Solving Geometry Problem: Disparity in Terms of a, D, d, e, & f

[SoggyBottoms]

Homework Statement

Disparity is defined as $\delta = \alpha - \beta$. Find $\delta$ in terms of interocular distance a, viewing distance D and d, e and f.

http://img220.imageshack.us/img220/7576/43519392.jpg

The Attempt at a Solution

I'm not getting anywhere. Any tips to get me started?

 

[bbbeard]

I would start by writing down the tangent relations. For example, α is the difference between the two angles that Q and P make from the vertical (forward?) direction. So

α = atan(f/(D+d)) - atan(e/D)

and so forth.

BBB

 

[SoggyBottoms]

Thanks. Then:

$\beta = \arctan(\frac{a - e}{D}) - \arctan(\frac{a - f}{D + d})$

Correct? Then I already have my answer it seems.

 

[bbbeard]

Yes. It seems like you're about there. Don't forget the problem asks for δ=α-β.