Diameter of central zone of diffractive IOL and its relationship with far MTF

Does a smaller bull's eye or diameter of central zone of the diffractive IOL mean less tolerance to decentration?

Two comparative studies on the role of decentration on Far MTF of diffractive IOLs show interesting results. The first study on diffractive bifocal ReSTOR, shows improvement in Far MTF with decentration, with a drop in near MTF, while a second study on PanOptix showed notable affects of decentration on Far MTF. quickguide.org

It has often been assumed that smaller, central “bull’s-eye” of a diffractive IOL, would be less forgiving of decentration, particularly with respect to distance vision.

The clinical concern is certainly understandable. However, optical performance of the diffractive IOL may not be solely dependent on the size of the bull's eye alone.

Several factors may play an equally important, or potentially greater, role:

• Overall diffractive zone diameter

• Distribution of light among the diffractive orders/focal points

• Light scattering and losses within the IOL optics

• And perhaps most importantly, where the light is directed when it passes through the centre of the diffractive zone.

Two interesting studies provide a thought-provoking comparison:

One study evaluated a well-known apodized diffractive bifocal IOL with a relatively smaller diffractive zone (~3.6 mm) and a central diffractive element directing light predominantly toward distance.

With increasing decentration, the study found that far MTF, increased, while near MTF showed a notable reduction.

A more recent study evaluated a non-apodized diffractive trifocal IOL, with a larger central diffractive element (1.164 mm) and a much larger 4.5-mm diffractive zone.

Interestingly, with 0.5-mm decentration, far MTF showed a substantial relative reduction compared with its own on-axis performance, particularly at larger apertures.

So, does a larger bull's eye necessarily mean greater tolerance to decentration? Not necessarily.