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Your Normalized and Differentials Lenses power should steadily reduce in power over time. (+0.25 for myopes, -0.25 for hyperopes, moving towards 0)

Generally, the correction should be a quarter diopter weaker than needed to see clearly, which provides a slight blur stimulus for change, although trading 0.5 cyl for 0.25 sph equivalent is also possible.

When to reduce

As your eyes improve, you will need to reduce again to maintain that slight blur. These reductions typically occur every 3-4 months for sphere and at least 6 months for cylinder. Alternate between differentials and normalized with 4-6 weeks of adjustment period between changes to your focal plane. Do not reduce your sphere and cylinder at the same time. It is important to be sure you are ready to reduce and that your measurements justify reduction. Reducing too soon is never a good idea (see blur adaptation). Ideally, you should be consistently (for a week or two) seeing 20/20 in your current correction, indoors, with reasonably good lighting, on a 20 foot or 6 meter chart. The 20/20 doesn't need to be ultra sharp, remember being able to identify half or better on a line is a pass, but make sure you are only giving yourself credit for letters you can actually see; preferably without active focus.

Example reduction schedule

See also: Guide:Reducing lens complexity

If you work around 70 to 90 cm,

  • Reduce norm at 80cm through diff (gap of 1 D for first half of norm, second half of diff)
  • Reduce diff at 88.89 cm through old diff, 72.73 cm through new diff (gap of 1.25 D for second half of norm, first half of diff)
Normalized Differentials
-2 -1
-2 -0.75
-1.75 -0.75
-1.75 -0.5
-1.5 -0.5
-1.5 -0.25
-1.25 -0.25
-1.25 0
-1 0

This can be generalized to alternating a gap of G-0.25 D and G, with these properties:

  • Reduce norm at 1/G through diff (gap of G-0.25 D for first half of norm, second half of diff)
  • Reduce diff at 1/(G-0.125 D) through old diff, 1/(G+0.125 D) through new diff (gap of G for second half of norm, first half of diff)
  • Norm always has 0 to 0.25 D of undercorrection.
  • Diff always gives cm within 0.125 D of 1/G.

How to reduce

Once your measurements justify that reduction you want to start out with the new correction properly, which is why it is best to perform a "zero diopter reset".

Further Reading