What does extended depth of focus mean in contact lens designs?

Extended depth of focus (EDOF) designs in soft contact lenses are employed for presbyopia correction as well as myopia control. These designs may also be described under the catch-all terminology of 'multifocal' - there are significant differences in the optical profiles of these designs. In myopia management, just as we understand there are different types of myopia controlling spectacle lenses or atropine concentrations, similarly there are different types of soft contact lens optical designs. Here we explore what 'extended depth of focus' means in terms of soft contact lens design and how it compares to other available options for myopia. 

(Note that the image above does not depict any type of contact lens design - we just liked the look of it!)

EDOF contact lenses create a range of clear vision at different distances on the visual axis. While this has obvious applications for presbyopia, for myopia the purpose is not so self-evident. Typically, myopic children do not need assistance in managing clear vision at different distances other than distance correction, so in this case EDOF aims to provide competing signals on the retina - one to correct vision, and another in front of the retina, creating relative myopic defocus to slow eye growth.¹

The concept of utilizing EDOF contact lenses for myopia control is based on the relative myopic defocus theory. This slow-down signal for eye growth has been demonstrated in animal models as both 'peripheral defocus'¹ and 'simultaneous defocus'.² This can be imagined as either off-axis (peripheral) or on-axis (simultaneous) contrasting signals, and both will occur in practice, as eyes receive central and peripheral visual information at once.

A center-distance multifocal can be easily pictured as a smooth and regular transition of power from the central optic carrying the distance correction, through to a peripherally concentric 'add' zone with relative plus power. A concentric design contains abrupt changes in discrete power zones, alternating between the distance correction and the relative plus or 'add' power.³ An EDOF contact lens, by comparison, doesn't fit into either of these categories. The optical design may not have discrete power zones, and may vary above and below the mean power of the lens.⁴

Multifocal soft contact lens designs typically rely on spherical aberration to create their depth of focus effect on-axis. The optical effect of spherical aberration is to refract peripheral paraxial (incident light that is parallel to the optical axis) light rays by greater or smaller amount relative to central rays with increasing distance from the optical axis. In the case of positive SA the outer paraxial rays are refracted by a greater amount than central rays - this can be pictured as a center-distance multifocal effect. Read more on this in our article Understanding Spherical Aberration.

EDOF contact lenses can manipulate spherical and other higher order aberrations to create a specific optical effect and image profile.⁵ Optical profiles can be characterized by power maps, which use aberrometry measurement to demonstrate the differential power profile of a lens across a set diameter.³

The image below is Figure 1 from the open access paper Modelling the refractive and imaging impact of multi-zone lenses utilised for myopia control in children's eyes.³ This demonstrates power profile measurements of three types of multi-zone contact lenses and one spectacle lens, all of which have been researched for myopia control.

• Figure 1(a) shows MiSight 1 day, which is a concentric, dual-focus design which alternates between the distance and 'add' power.
• Figure 1(b) is the Biofinity center-distance multifocal
• Figure 1(c) is the Defocus Incorporated Multiple Segments (DIMS) spectacle lens, described as a 'dual-focus spectacle lens', imaged through the 'lenslet' peripheral zone of the lens
• Figure 1(d) is the NaturalVue Multifocal 1 day contact lens.

The caption states: Ex-vivo, aberrometer measured power maps of four myopia control lenses. (a), (b) and (d) represent sagittal powers maps for MiSight 1 day, Biofinity Multifocal centre-systematically distance (CD) and NaturalVue Multifocal 1 Day contact lenses, respectively, whereas (c) represents the curvature power map of the peripheral region of a Defocus Incorporated Multiple Segments (DIMS) spectacle lens. The red and blue triangles on the colour bar scale indicates the measured distance and add power, respectively. Note that these measured powers served as the basis for optical modelling as opposed to the nominally labelled powers on the packaging.³

A catenary curve can be pictured as the U-like shape seen when a chain or cable hangs freely, under its own weight. The terminology of catenary optics is more at home in optical imaging and engineering than in contact lens design, being a relatively new direction in micro-imaging and wide-angle imaging with large lenses such as in microscopes, cameras and telescopes. In these applications, catenary optics can permit a larger gradient (change) of power than a traditional surfaces for wider angles or larger fields of view.⁶

Catenary curves also have applications in architecture and even orthodontics and prosthodontics, in modelling the shape of the mandibular arch. See the images and links below to fall down a catenary-shaped rabbit hole!

The NaturalVue Multifocal 1 day has been newly described as a 'contact lens design using catenary optics' in a presentation at the International Myopia Conference in September 2022. (See page 268 of the Abstract Book) The design aims to create a rapid and smooth power progression, with a "spherical-like.. smaller central region and a higher level of peripheral steepening. It effectively extends the depth of focus while producing a gentle halo." This is theorized to create myopic defocus over a large area of the retina to provide a large 'dose' for myopia control.

An optical engineering review stated that the two ends of a catenary curve must be truncated at the edge as the mathematical equation approaches infinity.⁶ This indicates that rapid shifts in the curve, or the optical power, are possible and could potentially contrast with a continuously aspheric curve. In the Figure 1(d) above, a sharp delineation is seen between the edge of the 'add' zone at around 6mm diameter and the rest of the lens, in contrast to the aspheric multifocal design above it in 1(b) which features a continuous gradient of change.

Note that there has been no published data yet correlating any changes in central or peripheral optical profile and the myopia control effect with the NaturalVue Multifocal 1 day, nor any direct comparisons between it and other soft contact lenses investigated for myopia control. A randomized clinical trial named PROTECT is underway for this lens design.

Animal studies have shown a dose-response effect with this lens design. The first compared two lens designs to determine the most effective peripheral add gradient.⁸ The second study showed that lens-induced myopia in chicks could be slowed in axial progression and halted or even reversed in refractive progression with the two test lens designs.⁹

The BLINK study which investigated center-distance (CD) multifocal contact lenses showed that a higher add (+2.50D) provided a myopia control effect in children while a lower add (+1.50D) did not.¹⁰

There is still more to learn about the amount and location of myopic defocus required to optimize the 'dose' effect for myopia control.^1,2 Achieving a lens design which optimizes the 'dose' while maintaining good vision outcomes is a key goal. In head-to-head comparisons of visual outcomes in young adult myopes, the NaturalVue Multifocal 1 day showed a similar distance vision outcome to MiSight 1 day, which were both only 2 to 3 letters worse than single vision contact lenses and at least a line better than Biofinity CD +2.50 add. Near acuity was within 2 letters for all tested lens designs.¹¹

Learn more about the NaturalVue Multifocal 1 day contact lens via this link.

• Understanding The NaturalVue Multifocal Contact Lens

• Is there a dose-response in myopia control?

• The NaturalVue Multifocal 1 day in practice: Q&A Interview with Dr Carolyn Lederman