# Lens question

1. Nov 8, 2009

### mustang1988

1. A 70 mm focal-length lens is used to focus an image on the film of a camera. The maximum distance allowed between the lens and the film plane is 120 mm. How far ahead of the film should the lens be if the object to be photographed is at the following distance from the lens?When it is 13.2m away, 3.2m away, and 1.9m away

2. 1/f=1/di+1/d0

3. I am not sure which numbers to plug in. Is the film plane significant?

2. Nov 8, 2009

### mgb_phys

di is the image (ie film) to lens distance, do is the object to lens distance.

Hint - make sure that f,di and do are the same units

3. Nov 8, 2009

### mustang1988

Ok so I know f is 70mm and do is the 13.2,3.2, and 1.9 and I am supposed to solve for di but I dont understand what the film plane of 120mm is for.

4. Nov 8, 2009

### rl.bhat

The object distances are given from the lens when it is at 120 mm from the film. To focus the image on the film, you have to move lens back. Let x be the distance moved by the lens to focus the image.
Now in this position what is the object distance and the image distance.
Substitute these values in the formula and find the value of x.

5. Nov 8, 2009

### mustang1988

Ok, that makes a lot more sense. Thanks