308 F. Schaeffel

Figure 2. Frozen sections of mouse eyes at the ages of 26 and 44 days (re-plotted from Ref. 15). Note that the lens is growing faster than the globe, reducing vitreous chamber depth with age. Note also that accommodation is very unlikely since the large lens can neither be moved nor deformed. Note also the relative thickness of the retina, which should give rise to a large “small eye artifact” in retinoscopic measurements.26 Scale bars denote millimeters.

Schematic eye data

To make a mouse eye more myopic by one diopter, axial length has to increase by 5.4 m at the age of 22 days and by 6.5 m at the age of 100 days. Retinal image magnification increased from 0.032 mm/deg at 22 days to 0.034 mm/deg at 100 days in the study by Schmucker and Schaeffel15 — not too much of a difference. Using the data by Zhou et al.,18 an axial length of 2.86 mm at P22 converts into an image magnification of about 0.030 mm/deg, and an axial length of 3.34 mm at P102 into 0.034 mm/deg, similar to Schmucker and Schaeffel.15

As in other animals (e.g. chicken, barn owl),28,29 the retinal image brightness increases with age. Image brightness is proportional to the inverse squared f/number, the ratio of anterior focal length to pupil size. Typical for humans are f/numbers around 5 (for a 3.3 mm pupil), but for the mouse, the f/number is as low as 1.02 at day 22. This produces a retinal image 25 times brighter than in humans. Until the age of 100 days, the f/number declines even further, to about 0.93, providing another 20%