In photography, exposure is the amount of light reaching the sensor and therefore the exposure is responsible for the brightness of a picture. It is absolutely neccessary to control the exposure to get reasonable images. For bright scenes, the number of photons must be limited to a sensible measure to prevent the sensor from oversaturating resulting in burnt highlights. By contrast, photographing dark scenes will require more photons to be captured and therefore a higher exposure is needed. The brightness of a scene – also referred to as ambient light – can be expressed by the illuminance of a scene and is given for a particular setup.
There are two fundamental controls regulating the illumination of the sensor:
- As already described, the shutter – regardless of its design – is one unit controlling the amount of light reaching the sensor and therefore controlling the exposure. For bright scenes, it is required to reduce the shutter speed while for dark scenes a longer shutter speed is needed.
Another elementary factor to influence the exposure is the aperture, a circular (or more precisely, polygon-shaped) opening in the lens that can either increase or reduce its diameter. For bright scenes, it can be used to reduce the aperatures diameter while for dark scenes it can be increased. However, the aperture primarily serves another purpose and the aperture’s impact on the exposure is more a side-effect. In general, this unit is not as straightforward to comprehend as the shutter and will be described in the lens articles in detail.
In addition to these fundamental controls, there is another factor to infuence the exposure, at least indirectly. The ISO acts as a post amplifier to multiply the sensor’s signal until the brightness is at an appropriate level. Low ISO means low amplification while high ISO means higher amplification. Higher ISO levels allow the actual illumination of the sensor to be lower because of post amplification. For that reason, the ISO has an indirect effect on the shutter speed and aperture.
Knowing that these factors can be controlled while illuminance can’t, it is easy to understand that different photos with each a shutter speed of 1/1000s, an aperture of f7.1 and ISO 100 will not all have the same brightness when shot on different locations. In order to know what settings to use, it is important to measure the illuminance of each location, called metering. For some more details on the metering mechanism, feel free to read my metering article.
In photography, the term “stop” describes a logarithmic brightness setting on digital cameras. In general, brightness can be expressed in candela (cd) – a physical unit used to describe the intensity of light that is emitted by a light source. However, a stop is not an absolute value such as candela but rather a relative unit that is used in every aspect of photography to change the brightness of a picture. An alteration of one stop either halves or doubles the exposure of a photograph.
Full stops: A change by one full stop doubles or halves the exposure, depending on the direction of the change. Thus, if n is the number of stops, the exposure factor is 2n. Here is an example of the effects of full stops:
|Increasing the exposure by one stop||The light intensity in your photo will be doubled|
|Reducing the exposure by one stop||The light intensity in your photo will be halved|
|Increasing the exposure by two stops||The light intensity will be quadrupled|
|Reducing the exposure by two stops||The light intensity will be quartered|
Fractional stops: To adjust the intensity of light more detailed, fractional stops can be used. The following shows the exposure values depending on full and fractional stops
exposure factor (approx.)
Stops are interchangable
The settings for shutter speed, aperture value and ISO can all be adjusted in stops, even though the numbering systems are different. The brightness of a photo can be controlled by more than one instance:
- changing the shutter speed will have the most logical impact on the brightness.
- changing the aperture has a direct impact on the brightness, but also changes the depth of field on a photo (see lens chapter).
- changing the ISO has an impact on the brightness, but can cause additional problems (see ISO chapter).
A photo always is a combination of these settings. Here is an example of three images of the same brightness:
If the brightness of the photo should be reduced effectively, either one of these values must be changed, e.g. reducing the exposure from 1/500s to 1/1000s – or decreasing the aperture from f11 to f16, or going from ISO 200 to 100.
Contrast and Dynamic Range
Contrast is the difference between the lights and the darks of an image. Without contrast, no structures or shapes could be recognized. For this reason, contrast has the greatest signal effect on the eye and therefore the greatest impact on the perception of the image. The contrast ratio, also referred to as dynamic range, is the quotient of the maximum and minimum intensity of light in a particular scene. For example, a dynamic range of 10.000:1 explains that the brightest area is 10.000 times brighter than the darkest area.
In order to bring these relations into conformity with photographic terminology, contrast ratios and dynamic ranges are often described in stops. A sunny day for example features a dynamic range of around 14 stops between bright sky and dark shadows. The comparison of different dynamic ranges will put this number into a more transparent context:
Nature: In theory, any dynamic range is possible – the dynamic range in nature can be starting at an absolute pitch-dark area in the universe with no photons available up to the exposure to a hypernova.
Human eye: In everyday life, the human eye usually can adapt to dynamic ranges of 1.000.000:1 (about 20 stops). However, it cannot perform these feats of perception at both extremes of the scale at the same time. The retina of a human eye only has a static contrast ratio of around 100:1 (about 6.6 stops). The eyes need some time to adjust to different light levels. In bright scenes the iris (which regulates the size of the pupil) closes down and enables the perception of detail in bright areas, but simultaneously restricts our ability to see dark shades. In dark scenes the eyes iris opens up dramatically and improves the ability to see in the dark, but simultaneously limits the perception of bright detail. This adds up to a dynamic contrast ratio of 1.000.000:1. Left in the dark for longer, the eyes perform some long-time-adaption to dark areas including chemical processes. This long-time adaption will increase sensitivity to dark areas even more and can expand the dynamic range up to 30 stops. This long-time-adaption allows the human eye to see objects in starlight, although color differentiation is reduced at low light levels. While the adaption to darker areas usually takes a few seconds, long-time-adaption in starlight surroundings usually takes half an hour. Hence, this dynamic range of 30 stops is more a theoretical range that is not instantly achievable by the human eye. The figure below displays the relations described above. Please note that every neighboring rectancle symbolizes a full stop. For demonstration purposes, the brightness int he figure rises in linear fashion, whereas a stop in reality always doubles or halves the intensity of light.
Camera sensors: The dynamic range of camera sensors used in digital photography is surprisingly larger than the static contrast ratio of the human eye, however the sensor itself cannot adapt to different levels of brightness within a scene. Today’s camera sensors usually have dynamic ranges between 10 – 14 stops which is less than the dynamic contrast ratio of the human eye. The dynamic range can however be extended manually by shooting photos at different exposures and applying some dynamic range fusion later. This relation is depicted below. Again, the increase / decrease of brightness is displayed in linear fashion for demonstration purposes, but they refer to a logarithmic function.
to be cont’d