6 HOW the camera controls light


  • Understanding Aperture and F stops
  • Understanding Shutter and Shutter speeds

The camera controls the amount of light entering it in two ways, the aperture and the shutter.

Understanding aperture and shutter speed are the keys to driving the car, which is why there is a separate chapter on these functions named Film Exposure. So don’t worry if it doesn’t make sense yet, just remember the terms, and be sure you know where the controls are to access them on your camera.


Small size aperture or lens opening
Large size aperture or lens opening

The lens of the camera has an opening called the aperture. The size of the aperture can be adjusted by rotating a portion of the lens (mechanical cameras), or by turning the command dial (electronic cameras) when it is in aperture mode. You will know you have found the correct dial for changing the aperture when you see more or some of the following numbers: 2, 2.8, 4, 5.6, 8, 11, 16, 22.  These numbers are called f stops, and indicate the size of the opening.

top:50mm lens showing f stops from f2 to f22bottom: rotating ring for aperture size selection
50mm lens showing F stops from F 2 to F 22

The bigger the opening the more light will enter the camera, making your image brighter. Although it gets a little confusing, because the bigger the number, the smaller the opening….go figure!

For example, f 16 is a small opening and therefore will not let in a lot of light. This might be a good setting for shooting outdoors in bright light.  f 2.8 is a big opening and lets in a lot of light. This might be a good setting for shooting indoors.

To make it even more confusing, your camera lens may not have all these numbers and if you have an electronic camera you may have additional numbers in between the numbers shown above.  Your manual will be helpful here.


Because the aperture is always open, light is always coming into the camera….however it gets stopped from touching the film by a curtain that is called the shutter.

The shutter curtain is always closed protecting the film from the light coming in through the aperture. What happens when you push the shutter release button?…If you guessed that the curtain opens, you were right! 

As soon as the curtain opens, the light coming in from the aperture will expose the film for as long as the shutter stays open.


35mm SLR camera, back open showing shutter curtain close
Camera showing the shutter curtain closed
shutter open while shutter release button is pressedshutter release button
Camera showing the shutter curtain open
Shutter speed

There is a dial on your camera that allows you to set the time for how long the shutter will stay open. The longer it stays open the more light will get on the film.  clockwise from top left. shutter speed set at 125/sec, ISO set at 400, shutter release button, frame counter, film advance lever, shutter speed dial

To locate the dial that controls the shutter speeds, look for these numbers: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. 

These numbers represent fractions of a second. 1 is really 1/1  (one second) and 2 is 1/2 (half of a second), 4 is 1/4  (a quarter of a second), etc.  As the numbers get bigger, the time the shutter stays open gets shorter.

For example, a shutter speed of 60 (which is 1/60th of a second ) will let more light in than a shutter speed of 500 (1/500th of a second).

If you are in a situation with low light, you will need to keep the shutter open longer, to let more light in, and you will want shorter times if you are in bright sun.

ACTIVITY :  Use your camera for this activity
  •  Set the aperture to f 5.6 and the shutter to 125, now change that to Aperture f 8 and shutter to 15, f 16, and  250………. You’re getting the hang of it.  if you are using a mechanical camera you need to use the film advance lever, after you click the shutter, even if you don’t have film in the camera. The camera does not allow you to take a second picture without doing this in order to prevent accidental double exposures.


  • Aperture controls the quantity of light entering the camera
  • F stop is the measurement of the aperture or lens opening
  • Shutter controls how long the light hits the film
  • Shutter speed is the speed with which the shutter opens and closes




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Photography: What, How, Why Copyright © 2023 by Maria Politarhos and Randy Matusow is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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