All intraocular lenses provide a relatively narrow range of acceptable focus. This range is called depth of field or depth of focus. Photographers are very familiar with depth of field.
Here's an example of how depth of field applies to cataract surgery. Let's say you choose to be slightly nearsighted. The surgeon may choose a lens that focuses best at 1 meter. With this lens, you may still see quite well as close in as half a meter, and as far out as 2 meters. The distance between the closest acceptable focus of half a meter and furthest acceptable focus of 2 meters is the Depth of Field of the lens.
One presbyopia correcting strategy is to focus the lenses differently in each eye. One eye is focused further out than the other. Combining the depth of focus of these two lenses expands the depth of focus that you experience. Depending on how far apart the eyes are focused, this technique may be called Micro Monovision, Mini Monovision, or Full Monovision. These are discussed in more detail below.
With Micro Monovision, the eyes are focused similarly enough that they continue to work together. This has the advantage of retaining binocularity (3D vision). The depth of field of each lens, combined, provides the Total Depth of Field of the binocular lens system. In most cases, you become quite good at paying attention to the eye that sees slightly better, without even being aware which eye is seeing best at any given moment. The other eye still provides significant visual input, including 3D vision. Micro monovision will not completely free you from glasses for all tasks, but can provide significant glasses freedom at the range that is most important to you.
Because micro monovision typically does not give you full depth of focus from distance to near, you have to decide your priorities.
Three basic refractive paths can be considered: Distance, Middle, and Near. Each has unique advantages and disadvantages. Which refractive path you choose depends on your goals. The bottom line: you can have a lot with micro monovision, but you can't have it all.
Near Refractive Path .
Advantages: Good vision without glasses for reading or crafts such as knitting and artwork.
Disadvantages: You will need glasses for driving, watching TV and movies, and you may feel like your surroundings in a room are blurry if you don’t wear glasses.
The upside of excellent reading vision is balanced by the downside probably needing a small glasses prescription for distance. It is likely that you will prefer to wear glasses when you watch TV and drive, especially in dim light.
Middle Refractive Path
Advantages: Good all around vision. In good light, you may only sometimes need glasses to read. Many tasks at distance and near can be performed without glasses, but the most demanding tasks, such as driving at night and reading a book, will probably require glasses. This is a popular choice for people who want to be glasses free for general tasks such as working around the house, shopping, texting, and talking to people.
Disadvantages: Although you may spend most of your day without glasses, you may need glasses for reading and driving.
Distance Refractive Path
Your eyes are focused like a camera at distance.
Advantages: Good vision without glasses for driving, hiking, skiing, watching movies.
Disadvantages: You will need reading glasses for most close-up tasks, like reading a book. You might feel comfortable using a computer without glasses in a well lit room.
Mini Monovision and Full Monovision
Mini Monovision and Full Monovision can also be considered. Larger amounts of monovision have the advantage of allowing you to perform more tasks- perhaps all tasks- without glasses, and the disadvantage of reducing stereopsis (3D vision). If you have been happy with monovision contact lenses in the past, you may like this option.
With Micro Monovision, the two eyes continue to cooperate with one another, but your brain naturally pays a little more attention to one eye depending on the situation at hand. You may not even be aware of which eye is giving you the best focus at any given moment. Depth of focus is enhanced while retaining cooperation between the eyes. Mini Monovision and Full Monovision can also be considered; these methods increase depth of focus, and reduce stereo 3D vision.
The tasks that are important to you will help you determine whether you want to apply Micro Monovision to the Distance, Middle, or Near path. Take this brief refractive questionnaire to help you decide what might be best for you.