Physics of a Film Camera Lens


Have you ever wondered how a camera is able to focus in order to take photographs?

The answer is simple physics.

Photo taken by Jonathan Gates

The Basics

A camera at the very basic level is a light proof box that will allow for light to enter in one place. A camera will open its’ shutter, which is a doorway into the camera, and allow for a split second of light to be transmitted into the camera. That light contains the image you wish to capture. Since we can only see things that light reflects off of. Once the light hits the back of a camera it will hit a light sensitive material made of silver nitrate which will cause a chemical reaction to take place making the places where light touched dark and the unexposed film to remain transparent.



The Convex Lens

Cameras use convex lens to take real inverted images. This is because light rays always travels in a straight line, until a light ray hits a medium. The medium in this case is glass. The glass causes the light rays to refract (or bend) this causes them to form inverted on the opposite side of the medium.

  • Aperture              

  • Shutter

  • Lens

Camera Lingo

  • A hole which size can be changed to allow light passes out of the lens, and into the camera. This is important for clear images without distortion around the edges.
  • The shutter is a doorway that will allow for light to pass through out of the aperture. The shutter speed will allow for long or short exposures thus allowing for pictures of fast moving object or low lighting photos to be taken.
  • A piece of curved glass that will focus light allowing for clear images to be transmitted onto the unexposed film 

f= Focal length
do= Object  distance 
di= image distance                  
This formula is how a lens is able to focus on an object. The problem is this equation is for ideal lenses, strictly speaking for lenses that have "zero thickness".  However lenses in cameras can be manufactured to act almost as though they are "perfectly thin."

Focusing with your feet?

Lets say you want to take a photo of a tree, you pull out your camera and try to snap a photo, the problem is that the tree is blurry and out of focus. You could say walk towards the tree and it would cause the image to come into focus,  or using our handy dandy formula we can calculate how much the lens needs to move to cause the camera to focus. Lets say you have a lens with a focal length of 10mm, and you want to take a photo of the tree that is one meter away (1000mm) using some simple algebra we can calculate that the lens needs to move only 10mm to focus on the tree. Conversely, if you could not adjust the lens you would have to move back over a meter in length to cause the tree to come into focus.

Focus Time

Light travels slower through different mediums like for example glass or whatever the lens is made of, sometimes plastic. When one side of a light ray hits the lens, part of it slows down and the ray bends. Think of it like a car, “If the wheels on one side of the car turn slower than the other side, the car turns towards the slower wheels. When one side of a light ray travels faster than the other, the light will bend towards the slower side.” The lens will cause the light to bend and focus as a point on the other side.