Types of measurement of a photometer
Photometers, except for those in mechanical cameras, have different measurement systems. This means that they will give priority to the entire or a part of a scene when measuring.
The different measurement systems are:
1. General measurement: it runs a general average of every light value in a scene. Generally, mechanical cameras only have this system.
2. Centred weighted: it prioritises the light values found in the centre of a scene when measuring.
3. Spot: these only measures one point in the scene. They are used for accurate readings.
4. Multisegment: it measures areas individually and it compares them to data it has stored in its memory. Then, it gives you, as a result, a combination for this specific situation.
Generally, photographers try to use the most accurate system possible, this being spot measurement. However, this type of measurement usually requires more time. As a result, the way we use the photometer will depend and vary according to the type of photograph we want to take. It is not the same to shoot for the news, in which sometimes we need to be quick, than to shoot commercially (products), in which we can take all the time in the world.
The photometers found in cameras, which as we mentioned previously measure reflected light, usually have the tendency of making mistakes. Why does this happen? It happens because they are programmed to measure a medium gray. A medium gray, the one from the printing industry, is a surface which reflects 18% of the light it receives. The problem lies in the fact that not every surface or object we photograph is a medium gray.
To understand this concept better, imagine we shoot 3 photographs. In the first picture, we shoot a gray piece of cardboard - 18% (just as the photometer is programmed); in the second one, we shoot a white piece of cardboard; and in the third, we shoot a black piece of cardboard. Can you guess how these 3 photographs will come out? They will be almost identical. The 3 pieces of cardboard will come out at an approximate medium gray - 18%, just because the photometer is calibrated to transform everything to medium gray.
In reality, we rarely need to photograph a scene which only has white or black colours, or even just 1 colour. In the majority of cases, the scene is diverse and it holds different highlights and shadows (dark and bright areas) and a countless quantity of mid-tones. In these scenes, a photometer works well.
The problem is found when there is a scene in which bright tones or dark tone predominate. This is when the photometer produces mistakes. For example, if we shot the picture of a white dove over snow, or a black chair in front of a black (or dark) background.
In the case of the dove and snow, the photograph would be under-exposed, in other words, it received too little light. Why does this happen? Since everything is white in the scene, there is a lot of light reflected and reaching the photometer. The photometer doesn't know the scene is pure white and, as it needs to transform everything to medium gray, it believes there is plenty of light. Thus, it tells you to use a diaphragm and shutter speed combination which will allow less light into the camera than the actual amount the photograph needs for a proper exposure.
In the case of the black chair and dark background, it is the opposite. There is very little light reflected and reaching the photometer. As a result the photometer believes there isn't enough light and will give you a diaphragm and shutter speed combination which will allow more light into the camera than the right amount the scene need for a proper exposure. In this case, the photograph will be over-exposed.
In digital cameras, this problem is easy to correct because we see the result right away. On the other hand, if we are using film, we need to understand how the photometer works to compensate any mistakes it can produce.
Whether we are using a digital camera or a film camera, I sincerely believe we need to understand how every important aspect when shooting photographs works to achieve better results. Understanding how the photometer works not only will allow you to expose a picture properly but it will also give you the freedom of shooting pictures more creatively.
Photometers which measure incident light are not affected by the different capabilities of reflection surfaces possess. They also render a more precise measurement regardless of the colour or shape the objects found in a scene have.
