Sebuah benda terletak 30 cm di depan cermin cekung. Jika tinggi bayangan sama dengan tinggi bendanya maka jarak titik fokusnya adalah

M = -s' / s = h' / h
    = -h / h = -1

1 / f = 1 / s + 1 / s'
1 / f = 1 / s - 1 / (Ms)
1 / f = (Ms - s) / (Ms²)
1 / f = (M - 1) / (Ms)
f = Ms / (M - 1)
  = -1(30 cm) / (-1 - 1) = 15 cm

s = 30 cm
h = h'

s' = h'/h . 30 cm
s' = 30 cm

1/f = 1/s + 1/s'
1/f = 1/30 cm + 1/30 cm
1/f = 1/60 cm
f = 60 cm