It can be done by looking at your eye’s light reflex by a trained Optometrist with a specially designed light source. This is the objective measurement.
It can be refined by your own responses to which lens makes the image on the eye chart clearer. The two eyes’ power can be balanced by taking your eyes' accommodating factor in. The vergence powers can be considered with further tests with phoria tests or prisms in a binocular vision exam if there are strabismus or large phorias.
It can be further refined by your responses to which lens makes the image on the eye chart larger. With accommodation and vergence factor in again.
These are the subjective measurements.
The clearest two are made when you choose better one or two, so you would get a glasses prescription that you can see clear, comfortable and consistently for distance. When you are squinting or stressed, or have dry eyes, you may change the answer slightly.
The same can be repeated for near, to give you clear, comfortable, and consistently images for desk or near work. Viola!
Your power for far (6m and beyond), and for near (1m or less) could be different.
If you wear the same lens for far and near, it can sometimes cause vision fatigue, especially in those high achievers who focus on the computer all day, 5 days a week.
From time to time, there are those with amblyopia/lazy eyes, strabismus/crossed eyes, two eyes that are very different (-2.00 difference and beyond), and eyes with astigmatism. Those eyes need more tender loving care, lenses with better Optics in the peripheral, and sometimes vision therapy. See an Optometrist for details.
Dr. Yan L. Liang, OD
Contact Warden Optometry to book your vision and health consult.