# Why 0.25D every 3-4 months?

Lately, I came across this study: The time course of the onset and recovery of axial length changes in response to imposed defocus (open source) where they basically put -3D or +3D lenses in front of the subect’s right eye for 60 minutes and measured the change in axial length.
Interestingly, for myopic defocus, they found the eyeball to shorten with a reduction of -10µm in length after 40 minutes or so. This seems to stay constant even for longer defocus periods (Fig. 2 in the paper) and recovers slowly after the lens is removed.

I always wondered WHY the “drop a quater diopter every 3-4 months” rule seems to apply to almost everybody. Here is my attempt to explain it:

Assume that, with active focus and normalized correction, you get enough myopic defocus to achieve the maximum value of -10µm of axial change each day. After enough recovery time (sleep, eat, rest) a fraction x of this change becomes the new normal the next day.

So, -10µm per day seems to be the maximum and roughly translates to 0.03D. The only variable is the fraction x that can be carried to the next day. If we assume x = 7.5% (which is just a random guess) we get pretty close to the 0.25D per 3 months.

3 - 4 months (90 -120 days) times 0.0023D = 0,20D - 0,27D per 3-4 months

Of course, the numbers are somewhat arbitrary but, nevertheless, the limit of improvement is directly linked to the possible axial change per day.

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I read this paper awhile ago and this is some wonderful insight! I just wonder how it is different for us with only -0.25D blur.

Based on Jake’s experience (with a lot of people) it’s not different. The only thing matter is the presence of the stimulus, the quantity of it doesn’t matter (hence there is no reason to be undercorrected more).