Any photographic film needs an optimum amount of light energy to achieve the chemical reactions needed for a good image. The same is equally true of a digital camera’s sensor.
Exposure is a measure of how well the amount of light energy falling onto the sensor matches the sensor’s needs. Under-exposure occurs if too little light energy falls onto the sensor and the result is a dark, grainy image with little or no detail in the shadows. At the other end, over-exposure occurs when too much light energy falls on the sensor and the result is a washed-out image with little or no detail in the highlights.
The amount of light falling onto the sensor is determined by the shutter speed and the area of the lens aperture. Using a slower shutter speed or opening the aperture will allow more light to reach the sensor. On the other hand, if we open the lens by one stop but set the shutter speed one stop faster, the light reaching the sensor will be unchanged. It's like filling up a kettle; we can open the tap fully for a short time or half-way for twice as long. This is represented below by a balance; to keep the same exposure, if f/number goes up, shutter speed goes down.
There will always be a range of possible f/numbers and shutter speeds that will allow the correct amount of light to fall onto the sensor. Modern digital cameras have a number of shooting modes and depending on the mode selected, the camera will set the aperture and/or shutter speed to achieve the exposure. See: Shooting modes
Film speed – ISO
Photographic film is manufactured with different sensitivity for different purposes. Film sensitivity is known as “ISO film speed” and is a number usually in the range 25 – 1000, with a typical film for everyday use having a sensitivity of about ISO 100. Lower sensitivity film will give finer grain and better colour but at the expense of needing lots of light. High speed film can be used with less light but at the expense of noticeable grain and less pleasing colour.
Happily, digital cameras are designed to respond in a similar manner to conventional film cameras so the same thoughts apply. In addition, digital cameras allow user selection of ISO film speed. This doesn’t actually change the sensitivity of the light sensor but it tells the camera to simulate a more sensitive sensor. A typical digital camera will have an “automatic” ISO setting as well as a manual range of about 100 to some 25,600 or more. A detailed discussion of how digital cameras simulate variable ISO is provided here.
ISO = International Organization for Standardization
Setting the correct exposure is a two-stage process - first, measure the light and then select the camera settings. In the first stage, the exposure meter measures the sunlight being reflected from the subject and, taking account of the selected ISO, gives an Exposure Value.
The Exposure Value (EV) is a number representing a combination of shutter speed and f/number. All combinations that have the same EV will let the same amount of light onto the sensor. An EV of 0 (zero) is defined as a shutter speed of 1 second and an aperture of f/1. From that point, stopping down the lens or using a faster shutter speed increases the EV as shown in the table below. The blue diagonal shows that for an example EV of 9 there are many combinations of shutter speed and f/number - and each will let the same amount of light onto the sensor.
The actual camera settings used by the camera will depend on the camera mode. For example, let's say that the camera is in aperture priority mode with the aperture set to f/4. If the exposure meter gives an EV of 9, we enter the table above at the column for f/4 and go down to EV 9 and then left to the shutter speed - in this case 1/30 sec. If we then alter the f/number, the camera will adjust the shutter speed to maintain an EV of 9.
Notice that there is a strong relationship between EV and the strength of the ambient light on the subject, the luminance. If we take a standard ISO of 100, subjects in outdoor light will generally fall into the range 12 to 16 EV and interior light in the range 5 to 8 or so. A night lit by a full moon might be -2 to -3 EV.
We know that the camera's exposure meter measures the light reflected from the subject. This reflected light depends on both the level of illumination of the subject and its reflectance. A dark subject in bright light might reflect a similar amount of light as a light-coloured subject in poor light. The camera cannot differentiate the two cases, so camera designers assume that the subject is overall a mid-grey colour - specifically that the subject reflects about 18% of the incident light. Both the dark subject and the light-coloured subject will appear to have similar tones in the photographs.
While this approach works well for general subjects such as landscapes, there are times when the result is not what we expect. For example, if we photograph a snowy mountain on a sunny day, we expect the snow to look bright in the resulting photograph. However, the camera does not know that this particular subject should be exposed to a brighter image than average - and it will endeavour to portray the snow as mid-grey. Similarly, if we take a street scene at night, we expect the average tone of the photo to be dark - but the camera will again portray it mid-grey.
This is a perceptual dilemma. As humans, we expect certain scenes to be bright and others dark - but to the camera, all scenes are treated equally and rendered as mid tones.
The way around this is to compensate for the limitations of the light meter if the subject is of unusually light or dark tone. Most cameras have a readily accessible function to apply an exposure compensation of +/- 2 “stops” of aperture or shutter speed. So for the night scene, we would deliberately under-expose by about –1 stop, while for the snow-covered scene we would over-expose by +1 stop or so.
In the description above, I've assumed that the camera's light meter just measures the average brightness of the scene and sets the exposure to produce, on average, a mid-grey image. In many cases, this is not a smart approach as the illumination of the subject might be quite different from the background. To help us overcome this problem, modern digital cameras offer a number of metering modes – typically evaluative metering, partial metering, spot metering and centre-weighted average metering. See: Metering modes
The term “dynamic range” is used to describe the ratio between the minimum and maximum light levels in a scene, measured in f-stops. For example in a landscape on a sunny day, the scene might include dark shadow areas as well as bright clouds - and the dynamic range would be the difference in light levels between shadow and clouds. While it is not possible to define a typical scene, for illustration let's say that on a sunny day the dynamic range might be somewhere in the vicinity of 16 f-stops.
Now, the sensor in a camera also has a dynamic range. When we take a picture, the sensor is exposed to light and each picture element converts the light into an electrical signal which is then converted to a numerical value. Each picture element can only receive so much light before the signal reaches a maximum; it’s like a bucket – once it’s full, adding more water doesn’t increase the level. So the top end of the sensor’s dynamic range is defined by the exposure level that causes the picture elements to become “full”.
While there is a clear cut-off at the top end of the scale, the bottom end of a sensor’s dynamic range is less well defined. As the light energy reaching a picture element reduces, the signal generated by the element also reduces. But, as with any electronic system, there is a certain amount of “noise” inherent in the system – think of the background noise on a radio or telephone. As the electrical signal from the sensor reduces to near the level of the background noise, it becomes increasingly difficult to tell the signal from the noise.
Modern full-frame digital SLR cameras claim to be able to record a dynamic range of around 10 to 14 stops. While this is good for advertising, for practical purposes we should think in terms of the bottom end of this range. Tests on my Canon DSLR in RAW mode show that it is capable of good results across a dynamic range of about 10 stops.
Exposing to the Right
We can see that there is a dilemma when photographing a scene with a dynamic range of 16 f-stops with a camera that can only record a range of perhaps 10 stops. Either the upper light levels will have to be allowed to become “blown” to white or the lowest light levels must be allowed to go to black – or both.
In some cases this might be acceptable. Especially in black and white photography, it is considered perfectly good creative practice to have rich, dark blacks and pure whites. We have come to accept that this is what a "good" black and white photo looks like. However, we tend to be more critical when looking at colour photos - and blown highlights in particular can ruin the look of an otherwise good image.
A classic problem is that of taking a landscape on a sunny day with bright clouds or snowy mountains in the scene. How would we choose an exposure to keep detail in the clouds without driving too much of the shadows to black? An approach would be to take an exposure reading from the clouds using spot metering and then set the camera to overexpose the clouds by 2 to 2½ stops. For example if the exposure reading for the clouds was 1/500 sec at f/8, we could set the camera to 1/125 at f/8.
If we looked at the photo on the back of the camera and brought up the histogram, it would look like the diagram above. The bright areas have been placed to the right-hand side of the histogram but without going off the side and becoming blown.
This is known as exposing to the right and is an example of using a simplified version of the “zone” system of setting exposure, as brought to perfection by Ansel Adams (1902–84). We take a spot reading on a significant part of the scene and then “place” it at a chosen point in the dynamic range of the camera. If shadow details are important, ensure they are not underexposed by more than 3 stops or so – while if highlight detail is needed, make sure they are not overexposed by more than 2 stops or so.
High Dynamic Range (HDR)
If we absolutely must get both shadow and highlight detail in a scene that exceeds the dynamic range of our camera, we have to adopt the techniques of high dynamic range (HDR) photography - as described here.