设最初的物距为 u,焦距为 f.
则有：
1/u + 1/(u/2) = 3/u = 1/f
则有 f = u/3
1/(u-100) + 1/(u-100) = 2/(u-100) = 1/f
则有 f = (u-100)/2 = u/3
可以解得：
u = 300 mm,f = u/3 = 100 mm