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Lenses

Near insets for ophthalmic lenses

04/10/2018
By Grant Hannaford
While the facial measurements used in dispensing are well established, the derivations and purpose driving them are often poorly understood. GRANT HANNAFORD explores this topic in a two-part series.

Clearly the easiest method for determining inter pupillary distances (PD) is direct measurement with the distance (or at infinite gaze) measurement being the most familiar.

Means for determining this parameter will vary, from rulers and pupillometers to the more complex digital imaging devices that have been developed in the past decade.

These devices vary, both in conceptual terms and also with regard to accuracy – some are more successful than others.

"A straw poll in a lecture recently revealed that while there was agreement that a patient’s eyes converge when performing near tasks, there was no real consensus on how much."

However, they all have the same goal of finding a parameter that may be used to accurately decentre a lens so that it aligns with the visual axis of the optical system in the eye.

It is of course possible with pupillometers to obtain accurate and usable measurements for distance, gaze and a variety of near working distances.

However, there are rules of thumb which have crept into practice over time that attempt to shortcut the process, most commonly in the area of determining the change in centration distances for near vision at 40 cm (a special case exists for aspheric lenses in which there is no modification required for pupillary distances between distance and near vision, as the asphericity of the lens surface precludes movement away from the visual axis).

A straw poll in a lecture recently revealed that while there was agreement that a patient’s eyes converge when performing near tasks, there was no real consensus on how much.

Numbers for convergence varied from 1–3 mm per eye, which implies a middle ground of reducing the PD by 2 mm per eye for near centration.

This is also the figure quoted to the author in his early days in practice as being the ideal figure for modification of the PD to achieve near centration.

Investigation of the layout of a visual system indicates that there is a geometric solution that offers a partial explanation (Brooks and Borish 2007, Jalie 2008). Figure 1 shows a simple representation of such a system.

If we take ‘d’ as the distance from the spectacle plane to the plane of the centre of rotation of the eyes, and ‘w’ as the working distance from the spectacle plane to the object of interest, we can begin to construct an expression that will determine geometrically the near centration or ‘near PD’.

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The angle theta may be used to describe similar triangles as follows:



Therefore, in the case of a patient with a PD of 32 mm, working distance of 400 mm, and distance to centre of rotation 28 mm, we obtain this result:



The final answer agrees fairly closely with the 2 mm rule of thumb. It should be evident now that this relationship, while linear, is not constant.

That means a PD of 35 mm will experience a reduction of 2.29 mm, while a PD of 28 mm will be reduced by 1.83 mm leading to a total variance of ~1 mm across both eyes.

Jalie (2008) uses the above method to generate a coefficient that may be applied to a given PD.

Finally, using this technique, it’s possible to generate a simple table of coefficients that can be applied to a given PD.

Table 1. Coefficient method for determining near centration
Table 1. Coefficient method for determining near centration


Part two will review alternative methods for determining near centration and varying working distances.

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