Question

Correct exposure for a photographic print is 10 seconds at a distance of one metre from a point source of 20 candela. For an equal fogging of the print placed at a distance of 2 m from a 16 candela source, the necessary time for exposure is

• A. 100 sec
• B. 25 sec
• C. 50 sec
• D. 75 sec

Answer 1

Answer: (C)

50 sec

Answer 2

For equal fogging $I_{2} \times t_{2} = I_{1} \times t_{1}$ $\Rightarrow \mspace{6mu}\mspace{6mu}\frac{L_{2}}{r_{2}^{2}} \times t_{2} = \frac{L_{1}}{r_{1}^{2}} \times t_{1}$

$\Rightarrow \mspace{6mu}\mspace{6mu}\frac{16}{4} \times t_{2} = \frac{20}{1} \times 10$ $\Rightarrow \mspace{6mu}\mspace{6mu} t_{2} = 50Sec.$