Nigel Wood Photography - cobwood studio

Metering Light

Film speed – ISO

The human eye can adapt to be more sensitive at night – and cats’ eyes are more sensitive than ours. Similarly, 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 sensors respond in a very similar manner to conventional film so the same thoughts apply. However, 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 do its best with more or less light. 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.

[ISO = International Organization for Standardization]

Exposure

Any 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 grainy, dark 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.

Exposure metering

When setting the exposure we ideally want to measure the amount of light falling onto the subject. To do this we would need to use an incident light meter, which will give very accurate exposure settings. However, more generally we rely on the light meter in the camera itself, which measures the light reflected from the subject. As the light reflected from the subject depends both on the level of illumination and the reflectance of the subject, the camera cannot directly determine the level of illumination. To get around this problem, camera designers assume that the subject is overall a mid-grey colour; specifically that the subject reflects 18% of the incident light. Armed with this assumption the camera can make a best guess at the amount of incident light and the exposure required.

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. A wide aperture and a fast shutter speed could provide the same exposure as a narrow aperture and a slower shutter speed. For example, settings of f/4 and 1/125 sec would provide the same exposure as f/5.6 and 1/60 sec – and so on. 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

Exposure compensation

Noting that the camera will always try to expose the subject as a mid grey, compensation may be required if the subject is actually very dark or very light.

If we take a picture of the family pet black Labrador, the dog’s coat reflects little light into the camera’s light meter and so the camera thinks there is less daylight and incorrectly adjusts the exposure. The opposite will occur when photographing a snow-covered scene, which will reflect lots of light and the camera will be fooled into under-exposing the picture. In both examples, the camera will try to make the scene a mid-grey – which will look too light for the dog and too dark for the snow.

There is no way around this and we have to compensate for the limitations of the light meter if the scene is of unusually light or dark shade. Most cameras have a readily accessible function to apply an exposure compensation of +/- 2 “stops” of aperture or shutter speed settings. So for the black Labrador, we would deliberately under-expose by about –1 stop, while for the snow-covered scene we would over-expose by +1 stop or so.

Metering modes

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

Colour of light

We are familiar with the idea that very hot objects emit light in differing colours. On a gas stove, the hottest part of the flame shines blue and less hot areas shine orange or red.

The coloured bar below depicts the colour of light emitted by an object or flame at different temperatures (measured in the scientific unit of “Kelvin”). Each colour is associated with a temperature - its “colour temperature”.

colour temperature
colour temperature

Light from the sun has a mixture of these colours but as sunlight hits the Earth’s atmosphere, the blue light tends to be scattered more than the red light. The scattering of blue light in the atmosphere makes the sky look blue, while the direct light from the sun at sunset looks red.

If we take a photograph on a sunny day, the light is a mixture of direct and scattered light, tending neither to red nor blue. This white light has a colour temperature of about 5200K. If we step into the shade, we block the direct light from the sun and our scene is lit by the bluish light from the sky. This slightly bluish light has a higher colour temperature of around 7000K. If we step indoors, into a room lit by standard tungsten lightbulbs, the reddish light from the bulbs comes from the filaments glowing at around 3200K.

Light Source Temperature
Sunrise / set 2000 K
Tungsten 3200 K
Fluorescent 4000 K
Sunshine 5200 K
Cloudy 6000 K
Flash 6000 K
Shade 7000 K

White balance

When we take a photograph, white objects in our scene will reflect the colour of the ambient light and so may take on a blue or reddish tint.

To correct for this source of colour error, most cameras allow us to set a “white balance” of: daylight, shade, cloudy, tungsten, fluorescent etc. This tells the camera by how much to shift the colours in the image to remove the tint and record the white areas as true white.

Most of the time it suffices to set the white balance to automatic and allow the camera to do its best. Any residual tint can be removed later when processing the pictures. However, if we want to get the colours correct in camera, especially under artificial light, it helps to set the correct white balance.

When processing the pictures in Lightroom or other software, we can correct any error in the white balance by adjusting the Temperature slider or selecting the appropriate light source in the temperature options. Notice that these sliders seem to work the “wrong” way around; if we increase the temperature, the picture shifts to “warmer” (more red) tones – because we are correcting for the colour cast produced by higher-temperature (more blue) light.

Dynamic Range

The term “dynamic range” is used to describe the ratio between the minimum and maximum light levels in a scene. 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. It’s convenient to measure dynamic range in “f-stops”, where each f-stop represents a doubling of light level. 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 13 stops. No doubt this is true in laboratory conditions but for practical use, the range is likely to be less.

High Dynamic Range

We can now 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.

This may not be a problem if we treat photography as a creative art. Creatively we are entirely at liberty to let highlights go to white or to let shadows drop into black. Photographers have been happily doing this since the beginning of the technology – and as viewers we accept these characteristics as perfectly normal for the medium.

The question for the creative photographer is where to place the limited sensor dynamic range on the wider dynamic range of the scene? If we are taking a portrait of somebody, we may wish to place the light levels from their face in the middle of the sensor dynamic range – i.e. expose for the skin tones of the face. On the other hand, if we wish to have lots of details in the shadows, we need to place the dynamic range of the sensor towards the bottom of the range in the scene. Conversely, to get detail in brightly lit clouds, we need to place the sensor range towards the top of the scene’s range.

Taking this last example, how would we choose an exposure to keep detail in brightly lit 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 or partial 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.

This 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 or partial reading on a significant part of the scene and then “place” it at a chosen point in the dynamic range of the camera – somewhere in the range –4 to +2 stops of exposure compensation. If shadow details are important, ensure they are not underexposed by more than 4 stops – while if highlight detail is needed, make sure they are not overexposed by more than 2 stops or so.