# Does 0.25 diopter reduction equal 4 meters shrinkage of dioptic bubble?

The standard advice is to lower spherical strength by 0.25 diopter at a time.
How does that correlate to the 20/20, 20/30, 20/40… eye chart?
How much distance will I see less clear of (how much does the dioptic bubble shrink)?
1/0.25=4 meters Is that right?

If you see 20/20 with full prescription and you reduce that by 0.25 then your edge of blur (size of the blur bubble) will be around 4m, yes. But it differs when you reduce not from full prescription, because edge of blur does not change linearly with diopter. If you want to calculate your edge of blur of any glasses you have to calculate it compared to your full prescription. For example if your full prescription is -3.00 and you use:

• -1.50 differential, then your blur bubble is 1/(3.00 - 1.50) = 1/1.5 = 0.66 m
• -2.50 normalized, then your blur bubble is 1/(3.00 - 2.50) = 1/0.5 = 2m
• -2.75 normalized, then your blur bubble is 1/(3.00 - 2.75) = 1/.025 = 4m
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There is no direct link between visual acuity and refractive state. Some of us start with more or less best corrected acuity.

@halmadavid @Dlskidmore Thank you so much for clarifying! This is why it takes trial and error to find the best differentials/normalized, or a test lens kit.