Question

Your digital camera lens, with an effective focal length of 10.0mm, is focused on a flower 20.0cm away. You then turn to make a picture of a distant landscape. How far, and in which direction, must the lens move to bring the landscape into focus?

Answer 1

Assuming the lens is of negligible thickness, You first need to find the location of the image (where the picture shows up) you can do this either by drawing a ray diagram, or by using the thin lens equation: \[s' = (f+s)/(f*s)\]
Next, you need to find out where the image of the "distant landscape" will appear. Since no object distance is given to you for this, you need to assume that the landscape is essentially an infinite distance away. Paraxial rays (rays coming from an infinite distance), will always come into focus at the focal point, so all that's left is to find the difference between the image location of the flower, and the focal point.