How Is Angular Width Calculated in Lens Optics?

[SamTsui86]

Homework Statement

A converging lens with a 50.0 mm focal length is used to focus an image of a very distant scene onto a flat screen 37.0 mm wide. What is the angular width of the scene included in the image on the screen?

Homework Equations

The Attempt at a Solution

Basically I just did tan-1(37/50) = 36.5

What are I doing wrong?

 

[daniel_i_l]

Do you know how to calculate the height of the image as a function of it's distance form the lens? Use that to find get the distance and then use the distance and width to get the angle. Your problem is that you assumed that the distance was the focal length which isn't true.