BASICS: Understanding Exposure.

Getting a firm grip on exposure is vital for any new photographer. With the inception of fully automatic cameras however, many photographers simply do not take the time to learn about this integral part of photography.

In this brief summary we will discuss the basics of exposure.

Terminology: Exposure:
The amount of light which enters the camera, controlled by the aperture and shutter speed.

Terminology: Aperture:
The hole through which light enters the camera. Sometimes known as f-stop.

Terminology: Shutter speed:
The length of time the shutter is open, exposing the sensor (or film) to light.

If in an exposure not enough light enters the camera, the resulting image will be dull and dark. This image can be referred to as under-exposed.


under-exposed

If in an exposure too much light enters the camera, the resulting image will be excessively bright. This is known as over-exposed.


over-exposed

To get a good exposure, we need to find the correct balance between shutter speed and aperture.


well-exposed

This can be done using a light meter. You can use a separate light meter or the one that is built into your camera.

If using a separate light meter you will need to set your ISO and shutter speed and the meter will give the appropriate aperture for your exposure.

The light meter in your camera will work in exactly the same way, allowing you can set either aperture (in aperture priority mode (Av)), shutter speed (in shutter speed priority mode (Tv)) or both (in manual mode (M))

In aperture priority mode (Av) you will set the aperture to the desired setting, and the camera will calculate the appropriate shutter speed to give a good exposure.

In shutter speed priority mode (Tv) you set the shutter speed and the camera selects the appropriate aperture.

In Manual mode, you control both aperture and shutter speed and you will need to use the guide in your viewfinder to ensure that your exposure will be consistent.

The display:



The above image shows the LCD panel of your DSLR. The important parts to take notice of for this tutorial are highlighted in red.

Shutter speed: shows the shutter speed that either you have selected (Tv or M mode) or the shutter speed the camera has selected based on the aperture you have chosen (Av mode).

Aperture: shows the aperture that either you have selected (Av or M mode) or the aperture the camera has selected based on the shutter speed you have chosen (Tv mode).

ISO: the ISO you have chosen.

Exposure: this meter shows the exposure compensation you have chosen detailed below:

: the meter shows that the image will be under-exposed by 2-stops.

: the meter shows that the image will be under-exposed by 1-stop.

: the meter shows that the image will be correctly exposed.

: the meter shows that the image will be over-exposed by 1-stop.

: the meter shows that the image will be over-exposed by 1-stop.

In Av and Tv modes, the camera will automatically choose the setting for optimum exposure based on the lighting conditions. In these modes, you can override the exposure by turning your click wheel to choose either under or over exposures. Under exposure can be used to give a more even exposure if there is a strong light source that is affecting the camera's reading. Over exposure can be used to give a more even exposure if there is an area of extreme darkness that is affecting the camera's reading.

In Manual mode (M), you adjust your aperture and shutter speed and should use the exposure meter (on your LCD or in the viewfinder) as a guide to your exposure as per the above diagrams.

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So now we understand the principles of exposure. Let’s look at things in a bit more detail.

Now, if we have a consistent exposure at a given aperture, how will changing this aperture affect the shutter speed?

Reducing the size of the aperture will cause less light to enter the camera in a given time, meaning that the shutter speed will need to lengthen to ensure consistent exposure.

It is true that:
half the aperture size = double the shutter speed
and
double the aperture size = half the shutter speed.

Apertures are represented as numbers on your camera and typically range between around f2.8 and f22 (sometimes more). It is important to understand two points:
1. The lower the number, the bigger the aperture. So on a lens range of f4 to f22, the biggest aperture you have will be at 4 and the smallest at 22.
2. For every 3 increments in aperture size, you will have changed the size of the aperture by a factor of 2. Each increment of 3 is one f-stop.

Let us take an example the range f4 to f22:

Looking at the numbers on your camera between these limits you will see the following sequence:



Here, our full f-stops will lie at 4, 5.6, 8, 11, 16, 22. This means that f5.6 is half the size of f4 and will result in the shutter speed doubling. In turn, f8 is then half the size of f5.6 and so on. The same will be true for any increment of 3 in the range.

With this information we should now be able to calculate how a change in aperture will change the shutter speed.

Example:
Using the range f4 - f22 and given that our optimum exposure is at 1/60th second at f8.
What will be the optimum shutter speed at f16?

Solution:
f16 is a larger number that f8.

Therefore f16 is a smaller aperture than f8.

Shutter speed will be longer at f16, but by how much?

Number of increments between f8 and f16 = 6.

If 3 increments causes a change by a factor of 2, then 6 increments will create a change by a factor of 4.

Therefore the optimum shutter speed will need to lengthen by 4 times.

4 x 1/60 = 4/60

= 1/15th second.

So if optimum shutter speed at f8 is 1/60th second then our optimum exposure at f16 is 1/15th second.

Task:
Using the same principles, work out the optimum shutter speeds at each of the main f-stops in our range (i.e. f4, f5.6, f8, f11, f16 and f22).
scoll down to see solution

That's it. Hopefully this brief introduction gives you a better understanding of how exposures work.
















Solution:

f4 1/250th* (technically should be 1/240th)†
f5.6 1/125th** (technically this should be 1/120th)†
f8 1/60th
f11 1/30th
f16 1/15th
f22 1/8th*** (technically should be 1/7.5th)†

† These shutter speeds although technically slightly incorrect are close enough to give very accurate exposures and vary simply to allow the numbers to factor better. The numbers I have given will be mirrored by your camera’s settings.