Question

A person using -5D lens can see clearly upto 500cm.at what distance he can see without using spect

Answer 1

He should use concave lens. 
P = 100/f(in cm)
   = 100/50
   = 2 D

1/f = 1/v - 1/u
1/f = -1/50 - (-1/25)
1/f = -1/50 + (1/25)
1/f = (-1 +2)/50
1/f = 1/50
f = 50cm or 0.5m
I hope it will help you