond which fine detail will not be resolved. MTF data are generally presented in the form of a graph in which the MTF for a particular wavelength and spatial frequency is shown over diopters of defocus or over distance. A reasonable simplification would be to understand these graphs as describing an IOL’s performance over a range of distances.
It is important for the surgeon to be able to evaluate differences in IOL designs by their respective MTF curves. Bifocal IOLs demonstrate 2 distinct peaks in their MTF curves, 1 at distance and 1 at a near focal point that differs depending on IOL design. Trifocal IOLs demonstrate 3 peaks of varying heights.
The area under the MTF curve represents the total light employed in imaging. This is always less than the total amount of light entering the eye because of absorption and, in the case of diffractive IOLs, destructive interference. The area under the MTF curve can be thought of as a photon budget and different IOLs spend this budget differently. Bifocal IOLs with higher near peaks can only improve MTF at near at the expense of distance MTF. This is clearly demonstrated in the comparison of the MTF curves of the Acrysof ReSTOR +2.5 and ReSTOR +3.0 bifocal IOLs (Alcon; Fort Worth, TX; Fig 6-13). Not only does the ReSTOR +2.5 IOL have its near peak at a greater distance than the ReSTOR +3.0 IOL, but the value of the near peak is itself lower. We can interpret this as the ReSTOR +2.5 IOL spending more of its photon budget on distance than does the ReSTOR +3.0 IOL.
MTF is of greatest utility in comparing conventional multifocal IOLs to newer extended-depth-of-focus (EDOF) lenses. A common feature of bifocal and trifocal IOLs is the very low MTF between the focal peaks, representing poor visual performance between the distance and near peaks. The MTF curves of EDOF lenses, like the TECNIS Symfony IOL (Johnson & Johnson Vision, Jacksonville, FL; Fig 6-14), do not have a precipitous drop between peaks and therefore demonstrate better performance at focal lengths between distance and near. However, it is important to remember that the total area under the MTF curve is not increased in EDOF lenses and the improvement in intermediate vision necessarily comes at the expense of performance at the peak focal distances.
Figure 6-13 Through-focus MTF values of the AT LISA tri, FineVision, ReSTOR +2.5 D, and ReSTOR +3.0 D IOLs, at 20/40 Snellen visual acuity equivalent (50 line pairs per mm). Manufacturers: AcrySof IQ ReSTOR IOLs, Alcon Laboratories, Fort Worth, TX, USA; AT LISA tri 839MP IOLs, Carl Zeiss Meditec AG, Jena, Germany; FineVision Micro F12 IOLs, PhysIOL SA, Liège, Belgium. Abbreviations: INF, infinity; IOL, intraocular lens; MTF, modulation transfer function.
(Reprinted from Carson D, Hill WE, Hong X, Karakelle M. Optical bench performance of AcrySof® IQ ReSTOR®, AT LISA®tri, and FineVision® intraocular lenses. Clin Ophthalmol. 2014;8:2105–2015. © 2014, with permission from Elsevier.)
Figure 6-14 Through focus average modulation transfer function for all frequencies up to 100 cycles/mm of all intraocular lenses for the 4.5-mm optical aperture. Manufacturers: AT LISA, Carl Zeiss Meditec AG, Jena, Germany; FineVision, PhysIOL, Liège, Belgium; and TECNIS Symfony, Abbott Laboratories, Abbott Park, IL.
(Reprinted from Esteve-Taboada JJ, Domínguez-Vicent A, Del Águila-Carrasco AJ, et al. Effect of large apertures on the optical quality of three multifocal lenses. J Refract Surg. 2015; 31: 666–676. © 2015, with permission from Slack.)
Artigas JM, Menezo JL, Peris C, Felipe A, Díaz-Llopis M. Image quality with multifocal intraocular lenses and the effect of pupil size: comparison of refractive and hybrid refractive-diffractive designs. J Cataract Refract Surg. 2007;33(12):2111–2117.
Carson D, Hill WE, Hong X, Karakelle M. Optical bench performance of AcrySof® IQ ReSTOR®, AT LISA®tri, and FineVision®intraocular lenses. Clin Ophthalmol. 2014;8: 2105–2115.
Esteve-Taboada JJ, Domínguez-Vicent A, Del Águila-Carrasco AJ, Ferrer-Blasco T, Montés-Micó R. Effect of large apertures on the optical quality of three multifocal lenses. J Refract Surg. 2015;31(10):666-676.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.