From this photo it is obvious that the aperture size of a camera makes a large difference in the depth of field. And while the photo on the left with the narrow depth of field makes for a compelling photo, that limited range is not as functionally appealing as the wide depth of field of the right photo. In cataract surgery we also want to target the post-op refraction that is most useful for our patients.
When we perform cataract surgery, we can optimize refractive outcomes by targeting specific focal points. This allows our patients the benefit of reduced dependence on glasses as well as removal of the cataract. Where we target our patients’ postoperative refractions depends largely on their daily activities, with a distinction being made between distance vision vs. near and intermediate vision.
In the young normal eye, the range of vision without glasses is fantastic due to the high degree of accommodation, which can be more than 10 D. This range decreases with age and the onset of presbyopia until spectacles become required for near vision. The typical IOLs that are used with cataract surgery are monofocal with no ability to accommodate or change focus. While some accommodating IOLs are commercially available, their range is very limited and not comparable to a young crystalline lens.
The focal point of the eye in meters is calculated as the reciprocal of the refractive state in diopters. This means that an eye with a postop refraction of exactly 0.00 (plano) has an optimal focal point of infinity, which we can understand as far distance. The functional range then depends on the depth of field, which is related to pupil size, corneal aberrations, IOL characteristics and ambient lighting. The smaller the pupil size, the wider the depth of field. This is similar to a camera in which a large aperture lets in more light but decreases the depth of field.
The depth of field is the range of distance along the viewing axis in which the objects stay in focus. The depth of focus is the range around the image plane in which the object stays sharp, and it is influenced by magnification. While the terms are often used interchangeably, when describing the range of uncorrected vision that the patient can achieve after surgery, we are dealing with depth of field.
While achieving a perfect 0.00 post-op refraction will result in the best distance vision with a monofocal IOL, it may not be the best functional vision for people who do more work indoors or at a focal range of a few meters or less.
For most monofocal IOLs in the average eye, the range of acceptably sharp vision without glasses is between ±0.25 D to ±0.5 D from the focal point, although it can be more in certain situations, such as with small pupils and in the presence of corneal aberrations. These aberrations — including mild amounts of higher-order aberrations, such as spherical aberration, and even lower-order aberrations, such as astigmatism — can increase the range of uncorrected vision although they may slightly reduce image quality.
- Extended depth of field IOLs (EDOF IOLs) can help extend this range, but they are not a cure-all and they come with their own side effects and compromises.
- Multifocal IOLs (MF IOLs) typically have two focal points that are created by diffractive optics in order to provide a wider range, although often at the expense of a considerable reduction in image quality and contrast sensitivity.
Choosing refractive targets
Even with a narrow window of just ±0.25 D from the focal point, a wide range of sharp vision without glasses can be achieved by employing a degree of monovision in which a degree of anisometropia is intentionally targeted. This can be as little as 0.75 D difference between the two eyes, which will have only a mild effect on depth perception and stereoscopic vision. In the example in Figure 2, a wide range of vision is produced because the right eye is distance dominant and sees from about 2 m to far away, while the left eye is optimized for intermediate and arm’s length vision. This is using a target of –0.25 D for the right eye and –1 D for the left eye.
If we assume a somewhat wider range of ±0.5 D from the focal point, we could target –0.5 D in one eye, which would give a range of far distance to 1 m, and then target –1.5 D in the other eye, which would give a range of 1 m to 50 cm, with only 1 D of anisometropia between the eyes. Note that due to the reciprocal relationship between refraction and focal point, at larger degrees of residual myopia, the depth of field decreases to just a few centimeters. This means that for most patients, targeting more than –2 D of postop myopia is not as useful.
In our world in which we are spending more time doing near tasks such as using computers, tablets, and cell phones, there is an advantage to targeting some degree of postoperative myopia. By offering our patients a specific refractive outcome at the same time as cataract surgery, we can provide them a wide range of sharp vision without glasses.