Understanding exposure

About The Author

CNET Editor

Lexy spent her formative years taking a lot of photos and dreaming in technicolour. Nothing much has changed now she's covering all things photography related for CNET.

Understanding exposure header

Once you know the bits and pieces of your digital camera, you're ready to move into a bold new territory. Automatic mode? Out the window. Let the camera do the work for you? Forget it. We're going fully manual — welcome to the world of exposure.

What is exposure?

You've probably come across the terminology "obtaining a correct exposure" in your photographic travels. Exposure is the measure of light that reaches the image sensor in order to capture an image. There are three main elements that determine how either a camera (in automatic modes) or a photographer (in manual modes) determines exposure.

ISO, aperture and shutter speed all make up exposure

Exposure is made up of three elements, working together. (Credit: CBSi)

Definition: ISO

Back in the old days of film, each roll would have an ASA or ISO rating, which meant how fast or how sensitive to light it was. Digital cameras work on the same principle, but this time, replace film with the image sensor. ISO is the sensitivity level, so an ISO of 100 is relatively slow, and an ISO of 400 and above is considered fast.

You would use a "slow" ISO when there is a lot of light so the image sensor is less sensitive, and a "fast" ISO when there is less light and it is difficult to get a decent exposure without introducing camera shake into your images.

Definition: aperture

Think of aperture like the pupil in your eye. Aperture is a measure of how much light is let into the camera through the lens. Like your pupil, the lens can open up (widen its aperture) to let more light in, or close down (narrow its aperture) to let less light in. Aperture is measured in f/ stops and affects depth of field.

Using a wide aperture (small f/ stop) will produce an image with a blurred background and sharp foreground, or area of focus, and a small aperture (large f/ stop) will produce an image with sharpness across more of the image. This will be explained further when we discuss depth of field.

A diagram showing the differences between apertures

The lens on the left is stopped down to f/22 (letting in the least amount of light), in the middle is f/8, and on the right, f/2.8. (Credit: CBSi)

Definition: shutter speed

The length of time the shutter is open, measured in fractions of a second. A fast shutter speed will let in less light than a slower shutter speed. For example, a shutter speed of 1/60 will let in more light than a shutter speed of 1/125.

In the following sequence, each shutter speed lets in half the amount of light than the one that came before.

1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500

The same principle applies for aperture, but when you change the measure it's called "stopping up" or "stopping down", or just a stop.

f/2.8, f/4, f/5.6, f/8, f/11/, f/16, f/22

The same exposure can be reached through a combination of apertures and shutter speeds. For example, an exposure of f/8 at 1/125 is equivalent to f/5.6 at f/250.

Depth of field

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KevinH2 posted a comment   

very useful


CMc posted a comment   

informative and to the point. not too technical yet I feel like I reviewed / learned a lot


Ajith posted a comment   

Good writeup..easy to understand


Bec posted a comment   

Thanks for the write up. the explanations are very easy to understand. Many Thanks :-)


Aaron posted a comment   

Great article. Very helpful to understand some of the more technical terms that overwhelm new photographers


Nath posted a comment   

Great write up, thanks big help.


Mavis posted a comment   

Excellent write up and easy to understand. Thank you!


anysia posted a comment   

Good job on the write up. Many new to digital SLRs can be overwhelmed, and end up using the Auto settings only, because reading the manuals' same tech phrases over and over again can be mind numbing. This was written for the new users in mind.

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