the space between the front of the eye and the back surface of the lens. In glasses with powers beyond ±4.00D, the vertex distance can affect the effective power of the glasses. A shorter vertex distance can expand the field of view, but if the vertex distance is too small, the eyelashes will come into contact with the back of the lens, smudging the lens and causing annoyance for the wearer. A skilled frame stylist will help the wearer select a good balance of fashionable frame size with good vertex distance to achieve ideal aesthetics and field of view. The average vertex distance in a pair of glasses is 12-14mm. A contact lens is placed directly on the eye and thus has a vertex distance of zero. Refractive index In the UK and the US, the refractive index is generally specified concerning the yellow He-d Fraunhofer line, commonly abbreviated as nd. Lens materials are classified by their refractive index, as follows: Normal index: 1.48 ≤ nd < 1.54 Mid-index: 1.54 ≤ nd < 1.60 High-index: 1.60 ≤ nd < 1.74 Very high index: 1.76 ≤ nd This is a general classification. Indexes of nd values that are ≥ 1.60 can be, often for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials may be referred to as mid-index. Advantages of higher indices Thinner, sometimes lighter lenses (See below). Improved UV protection over CR-39 and glass lenses. Disadvantages of increased indices Lower Abbe number, meaning, amongst other things, increased chromatic aberration. Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection equation), increasing the importance of anti-reflective coating. Manufacturing defects have more impact on optical quality. Theoretically, off-axis optical quality degrades (oblique astigmatic error). In practice this degradation should not be perceptible – current frame styles are much smaller than they would have to be for these aberrations to be noticeable to the patient, the aberration occurring some distance away from the optical centre of the lens (off-axis). Optical quality Abbe number Of all of the properties of particular lens material, the one that most closely relates to its optical performance is its dispersion, which is specified by the Abbe number. Higher Abbe numbers mean a better lens material, and lower Abbe numbers result in the presence of chromatic aberration (i.e., color fringes above/below or to the left/right of a high contrast object), especially in