# Optical Behavior of an Enhanced Monofocal Intraocular Lens Compared with a Standard One
## 4 Discussion  
Optical aberrations are imperfections of an optical system that may degrade the image quality of that system. In the case of the eye and human optical system, this translates into reduced quality in the perception of the object being viewed. Ocular aberrations, which are measured at the exit pupil of the eye, arise from their different ocular structures, with pupil size being a main determining factor. Aging also causes a marked increase in the eye's total aberrations, and, to a lesser extent, in those of the cornea. This means that in most younger subjects, total ocular aberrations are lower than corneal aberrations, while in older subjects, the reverse occurs [18,19]. Thus, while in young eyes, some corneal aberrations are offset by those of the natural lens, in aged eyes, this situation is reversed [19,20]. After the implantation of an IOL, the aberration balance of the pseudophakic eye changes with respect to the preoperative eye. This is due to (1) the corneal aberrations being changed by the surgical procedure and (2) the geometry, thickness and location of the IOL are different from that of the removed lens, and thus, the aberrations of the IOL are different from that of the natural lens, thus changing the balance of aberrations for the whole eye [21]. Regardless of age, an optical system's aberrations are aperture-size-dependent such that in the eye, these increase as pupil size increases. Most manufacturers use a technology based on spherical aberrations in their enhanced monofocal lens designs. Compared with most standard monofocal lenses, for which the approach is usually to try to reduce the total spherical aberration of the system to improve the vision quality [22,23], with these newer designs, the objective is the opposite. That is, the aim is to induce spherical aberration, whether positive or negative, to increase the depth of focus and achieve better intermediate vision than with conventional monofocals. To do so, it is necessary that at least one of the lens surfaces be a highly aspherical surface. It should be considered that positive spherical aberration induces an extra positive power in the lens periphery compared with its central zone. Conversely, negative spherical aberration leads to greater power in the central zone. Whatever induced spherical aberration occurs, either positive or negative, it should be of such a magnitude that it has no significant impact on the image quality so that intermediate vision is improved without sacrificing the quality of distance vision. [PERSON] et al. [24] described that for presbyopia correction strategies based on the deliberate induction of aberrations designed to increase the depth of focus, positive spherical aberration values have the same power as their negative equivalents. Figure 5: RMS HOAs of orders 3 to 13 for the three powers of the two lens designs examined. As far as we know, this is the first study in which standard and enhanced monofocal lenses of the same material and with the same platform were compared in terms of radial and spherical aberrations and RMS HOAs for different powers. Here, we found that the radial powers for the standard monofocal IOLs of powers +10.00 D and +20.00 D were stable from the lens center to the periphery, while for the +30.00 D lens, we observed a lower effective power than the nominal in the central zone (0.00 mm). For the three enhanced monofocal IOLs tested, the radial power was noted to increase toward the periphery, while in the central zone (0.00 mm), the power was lower than nominal values in all three cases by an amount ranging from 0.2 to 0.5 D. We observed that the higher the nominal lens power, the greater the increase in power produced at the periphery. In deflectometry measurement systems, the power obtained is derived from the reflection. In these systems, a noisy signal is generated at the origin of the coordinates. However, as shown in reference [17], this central noise does not impair the reliability of the instrument.