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Exposure (Part 4)
Exposure values (EV)
We've already touched upon EV in a previous section as it is often used as a way of comparing one exposure with another (for example if your picture is too dark you could try increasing the exposure by +1EV or +1.5EV).
However EV is sometimes used as an absolute measurement and, in this case, a particular exposure must be defined as the zero point. The convention for this is that exposure value 0 (EV 0) corresponds to an shutter speed of one second at f/1.0.
Whether EV is used as an absolute value or a relative value, a change of +1EV always corresponds to a doubling of the amount of light, and -1EV to halving the amount of light reaching the sensor.
Here's a table of exposure values showing aperture across the top and shutter speed down the side. This is useful for seeing at a glance equivalent exposures.
|
|
1.0 |
1.4 |
2.0 |
2.8 |
4.0 |
5.6 |
8.0 |
11 |
16 |
22 |
32 |
45 |
|
8s |
-3 |
-2 |
-1 |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|
4s |
-2 |
-1 |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
|
2s |
-1 |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
1s |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
|
2 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
|
4 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
|
8 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|
15 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
|
30 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
|
60 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
|
125 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
|
250 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
|
500 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
|
1000 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
|
2000 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
|
4000 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
23 |
Before the days of modern cameras with built in exposure meters, it was always handy to have a rough idea of what sort of EV value corresponded to typical conditions. Here's a table showing some common lighting situations. The values are given for an ISO setting of 100.
| Lighting conditions |
EV (ISO 100) |
| Bright Sunlight - light sand or snow |
16 |
| Bright Sunlight (strong shadows) - general scene |
15 |
| Bright Sunlight (soft shadows) - general scene |
14 |
| Daylight, Cloudy (no shadows) |
13 |
| Daylight, heavy overcast |
12 |
| Daylight, areas in the shade |
12 |
| Sunset - just before sunset |
12 to 14 |
| Sunset - at sunset |
12 |
| Just after sunset |
9-11 |
| Moon (Full moon) |
15 |
| First / Third quarter moon phase |
13 |
| Crescent Moon |
12 |
| Moonlit scene (no artificial lights) |
-6 to -3 |
| Night - bright street scene |
7 to 8 |
| Night - vehicle traffic |
4 to 6 |
| Floodlit monuments, fountains / Christmas tree lights |
3 to 5 |
| Indoor: Sports events and stage shows |
8 to 9 |
| Indoor: Offices and work areas |
7 to 8 |
| Indoor: home interiors |
5 to 7 |
Note that for each stop you increase the ISO you would add one to the value above before looking up the aperture/shutter speed pair in the first table.
Exposure compensation 
This is one of the most important exposure tools you will have on your camera (it may not be available on the most basic point-and-shoot models). This allows you to take the camera's recommended exposure and either increase or decrease it by a specified amount. This is not the same as using a camera in manual exposure mode where you are setting the exposure. Here you are just shifting the cameras exposure.
For example, if you are taking a lot of shots in the snow, you may wish to set the exposure compensation to +1 EV. As you point the camera at different parts of the scene, the exposure will continually change, however the camera will always set it at +1 EV over what it thinks is the correct exposure. You are effectively telling the camera that you want the average tonal value in the scene to be lighter than mid-grey (which for a snow scene is most likely the case).
Auto Exposure Bracketing (AEB) 
This is an extension of exposure compensation and is available on some compacts and nearly all DSLRs (the notable exception here being the Nikon D40 and D40x - what were they thinking?).
Bracketing is the technique of taking more than one photograph of the same scene using different settings - the most common approach being to take three photos, the first with the 'correct' settings (as deemed by the camera or photographer), and two more with the settings either side of the first shot. This allows you the flexibility to pick your favourite image later. Common types of bracketing include exposure bracketing, focus bracketing, white balance bracketing and flash bracketing.
Exposure bracketing is the most common form and dates back to early film photography. Three pictures (or occasionally five) are taken with the cameras recommended exposure and then two more a fixed exposure value either side (± 1EV or ±2EV being common). Although this was traditionally done manually by the photographer, modern digital cameras often have auto exposure bracketing in which the camera takes the three pictures for you, adjusting the exposure by the required amount for each one, and often triggered by a single press of the shutter button (Canon DSLRs require you to press the shutter button 3 times unless you have continuous shooting mode enabled).
Overexposure
For any given exposure, there is a maximum brightness that the image sensor in the camera can record. If any parts of the image exceed that brightness, they will be recorded as regions of pure white. These are referred to as burnt highlights. These are generally considered very undesirable, especially if they occur in large patches. Because of this, many cameras have an option to show areas of burnt highlights when previewing the image on the LCD screen. They are often represented as flashing black and white regions and are commonly known as blinkies. If your camera has this capability, it is worth using it.
If you have been using the raw mode, you can often recover some areas of burnt highlight in the conversion process. Adobe Camera Raw (the Raw conversion utility that comes with all versions of Photoshop) has a slider called recovery which will attempt to reduce areas of burnt highlights.
Over exposure also destroys colour saturation. This is why colours often appear much richer in images that are a little darker.
Summary of exposure (Parts 1-4)
Exposure of an image is dependant upon the amount of light available, the selected ISO, aperture and shutter speed.
ISO is the sensitivity of the light sensitive medium (camera film or digital image sensor).
The aperture is an adjustable opening at the rear of the lens. Wide apertures (eg. f/2) result in a shallow depth of field, narrow apertures (eg. f/22) result in a large depth of field.
Shutter speed is the measurement of the amount of time the camera's shutter is open. Slow shutter speeds are where the shutter is open for a relatively long time: fast shutter speeds are where the shutter is open for a short time. Fast shutter speeds freeze motion while slow ones show motion.
For any given ISO, the shutter speed and aperture need to be balanced to allow the right amount of light onto the sensor.
If the camera's light meter gets it wrong, use exposure compensation or the exposure bracketing and try again.
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