A person cannot see objects clearly beyond 50 cm. The power of the lens to correct vision is

Since the person can't see clearly beyond 50cm, we can say that the person is suffering from myopia or nearsightedness. In this case, the image is formed before the retina; therefore, in order to form the image on the retina, a concave or diverging lens is used.
Its power can be calculated as:

$P=\frac{1}{f\left(m\right)}$ 

P= $\frac{1}{50 cm}$ $= \frac{1}{{\displaystyle \frac{50}{100}} (in m)}$ = 2 D

Since concave lens is being used, it is minus

P = -2D