First I highly recommend that you go through this tutorial. This is the best resource I have found on refraction. After you have gone through the plus and minus cylinder refraction tutorials read the rest of this post.
General Eye Shapes
Emmetropia: This is a normal eye “well-proportioned”
Myopia: This is nearsightedness. This is an eye that can see near objects well, but can’t see distant objects well. Myopic eyes may be longer than the normal eye. This means that light will converge before it hits the retina. A minus spherical power lens will help light converge on the retina.
Hyperopia: This is farsightedness. This is an eye that can see far objects well, but can’t see near objects well. Hyperopic eyes may be shorter than the normal eye. This means that light will converge after it hits the retina. A plus spherical power lens will help light converge on the retina.
Myopia is the short word. Think,”The point of light convergence stops short of the retina.”
Hyperopia is the longer word. Think, “The point of light convergence is long past the retina.”
Presbyopia: The inability to focus due to loss of lens elasticity.
Aphakia: Having no lens
Pseudophakia: Having a new lens such as the lens placed in the eye after cataract surgery.
Automated refractometry, AR, is the way that we get an objective refraction. To preform AR a patient is asked to look at a central target. This target is useful to help avoid accommodation. The AR gathers information about the eye. The AR pumps out a prescription according the the eye measurements that it makes. AR is normally used as a starting point for refraction if no previous glasses have been prescribed.
Manifest refractometry is the way the we get a subjective refraction. A subjective refraction is the refraction that the patient likes the most. An objective refractions is the refraction that the AR thinks would be good for the patient based on the measurements made.
Manifest refractometry is acquired by asking the patients which series of lenses they prefer. This requires a phoropter. For a complete tutorial on manifest refractometry click here. This is the same link as above. If you have already watched the tutorial and are confident in refracting then go and practice. That is the best way to understand manifest refractometry. If you can successfully refract multiple people and you understand the information in this post you will be able to answer the majority of questions regarding refractometry.
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