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. The film sensitivity is known as “ISO film speed” and is a number usually in the range 25 – 1000. A typical film for everyday use would have 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 “film” responds in a very similar manner to conventional film, so the same thoughts apply. Digital cameras allow 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 1600.

[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

To obtain a correct exposure, we first set the ISO film speed – or leave it on automatic. Then the camera is pointed towards the subject and the shutter release button pressed down half-way to activate the light meter.

There will always be a range of possible f/numbers and shutter speeds that will provide the correct exposure – i.e. the correct amount of light falling 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 a correct exposure. See: shooting modes

Exposure compensation

An important point to remember is that the camera’s light meter has no idea at what subject the camera is pointing. So it has to assume that the subject is “average”, i.e. that it reflects an average amount of the available daylight.

If we point the camera at the family pet black Labrador, what happens? The dog’s coat reflects little light into the camera’s light meter and so the camera thinks that there is less daylight and adjusts the exposure accordingly – resulting in an over-exposed picture. 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”, i.e. aperture or speed settings. So for the black Labrador, we want to deliberately under-expose by about –1 stop, while for the snow-covered scene we want to deliberately over-expose by +1 stop or so.

Metering modes

When setting our exposure we ideally want to measure the amount of sunlight, or artificial light, falling onto our subject. To do this we need to use an incident light meter, which will give very accurate exposure settings – and many photographers do just that. However, more generally we rely on the light meter in the camera, which measures the light reflected from the subject, through the lens onto the light meter. 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 and set the “correct” exposure. To get around this problem, camera manufacturers assume that the subject is overall a mid-grey colour; specifically manufacturers assume 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 correct exposure.

We saw in the previous section that this assumption will lead to our black Labrador being over-exposed and our snow scene being under-exposed. 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

The human eye adapts well to variations in the colour of daylight but film faithfully records these variations, which can make colours in a photograph look too “warm” or “cool”. As a guide, the colour of light, measured as colour temperature and shown in the strip below is about:

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
colour temperature
colour temperature

To understand why the colour temperature varies outdoors, we need to think about how light passes through the earth’s atmosphere. Light coming from the sun includes the whole range of colours. As sunlight passes through our atmosphere, the red light tends to pass straight through, while the bluish light tends to be scattered by particles in the atmosphere. We are familiar with the red glow around the sun at sunrise and sunset, which is light passing straight through the atmosphere. On the other hand, during most of the day, the sky is blue, which is light scattered by the atmosphere.

Looking at the table above, on a sunny day the colour temperature is about 5200 degrees, being a balance of direct and scattered light. If we move our subject into the shade on a sunny day, the effect is to block out the direct red light and the colour temperature moves into the blue region ( 7000 degrees ). A similar although less pronounced effect occurs on a cloudy day ( 6000 degrees ).

White balance

The colour of light reflected back from a subject to our camera is a combination of the colour of the ambient light and the actual colour of the subject. In order to record accurately the colour of the subject, we have to correct for the colour cast caused by the colour of the ambient light. This can be done with the “white balance” setting on the camera or later on in processing.

On a digital camera you can usually select the “white balance”. This tells the camera what lighting conditions prevail and the camera adjusts the colours in any photograph accordingly. For example if we set the white balance to “Shade”, the camera will correct for the bluish hue of the light, reducing the blue slightly and boosting the red. A typical camera will have white balance settings of “automatic” as well as most of the conditions listed above.

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 being photographed. For example in a landscape on a sunny day, the scene might include dark shadow areas as well as bright clouds. The dynamic range would be the difference in light levels between the shadows and the clouds. In photography it’s convenient to measure and discuss 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 me say that on a sunny day the dynamic range might be somewhere in the vicinity of 16 f-stops.

Now, the digital 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 level falling onto a picture elements falls, the signal generated by the element also falls. 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. I find that in practice the working dynamic range is more like 6 stops when shooting in Raw and about 4 stops in JPEG.

In Raw, I find that a modern digital SLR can cope with about 2 stops of overexposure and about 4 stops of underexposure. Beyond 2 stops or so of overexposure, one of the 3 colour channels (red, green, blue) will reach its maximum (become “full”) and the camera can no longer record the correct colour. At greater overexposure, the highlights become “blown” to pure white as all channels become “full”. At the other end of the scale, it is surprising by how much an image can be underexposed while still recording reasonably pleasing colour. Using Lightroom or similar software, areas of a picture underexposed by about 4 stops can be recovered using the exposure or shadows controls. At more extreme underexposure, the colours become increasingly “muddy” and significant noise intrudes. For JPEG, the equivalent range is about 3 stops underexposure to 1 stop of overexposure.

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 6 stops or so. 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 – see artistic photography. Creatively we are entirely at liberty to let highlights blow out 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.