Question

A photographer uses his camera, whose lens has a50 mm focal length, to focus on an object 3.5 maway. He then wants to take a picture of an object that is 60 cm away.

A.How far must the lens move to focus on this second object?

Answer 1

solution:

Use the lens equation with the data from the first object:
convert focal length to m
1/i + 1/O = 1/f --> 1/I + 1/3.5(m) = 1/(0.05 m) -->1/i = 20 - 0.5 --> I = 0.05072 m

Now for the second object O = 0.6 m so

1/I + 1/O = 1/f --> 1/I = 1/0.05 - 1/0.6 ---> I = 0.0545454m

So the lens must move away from the image plane (toward the object) by a distance of |0.05072 - 0.0545454| = 0.00383m ans