Presbyopia
Presbyopia is the hardening of the lens in the eyeballs such that it becomes difficult to see near work. This is common in older adults and is commonly treated with reading glasses (reduced minus for myopes), progressive lenses, or bifocals.
Measuring
To determine power of accommodation, also sometimes called amplitude of accommodation, measure far power and near power.
Far power is straightforward, and it’s the typical cm measurement: just measure the furthest distance you can see clearly without blur, with a lens to adjust as needed. The far power is 1/(distance to blur) - (lens power).
- For example, 80 cm with a -0.25 lens is calculated as 1/(80 cm) - (-0.25 D) = 1.25 D + 0.25 D = 1.5 D.
- It might be useful to use a plus lens to measure low myopia. For example, 80 cm with a +0.75 lens is calculated as 1/(80 cm) - (+0.75 D) = 0.5 D.
Near power is the same, but measure the closest distance you can see clearly without blur.
- For example, if the closest you can see is 10 cm without lenses, it’s calculated as 1/(10 cm) - 0 = 10 D.
- It might be useful to use a big minus lens to measure young people (don’t look through it for too long!). In the example above, you expect to see as close as 50 cm with a -8 lens, which is 1/(50 cm) - (-8 D) = 2 D + 8 D = 10 D.
The power of accommodation is (near power) - (far power). With the first example in each of the above, it would be 10 D - 1.5 D = 8.5 D.
A person is generally considered to have presbyopia iff his power of accommodation is less than 2.5 D.
Caveats
This technique might need to be refined for cylinder, if spherical equivalent is not used.
The cornea and lens actually have more power because the total power has to focus the image on your retina, but they cancel out in the subtraction step, so it’s easier to ignore them.
Videos
Resources
- Here is a resource that may be of interest to people with presbyopia that was found in the Discord chat. The resource is used for training convergence. http://www.i-see.org/rizzi_charts_readvertical.pdf