# A camera is being used with a correct exposure at f/4 and a shutter speed of 1/15 s.In addition to more standard f-numbers, this camera has f-numbers f/1, f/1.4, and f/2. To photograph a rapidly moving subject, the shutter speed is changed to 1/122 s.Find the new f -number setting needed on this camera to maintain satisfactory exposure.

A camera is being used with a correct exposure at f/4 and a shutter speed of  1/15 s.In addition to more standard f-numbers, this camera has f-numbers f/1, f/1.4, and f/2. To photograph a rapidly moving subject, the shutter speed is changed to 1/122 s.Find the new f -number setting needed on this camera to maintain satisfactory exposure.
f /

asked Apr 22, 2012 in Physics

The f-number of a lens is given by:

f-number = f / D

*where f = focal length, D = diameter

The total light hitting the camera lens is proportional to the product of the intensity and the exposure time:

I1*t1 = I2*t2

furthermore, since intensity is given by D^2, we get:

(D1)^2 * t1 = (D2)^2 * t2

The initial diameter is 4, since f-number = f / D , and question gives f/4

so D1 = 4cm,

t1 = 1/15 s

t2 =  1/122 s

plug in the formula above and solve for D2

Therfore the final answer will be

f-number = f / D2

answered May 5, 2012 by ~Expert~ (1,105 points)
edited May 5, 2012 by Gummmys