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Exposure

How does a camera decide what shutter speed, aperture and ISO to use when taking a picture? If the answer is not readily apparent to you, don't worry, for you are in good company, judging from the amount of questions we receive on this subject from our readers. Many who are new to photography believe the subject of exposure is too difficult to understand.

In fact, the concept of obtaining a correct exposure (the camera exposes the image sensor to light) is not really that difficult to understand. And a good grasp of the concept will go a long way to help you understand why some of your pictures are blurry and why some are underexposed. A solid grounding in understanding exposure is also the first step to start taking charge of your camera.

As we mentioned earlier, when you press the shutter release button, a camera exposes its image sensor to light, and thus records an image.

Your digital camera has a processor (a CPU, a 'brain') that uses a light meter to evaluate how much light is available, and then it uses that information to determine 1) how much light to admit through the lens and 2) for how long so as to obtain a correct exposure.

Aperture

To control how much light to admit through the lens, the camera varies the lens aperture -- which is simply a hole behind the lens to let light in. The camera can vary the diameter of the hole: the bigger the hole, the more light streams in (if it helps, think of photons streaming in); the smaller the hole, the less light streams in.

Some basic cameras have a fixed aperture, i.e. the diameter of the hole cannot be changed.

Other cameras have only two apertures to select from.

Most cameras can steplessly vary the aperture from large to small.

Of course, the larger the hole (aperture), the better your camera will perform in low-light situations.

Aperture is usually represented by a decimal number, called the f/value. The smaller the f/value, the larger the aperture. So, an aperture of F2.0 is larger than an aperture of F2.8.

Hint: a camera with a maximum lens aperture of F2.0 is better for low-light situations than one with a maximum aperture of F3.5.

Shutter Speed

Varying the aperture only limits the amount of light streaming in. The camera can also decide how long those light photons are going to expose the image sensor. That is, we can abstract it and think of exposure as: the camera is going to allow so many photons in (aperture) for a number of seconds (shutter speed).

So, the shutter speed is simply the amount of time (usually measured in fractions of a second) that the camera allows light to stream in.

A fast shutter speed means that the shutter opens and closes fast, thus allowing light in for only a very short time, such as 1/1,000 sec.

A slow shutter speed means that the shutter opens and closes slowly, thus allowing light in for a relatively longer time period, such as 1/20 sec.

Exposure

It's simple, Grade 3 maths.

Correct Exposure = # of photons * # of seconds

Not quite, but it helps to suspend reality for a moment and think of it this way. [If you really want to know the correct light equation, any college Physics text book should be able to bore you to tears on this fascinating subject.]

Looking at our simplistic (and absurdly incorrect, but sensible enough) equation above, it becomes clear that the camera can play with the aperture and shutter speed and still obtain correct exposure.

If it reduces the # of photons in (i.e. use a smaller aperture), then it needs to increase the time it allows them in (i.e. use a slower shutter speed) to maintain correct exposure.

If it uses a faster shutter speed (e.g. to catch action or to prevent camera shake), then it is decreasing the time light streams in, and therefore has to allow more light in (i.e. increase the aperture) to still obtain correct exposure.

Go back and get this concept clear in your mind before reading on.

Blurred Images

If your images are blurred, it is because:

1. your subject moved during exposure, and/or
2. you (the camera) moved during exposure (camera shake), and
3. you used a shutter speed that is too slow for either case above

In Auto mode, your digital camera will usually select a shutter speed that is fast enough to negate camera shake and slight subject movement. A rule of thumb is to use a shutter speed that is the reciprocal of the focal length in use. Usually, a shutter speed of 1/60 sec. and faster will suffice.

Say your digital camera has selected a shutter speed of 1/60 sec. To obtain correct exposure, the camera now needs to select an aperture.

In bright light, the camera selects an appropriate aperture and you obtain a correctly exposed image.

What happens in low-light? Your camera selects the maximum aperture and finds that this is still not enough to obtain correct exposure. Since it is already at the maximum aperture (the hole can't get any bigger), then it has no choice but to adjust the shutter speed to a slower one, perhaps 1/30 sec.

Now, unless you have very stable hands, it is very difficult for anyone to hand hold a camera at this shutter speed without introducing camera shake. And if the camera moves during exposure, you get blurred images.

Possible solutions:

1. The camera automatically fires the flash. By artificially adding more light, it can now select a faster shutter speed to negate camera shake.
2. The camera can automatically boost the sensitivity of the image sensor (i.e. use a higher ISO). However, by doing so, it introduces noise in the resultant images. Some cameras have aggressive noise reduction built-in, but this usually comes at the expense of image detail.

Underexposed Images

This is a variant on the above problem.

If your images are dark (underexposed), it usually is because there simply is not enough light.

In Auto mode, your digital camera will select the slowest shutter speed possible and the widest aperture available, but in low-light, that may still not allow enough light in for a correct exposure.

Possible solutions:

1. The camera artificially adds more light by automatically firing the flash, and/or
2. The camera can automatically boost the sensitivity of the image sensor (i.e. use a higher ISO). However, by doing so, it introduces noise in the resultant images. Some cameras have aggressive noise reduction built-in, but this usually comes at the expense of image detail.

If by applying these solutions, there is still not enough light, then your images will be underexposed.

There, you now understand how your digital camera uses aperture, shutter speed and ISO to obtain a correct exposure (i.e. to expose the image sensor with enough light to properly record an image).

Manual Mode

Many cameras provide a greater range of shutter speeds in Manual mode than is available in Auto mode. For example, in Auto mode, the slowest shutter speed available may be 1/4 sec. while in Manual mode, you can go as much as 30 sec.

So, if you are into night photography, switching your camera to Manual mode and experimenting with different shutter speeds will usually yield a nice picture. Remember, at these slow shutter speeds, a tripod is a necessity.

So, What Should I Do?

Sigh, you explain all that, and the question still is, So what should I do to obtain pictures that are not blurred and that are not underexposed in low-light situations?

To summarize:

• Use flash
• Dial in a higher ISO

If the results are still not good enough for you, then you should seriously think of moving to a digital SLR and obtaining a powerful external flash (with swivelling head) with it. A dSLR allows you to use high ISOs with negligible to acceptable noise; and the powerful external flash guarantees you'll always have enough artificial light to obtain a correct exposure. The swivelling head of the flash allows you to point the flash at the ceiling to obtain a diffused light for a pleasant picture.

So, here's our new practical equation:

Correctly exposed & unblurred images in low-light = dSLR + external flash

You can safely substitute the dSLR with any consumer digital camera as long as it accepts an external flash. Verify with the manufacturer to ensure the flash is compatible with the camera.

This combination of camera plus external flash is really a little known secret that too few amateur photographers use. If your main photography is to take indoor pictures, it is worth exploring this camera plus external flash combination.

Other articles on exposure:

Understanding... Exposure
ISO
Low-LIght Exposure

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