It is not uncommon to hear patients complain of inadequate correction of presbyopia when they are put in traditional bifocals. Here, the patient often finds that his or her vision may be clear in the distance as well as for reading, but when trying to focus beyond 14 to 16 inches, the patient experiences blur. It is a clear example of the fact that the true measure of successfully correcting presbyopia would entail a treatment that optimizes a patient’s near vision over an extended range of distances, and not just for one point of focus. As we search for the best treatment strategy for presbyopia, how can we compare one treatment to another in this respect of providing the broadest depth of focus? The answer involves an understanding of the defocus curve.
Let’s use the example of a multifocal IOL and assessing its ability to correct presbyopia. Traditional studies of visual function with multifocal IOLs involved measurements of a patient’s visual acuity at varying distances. However, doing so to compare various technologies can be fraught with error. We have discussed in previous articles in our segment that the measure of visual acuity can be affected by numerous factors, such as visual quality, reading speed, or neural processing. Additionally, measuring the visual function of multifocal IOLs using this method can be affected by the angular image size, contrast, or lighting conditions. To eliminate some of these sources of error, defocus curves offer a much more controlled means of evaluating visual performance by measuring a patient’s visual acuity with varying levels of defocus.
Defocus curves are created by presenting a series of positive- and negative-powered lenses in front of a patient’s eye and measuring the degree of “defocus” that is induced. The zero reference on the x-axis is controlled across patients by correcting for the best possible distance acuity. Using 0.50-D increments, the defocus curve measures a patient’s binocular visual acuity often from +1.00 D to -4.00 D. In doing so, the resulting acuity that is measured can be used to simulate what the patient’s visual acuity would be at different distances. To understand this concept, we have to remember the most basic formula in optics, the formula for focal length: f = 1/D. For example, when an emmetropic patient views a logMAR chart through a plano lens, the image is at infinity representing distance vision. Place a -2.00 D lens in front of the eyes, and this would essentially equate to viewing the chart at 50 cm (20 inches). When looking through a -4.00 D lens, it would be the visual acuity equivalent at 25 cm (10 inches). Thus, the defocus curve can be created in a more controlled means for evaluating visual acuity at various distances.
Defocus curves not only allow us to measure the range of focus achieved with various means of presbyopia correction such as multifocal or accommodative IOLs, but they can also be used to assess monofocal IOLs. More importantly, they allow us to better compare various multifocal technologies to one another. Beyond these more basic data, they can provide an incredible amount of insight into the effects of residual refractive error, corneal astigmatism, spherical aberration, pupil size, and numerous other factors that can affect visual performance of presbyopia-correcting strategies.
Defocus curves are not perfect and certainly need to be scrutinized when evaluated. We must remember the optical principles of telescopes and the resulting magnification effect a lens has when it is 13 mm in front of the cornea. As such, defocus curves can underestimate true visual acuity. Additionally, defocus curves may not be fully representative of reading visual acuity, as the effects of convergence and pupillary constriction are negated. Lastly, and most importantly, there exists a lack of uniformity and standardization in the methods by which defocus curves are generated, as well as in the analysis of data, making it difficult to directly compare results from one study to another.
When we evaluate the best strategy for presbyopic correction, each surgeon should understand not only the significance of the reported defocus curves associated with that technology, but also how these curves can vary with real factors such as pupil size or residual corneal astigmatism. Perhaps we will advance our understanding to be able to personalize the treatment strategy that is best for each patient. However, it is certainly my hope that with continued development of presbyopic treatment strategies, our discovery of the best correction of presbyopia will essentially eliminate the curve altogether and demonstrate a linear result through all increments of defocus.