Metering is used to determine an appropriate exposure for a scene; like much of photography there is no 'right' or 'wrong' exposure for a particular image. Being significantly off can lead to an image being too dark or blown out but a scene can quite easily have a range of exposure values that result in an acceptable image.


Many digital photographers on workshops I have been on can be heard commenting that the image they have captured does not look like the scene that they remember. This is most often the case when the scene involves a high dynamic range and they have been full sensor metering to make their exposure. The camera system has attempted to get an average best exposure that neither blows too much out nor results in areas of impenetrable shadow, and as a result the sense of the image, the thing that made it look memorable, has been diluted or even lost.


Camera sensors work within a limited dynamic range which is less than the dynamic range the eye sees. The human eye can discern about 22 stops of  dynamic range; the best camera systems at present are restricted to about 15 stops. So there can be more shadow or light detail which the sensor cannot capture every time the scene dynamic range exceeds that of the sensor. The camera, left to it's own devices will work to the compromises set up in its programming. Sometimes these work well, sometimes they are OK, sometimes they fail. By moving to manual metering as a technique this uncertainty can be removed.


Moving away from the automatic, sensor wide metering process lets the photographer take control of  the image and determine what exposure value should be assigned to which element of the image. Unimportant areas can be left to over expose perhaps - so, for example, a wedding photographer shooting in bright sunlight may choose to allow the sky to blow out in places in order to get the characteristics of the subject that are wanted.


Digital cameras provide a number of metering options. The most common types are;

- Matrix (Nikon) or Evaluative (Canon) which meter across a whole scene, taking into account the full sensor area.

- Centre weighted which meters a small area of the scene.

- Spot meters a single point in the scene.


The important thing to remember when using matrix/evaluative is the cameras programming will try to accommodate the widest dynamic range range that it can, and according to the way it has been programmed it will make a determination where to place the maximum highlight and work back from that. A technically optimised averaging of the whole it probably only works reasonably well when the dynamic range of the scene approximates that of the camera. Where the scene range  has greater or lesser range this option most probably will not produce the result the photographer was looking for.


To improve on the image data capture means working out what is optimal exposure for the scene as you visualise it. Where the scenes dynamic range is within the scale supported by the camera this is straightforward. Where it exceeds the camera's capability the photographer has some decisions to make;


- Whether to use HDR or blending in editing and to shoot the scene several times making exposure changes to capture the full dynamic range over a series of shots. Only possible if the subject is stationary in most cases.

- If a single shot is the option the question is where to place the camera dynamic range on the scene dynamic range, whether to accept blown highlights or lost shadow detail.

- If modifying the scene by using graduated filters will work, principally to reduce the brightness of the sky (or a polariser can be used to reduce 'hotspots' and reflections).


The job of the photographer is to work out what the important areas of the scene are and where, when the dynamic range exceeds the camera capabilities, detail can be lost or if all detail is required which of the tools available will provide the best answer.


This is where the use of a more precise means of metering and the selection of a suitable reference point in the image are required. The in camera metering system looks for this mid tone, the mid point grey, also known as the 18% grey, which is the reference it needs to provide the exposure. The photographer needs to select an area in the image that can be used to set this and to meter from that using a metering choice that works of a single small area such as centre weighted.


Finding a place to meter an image from will depend on the scene and the light conditions. On a bright day a grass lawn can provide this or the blue sky; it depends on the conditions and the skill of the photographer in recognising where the mid tone needs to be in visualising the image and calculate exposure from that.


The choice faced by the photographer is where to place the cameras dynamic range on the scale of the actual dynamic range. If the actual is the same as the camera no problem; if - as is quite common - it exceeds it then the choices are to set the camera up to capture the highlights, or to capture the shadows. Some part of the actual will not be able to be captured; the decision is for the photographer, and the technique used to image the scene will should be decided largely on this.


The type of image will influence the techniques available; action photography can require different techniques to landscape, the first concentrating on the subject and therefore exposing for that, whereas the more studied landscape environment provides time for techniques like multi shot blending to be part of the equation.


A good example is working in snowy conditions where the camera left to it's own programming will meter the white snow as mid point so under exposing it significantly - grey snow - which no one can argue is an artistic interpretation. If there is nothing in the image that can be used to provide a mid tone reference it is necessary to adjust the exposure to compensate by at least two stops.


Another example is imaging a white bird like a swan. Here the risk is that the evaluation of the white may result in everything around the bird being significantly under exposed so you are left with a white-ish bird floating in a sea of darkness; which can look quite dramatic but totally unrealistic.


There are a number of techniques that help calculate an exposure. One of the most important skills to develop is that of visualising the image intended and then calculating how the exposure should be set up to achieve this. When faced with say a sunset the exposure needs to capture the light from the sun; the sun itself may - probably will - blow out but the light in the sky should be captured perhaps at the expense of shadow detail being lost, so exposing for the highlights is required. Here the exposure decision is to move the cameras dynamic range to cover the high end of the scene scale, leading to the lower end of the actual scene disappearing into shadow. If shooting a moonlit scene (without the moon) the exposure will need to be set to render shadow detail or the image will appear as a large array of darkness, so the camera range is set to match the lower end of the scene range.



All Photography and Video Copyright David Hammant.

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