When a young animal wears a monocular minus (concave) lens that shifts the focal plane away from the cornea, the vitreous chamber elongates over a period of days, shifting the retinal location to compensate for the altered focal plane. We examined the effect of removing the lens for a portion of each day on the amount of compensation in tree shrews. Starting 24 days after natural eye opening, juvenile tree shrews wore a goggle frame that held a –5 D lens in front of one eye, with an open frame around the fellow control eye. The goggle was removed for 0, 0.5, 1, 2, or 7 h each day (N = 5, 5, 5, 5, and 3 animals per group, respectively), starting 0.5 h after the start of each 14 h light-on period. After 21 days of treatment, measures were made of the cycloplegic refractive state (streak retinoscopy) and the ocular component dimensions (A-scan ultrasound). Normal animals that experienced 14 h each day with no lens (N = 3) were also examined. The treated eyes of the 0 h group developed full refractive compensation for the lens (treated eye - control eye, mean ± SEM = –5.8 ± 1.1 D) and had increased vitreous chamber depth (0.13 ± 0.02 mm) and axial length (0.12 ± 0.02 mm) relative to the untreated control eye. The groups in which the lens was removed for 0.5 and 1 h each day showed partial compensation for the –5 D lens, both in refractive state (-4.2 ± 0.4 D; -2.9 ± 1.6 D) and in vitreous chamber depth (0.12 ± 0.02 mm; 0.09 ± 0.02 mm). The 2, 7, and 14 h (normal) groups showed no significant refractive or axial compensation. In the 0.5 and 1 h groups, A-scan ultrasound showed a thinning of the region between the front of the retina and back of the sclera. The eyes of tree shrews can tolerate altered monocular visual stimulation produced by a minus lens worn for 12 h of a 14-h light cycle without developing an induced myopia. However, when the lens is worn more than 12 of 14 h each day, compensation appears to increase linearly with decreased lens-off time. If the eyes of human children respond similarly to defocus from near work or other sources, it would seem that the defocus must be present almost all the time to induce myopia. If defocus contributes to human myopia through a compensation mechanism, then an increase in the amount of time that focused images are present should reduce myopic progression.