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.