To determine your power of accommodation, you need to measure your far power and near power.
Your far power is straightforward, and it’s the cm measurements you normally do: 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.
Your near power is the same, but you 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.
I’m not entirely sure how cylinder affects this measurement, so you’ll have to refine the technique if you have cylinder.
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.