Finding distance D and angle theta from a picture

[jcpwn2004]

Homework Statement

I'm supposed to quantify the picture below and show how to find D and the angle theta.

http://img10.imageshack.us/img10/7079/labg.jpg

Homework Equations

none i can think of

The Attempt at a Solution

I guess S and H are the same. I really can't think of a way to find D or theta though. Don't you need D to find theta?

 

[rl.bhat]

Opening of the camera, object and image from two similar triangles. Their sides are proportional.
Length of the camera and film height are known quantities. From that find D and theta.

 

[jcpwn2004]

Opening of the camera, object and image from two similar triangles. Their sides are proportional.
Length of the camera and film height are known quantities. From that find D and theta.

Alright so my camera's length is 14.6cm and the film height is 5cm. Even if I know they're proportional how can i find D?

 

[rl.bhat]

To find D you must know H. As your guess S need no be equal to H.

 

[jcpwn2004]

To find D you must know H. As your guess S need no be equal to H.

Alright well we aren't given H so how would I theoretically get H?

Would I do 5/14.6 = h/d? And after I find D how would i find theta?

 

[rl.bhat]

You can find theta. 2.5/14.6 = tan(theta/2). But that won't give you D unless you know the size of the object.

 

[jcpwn2004]

You can find theta. 2.5/14.6 = tan(theta/2). But that won't give you D unless you know the size of the object.

Alright but even if i know my H, isn't the theta different? let's say my H is 7.5 cm. I got theta/2 for the left triangle to be 4.86 degrees. Where do I go from there though?

 

[rl.bhat]

Yes. For same H theta depends on D. When D changes size of the image also changes.
Theta will be unique only for one setting.