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Managing equivalent exposures

By December 7, 2017 Photography No Comments

Exposure is determined by three variables: shutter speed, aperture, and ISO. Adjusting any of these three changes the exposure of a photo, but to complicate things further, changing one of the settings has effects other than just making a photo brighter or darker.

Shutter speed: A long shutter speed lets in a lot of light, while a shorter shutter speed lets in less light. A long shutter speed also allows for more movement, or blur, to be captured in your photos. A shorter shutter speed will ‘freeze’ a scene.

Aperture: ‘Aperture’ comes from a Latin word meaning ‘hole.’ It allows light to reach the sensor. A variable aperture will allow more or less light into the camera. But creatively speaking, a larger aperture will make for a shallower depth-of-field with less of the scene in focus.

ISO: The final variable in our exposure puzzle is ISO. In the days of film, ISO was a number describing how sensitive the film was to light—an ISO 400 film was twice as sensitive as an ISO 200 fillm. This isn’t quite how it works in the digital world, but for all practical purposes, it’s best to think of ISO as light sensitivity: higher ISO is more sensitive, lower ISO is less sensitive. But, a higher ISO will introduce more ‘digital noise’–a grainy looking effect–into your photos.

Equivalent exposures

Because the aperture of your camera changes more than just how much light reaches the sensor you might decide that you want a smaller aperture, in order to get a greater depth-of-field in the shot. A smaller aperture means that less light comes in to the camera, which might lead to an underexposed image. So how do we avoid this?

The three different exposure factors–aperture, shutter speed, and ISO–allow you to control for this. If you adjust one so that the scene will be brighter, you can adjust another to make it darker again.

Let’s start with an example. You have a shutter speed of 1/100 seconds, an aperture of ƒ/4.0, and an ISO of 200. You could change your shutter speed to 1/200 second, which would let in only half the amount of light because the shutter is only open for half the duration. Your photo will now be darker. If you change your ISO to 400, however, the sensitivity of the sensor is doubled, and the photo will come out looking the same as your original exposure (from a brightness point of view).

You can change any of the settings to compensate for any of the other settings: a smaller aperture can make up for a higher ISO; a faster shutter speed can make up for a larger aperture; and a lower ISO can make up for a slower shutter speed.

equivalent exposures

It can be useful to think of the three variables in an exposure as a slider: if you increase one and decrease another, you end up with the same brightness overall. However, as you will discover, adjusting one element, such as aperture, can have other effects on an image.

The Beginner’s Guide to Photography is Haje Jan Kamps’ straightforward, no-nonsense, jargon-busting guide to taking terrific photos. It starts with the basics of exposure and moves through to getting the best out of flash and taking pictures of tricky subjects. If you’re looking for the perfect book to start off your photography career, this is it!

The Beginner’s Guide to Photography, Haje Jan KampsThe Beginner’s Guide to Photography
Haje Jan Kamps

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