Labs will often supply lenses with compensations incorporated, even when the practitioner hasn’t specified any angular parameters. GRANT HANNAFORD explains why – and how dispensers can provide optimal outcomes using compensated powers.
A regular source of contention between practitioners and lens laboratories is the subject of compensated powers. Many practitioners complain that if they ordered a specific power, let’s say +5.00DS, then why did the lab send them +4.94/-0.16 x 163?
The primary source of these changes is the relationship between the refracting, or testing, environment and the ‘as worn’ position of the lenses. The most common setup for phoropter heads, and also most trial frames, is one that is perpendicular to the principal axis of the visual system. This results in tilt and wrap of zero degrees, and vertex distances that typically range from 12 to 18mm. In phoropter heads, depending on the design, the vertex distance can be up to 30mm, however the design of the units tends to have this in place for the lowest powers to minimise the impact on effective power.
If we wish to ensure the patient experiences the same power at the cornea (and in some lens designs at the fovea) between the refraction and their final fitted spectacles, it is necessary to compensate for position of wear.
The first of these compensations is typically vertex distance. The expression should be familiar from our studies and is a simple application of a power modification in the context of the change of the distance from the lens to the eye.
We then consider the effect of tilt on the power. The expression for effective power due to tilt is perhaps not as familiar as that for vertex power, but may be verified by taking a spherical lens and simply observing the effect of tilting the lens on the lens rest.
We should see that as the lens is tilted an amount of cylinder is induced, despite there actually being none in the lens.
Fsph = F 1+ sin2 θ
Fcyl = Fsph tan2 θ
Effective power for wrap is calculated using the same expression, but it is at this point that we encounter a problem. The overwhelming majority of spectacles have both tilt and wrap, so we need to consider the order of operations. If we take a +5.00DS lens and calculate for tilt first then apply those results back in for wrap, using a tilt of 10 degrees, a wrap of three degrees and n=1.498, we get a result of +5.06/-0.02, reversing the order of operations gives +5.23/-0.15, ignoring axis for the time being. Further complication arrives when we consider spherocylindrical prescriptions where we attempt to consider power sequentially in the vertical and horizontal meridians in short, it is not generally possible to reliably perform power compensations using the expression above.
How can we reconcile this? The solution is to not consider the tilt and wrap sequentially, rather we need to define the plane of the spectacles with both tilt and wrap in place simultaneously as described by Keating (1995), Harris (1998, 2002). Doing this we discover an effective tilt of 10.44 degrees, but perhaps more importantly the axis of our new reference plane is 16.79 degrees. We now have an angle in three-dimensional space that defines the relationship between the tilt/wrap of the spectacles and the plane of the phoropter head (or trial frame), and an axis that can be used in calculations.
This technique is significantly more powerful as it allows inclusion of spherocylindrical power sets without modification to the process. Understandably it is somewhat more involved, however the results are consistently in line with those that we see from the lens laboratories, allowing for variation due to proprietary considerations. In this case we achieve effective powers of +5.14/-0.17 x 163.5 and compensated powers of +4.94/-0.16 x 163.2.
When receiving our lenses from the lab we will often see that some compensations have been applied, even without having specified angular parameters. This is most often due to the lab understanding that, despite no tilt/wrap being specified, there will be tilt/wrap for almost every pair of spectacles. Experience tells us that these will range from eight to 10 degrees for tilt and two to three degrees for wrap. Importantly, if you don’t tell the lab the fitting conditions then average values may be applied, hence the arrival of the lenses with some variation from the order.
ABOUT THE AUTHOR: Grant Hannaford is a qualified lens designer, has completed an MSc (optometry) and is undertaking doctoral research in ocular biometry and emmetropisation. He co-owns a private independent optometry practice in the Southern Highlands of NSW and is the Director of the Academy of Advanced Ophthalmic Optics, is a Fellow of the ABDO and ADOA, and was recently nominated as a finalist in the 2022 International Optician of the Year Award. He is also the past Chairman of the NSW Optical Dispensers Education Trust and past Vice President of ADOA (NSW) and a current appointee to the Australian Standards Committee for Spectacles.
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