Managing Depth of Focus with Micro Monovision and Micro Astigmatism
In an recent post, I discussed the clinical experience of a patient who had cataract surgery and ended up with enhanced depth of field due to a serendipitous combination of micro monovision and micro astigmatism.
This article shows how, in theory, depth of field can be significantly improved with standard monofocal lenses by combining micro monovision and micro astigmatism. High quality depth of field spanning 1.40 Diopters can be achieved.
In this article, “depth of field” describes the range, both towards and away from the focal point, throughout which the viewer can comfortably perform moderately demanding tasks without difficulty. Examples of moderately demanding tasks include reading a text on a cell phone, or driving a car on a bridge at night. Depth of field can be stated in Diopters or meters depending on what concepts are being explored.
This is a subjective definition. What is considered comfortable and adequate for a task may differ between individuals for identical tasks and conditions. It can be estimated for populations.
For the general population, a conservative estimate of the depth of field for intraocular lenses might be 0.35 Diopter both toward and away from the viewer’s focal point, and I will use this estimate for the purpose of consistency throughout this article. Accepting this assumption, the depth of field of an aspheric intraocular lens would be 0.70 Diopters and can be expressed conceptually below:
Note that in these examples I will be using a decimal system rather than the quarter Diopter system that we use when refracting.
Imagine that we perform cataract surgery on both eyes. We aim for and achieve the exact same spherical refractive outcome without astigmatism or any other higher order aberration in both eyes. This could be expressed with the following diagram.
The depth of field remains 0.70 Diopters. The right eye (yellow) is above the line; the left eye (blue) is below the line.
Next let’s insert a small refractive difference between the two eyes of 0.40 Diopters, keeping the spherical equivalent of the entire system at 0.00. The patient now has micro monovision. Now our depth of field diagram looks like this:
Using micro monovision alone, the depth of field has been expanded to 1.10 Diopters.
Now let’s add micro astigmatism of 0.30 Diopters to both eyes, again keeping the spherical equivalents the same. When astigmatism is present, the eye focuses light equally and simultaneously at two different distances. Thus astigmatism is a form of monovision. Each eye now has two focal points. Putting the two eyes together, our system has four focal points, each of which has its own depth of field. This is shown below:
The next image summarizes the progression in depth of field when we progress from identical refractive outcomes, to micro monovision, to micro monovision with astigmatism.
By combining binocular micro monovision of 0.40 D (anisometropia) with monocular micro monovision 0.30 D OU (astigmatism) we have now expanded the depth of field from 0.70 D to 1.40 D.
Using Diopters is useful for this analysis but we need to define a specific refractive goal and convert to meters to demonstrate the clinical relevance of this expansion of depth of field. This is done next.
Imagine we create a binocular refractive system with expanded depth of field, in which the outer limit of our depth of field is 0.00 D. The depth of field spans 0.00 to -1.40 Diopters.
The inner limit of the depth of field is 0.71 meters (-1.40 D). The patient with expanded depth of field can be expected to see at distance but also perform many moderately demanding tasks on the computer without any glasses. This patient will experience no eye strain when performing daily tasks with their hands, or talking with another person.
The chart below demonstrates the ranges that can be achieved with 1.40 Diopters of depth of field.
Range in Diopters Range in Meters
-0.90 to +0.50 D 1.11 m to crisp infinity
-1.40 to 0.00 D 0.71 m to infinity
-1.90 to -0.50 D 0.52 m to 2 m
-2.40 to -1.00 D 0.42 m to 1 m
-2.90 to -1.50 D 0.34 m to 0.66 m
In summary, small amounts of astigmatism and anisometropia can be used to expand depth of field. This theoretical discussion implies but does not prove clinical relevance. The use of micro monovision and micro astigmatism to increase depth of field deserves increasing attention as we gain more precise control over our cataract surgical refractive outcomes.