Question

A lens with a focal length of 70 mm forms an image of a person on a digital image sensor. The person is standing 1.4 m away.

1. How far must the lens be from the image sensor in order for the image of the person to be in focus?

2. What kind of lens must it be to form the image: convex (converging) or concave (diverging)? Or could it be either kind?

Answer 1

2) since the image should be real to be formed on the sensor, it should be convex(as concave always form virtual)

and, for forming an real image , rays should converge at an point

* http://www.ehow.com/about_6469013_camera-convex-lens_.html

1)using, 1/v = 1/f + 1/u (this equation can vary depending on the sign you take for 'v' and 'u' . in my case equation, i always take right sign of 'v' and 'u'. if they on left of lens then '-' , if right then '+')

f = 70mm (positive as convex)
u = -1400mm (minus as left side of lens)

v = 73.68mm