# Thread: An 80 mm focal lenght lens

1. ## An 80 mm focal lenght lens

An 80 mm focal lenght lens is used to focus on an image on the film of a camera.The max distance allowed between the lens and the film plane is 120mm. Determine how far ahead of the film the lens should be if the object to be photographed is 10 meters away.

2. Originally Posted by westdivo
An 80 mm focal lenght lens is used to focus on an image on the film of a camera.The max distance allowed between the lens and the film plane is 120mm. Determine how far ahead of the film the lens should be if the object to be photographed is 10 meters away.
Let o denote the distance between the lens and the object, i the distance between the lens and the image and f the focal length of the lens.

Use the equation

$\displaystyle \frac1o + \frac1i=\frac1f$

Plug in the values you know to calculate i:

$\displaystyle \frac1{10,000} + \frac1i=\frac1{80}~\implies~\frac1i=\frac{31}{2500 }$

Solve for i. I've got $\displaystyle i\approx 80.645\ mm$