gms | German Medical Science

Visual acuity can either be focussed and optimized for one position (e.g. emmetropia), or distributed into different positions (foci), or into a continuous range, sometimes called “pseudoaccommodation range” or “depth of focus”. All optical errors apart from defocus, e.g. astigmatism, spherical aberration and all other kind of higher-order errors, increase the pseudoaccommodation width and decrease the optical in-focus quality correspondingly.

Developments of aspheric – particularly aberration correcting – and toric IOLs have improved the in-focus quality during the last years. “Extended depth of focus” (EDOF) IOLs do just the opposite. The results of such developments are virtually the same as leaving higher order errors (aberrations) uncorrected, but at a much higher financial expense.

Halos are an unavoidable disadvantage of all multifocal and EDOF IOLs because all non-focussed light produces a halo. Thus halos are an inherent logical problem rather than a problem that can be reduced by technical developments.

The chromatic aberration of diffractive IOLs has the opposite sign compared to the chromatic aberration of the refractive part of the system eye-IOL. Thus chromatic aberration can be reduced with such designs, and for particular data sets, it can even be corrected to zero. However, other than in technical optics, chromatic aberration is not only disadvantageous for human vision: it helps to distinguish between myopic and hyperopic defocus and thus to determine if objects are closer or more distant than the focus position. Surprisingly, human visual acuity is normally better or at least the same with white light compared to monochromatic (e.g. green) light.

Patients requiring spectacle-free vision for the major part of their daily activities can be supplied with Monovision. The resulting comfort for distant and near vision is quite similar to that of multifocal IOLs. But there are no halos with Monovision, and it can be converted into perfect distant and perfect near vision with spectacles.