Assessing Peripheral Focus in Myopes and Non-myopes: Introducing "Depth-of-refraction"

Peripheral image quality is of high relevance to myopia research, yet peripheral refraction is difficult to define due to aberrations affecting the depth-of-focus. This study investigated the peripheral image quality (±25° horizontal field) using three different image quality metrics, with added sphero-cylindrical wavefronts to find the best correction.

Nineteen adults (9 myopes, 10 emmetropes) and 33 children (5 myopes, 28 non-myopes) were measured using a dual-angle wavefront aberrometer as part of the Stockholm Myopia Study. The optical image quality was calculated from the wavefronts, for 10,000 different sphero-cylindrical corrections around the 2nd-order Zernike refraction, to find the best correction as well as the range of corrections with similar image quality ("depth-of-refraction").

Overall, the peripheral best focus was not distinct, with a large depth-of-refraction. Emmetropes/non-myopes had larger peripheral depth-of-refraction than myopes (mean values of 2.69 and 1.74 D, respectively (Strehl ratio metric)). For some subjects, this span of corrections was of a multifocal character. The prevalence of multifocality depended on the image quality metric but was generally more common in emmetropes/non-myopes than in myopes.

The peripheral visual field does not always have a clear best focus and can show multifocal properties in some individuals, with different corrections yielding similar image quality. As emmetropes/non-myopes had more multifocality and larger depth-of-refraction than myopes, this indicates that inherent peripheral optical properties can play a role in myopia development.