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2. Vertex distance - Wikipedia

   en.wikipedia.org/wiki/Vertex_distance

   Since most refractions (the measurement that determines the power of a corrective lens) are performed at a vertex distance of 12–14 mm, the power of the correction may need to be modified from the initial prescription so that light reaches the patient's eye with the same effective power that it did through the phoropter or trial frame.

3. Prism correction - Wikipedia

   en.wikipedia.org/wiki/Prism_correction

   Individuals with nystagmus, Duane's retraction syndrome, 4th Nerve Palsy, and other eye movement disorders experience an improvement in their symptoms when they turn or tilt their head. Yoked prism can move the image away from primary gaze without the need for a constant head tilt or turn. [1] Prism correction is measured in prism dioptres. A ...

4. Adjustable-focus eyeglasses - Wikipedia

   en.wikipedia.org/wiki/Adjustable-focus_eyeglasses

   Adjustable focus eyeglasses have one focal length, but it is variable without having to change where one is looking. Possible uses for such glasses are to provide inexpensive eyeglasses for people from low-income groups, developing countries, third world countries or to accommodate for presbyopia.

5. Corrective lens - Wikipedia

   en.wikipedia.org/wiki/Corrective_lens

   Switching between distance and near vision is accomplished by re-adjusting the lens, instead of by tilting and/or rotating the head. The need for constant adjustment when the person's attention switches to an object at a different distance is a design challenge of such a lens. Manual adjustment is more cumbersome than bifocals or similar lenses.

6. Dioptric correction - Wikipedia

   en.wikipedia.org/wiki/Dioptric_correction

   At the bottom of the image: the eyeglass Dioptric correction [ 1 ] is the expression for the adjustment of the optical instrument to the varying visual acuity of a person's eyes . It is the adjustment of one lens to provide compatible focus when the viewer's eyes have differing visual capabilities.

7. Astigmatism (optical systems) - Wikipedia

   en.wikipedia.org/wiki/Astigmatism_(optical_systems)

   Astigmatism causes difficulties in seeing fine detail. Astigmatism can be often corrected by glasses with a lens that has different radii of curvature in different planes (a cylindrical lens), contact lenses, or refractive surgery. [5] Astigmatism is quite common. Studies have shown that about one in three people suffers from it.