# Homework Help: How far from camera person should stand?

1. Jan 20, 2010

### agentas

1. The problem statement, all variables and given/known data

A 35 mm camera has a lens with a 50 mm focal length. It is used to
photograph an object 175 cm in height, such that the image is 30 mm high.
(a) How far from the camera should the person stand?
The person should stand 297 cm away from the camera.

2. Relevant equations
1/f=1/o+1/i
m=i/o

3. The attempt at a solution
1/50=1/o+1/i
m=30/175=i/o
I really do not know how to relate hight of the person to distance to the camera .please help someone!How to get this answer?297cm?

2. Jan 20, 2010

### tiny-tim

Hi agentas!
nooo … 3/175 … look at the units!

Try again!

3. Jan 20, 2010

### agentas

you look yourself. Iam using cm instead of meters.What does not change anything.

4. Jan 20, 2010

### tiny-tim

Yes, 3cm/175cm.

5. Jan 20, 2010

### rl.bhat

If u is the object distance and v is the image distance, then
m = v/u = i/o.
Now 1/f = 1/u + 1/ v
Multiply by u to both side. you get
u/f = 1 + u/v = 1 + o/i.
Now solve for u.

6. Jan 20, 2010

### DaveC426913

Uh guys? Zero.

The person should stand close enough to the camera to look through the viewfinder!

Of course, that doesn't say anything about how close the object should be...

7. Jan 20, 2010

### agentas

thanks I got the right answer :)