So how do light meters help, or hinder, us? Firstly, we need to look at the different ways there are of measuring the amount of light there is in a scene.
We can simply measure the amount of light falling on the subject using an incident meter. These usually have a white dome to collect the light from a wide angle and the sensor measures the light falling on the dome. Normally, they are pointed directly towards the camera lens from the subject's position, although some photographers prefer to point them directly at the main light source and make an adjustment. They measure the amount of light falling (incident) on the subject and are not influenced by the tones of the subject. They are often used with trannie film, particularly with shots including important light skin tones. To use them reliably, you need to be able to get right up to the subject to make sure that the meter is in the same light. Although outside in daylight, unless there are clouds or either the subject, or you, is in shadow, it is generally even enough to take a reading backwards from the camera position.
Reflected light meters measure the light reflected back from the subject. They vary in their angle of acceptance, how wide a view they take of the scene, from about the angle of view of a standard lens, down to only a few degrees. If the angle of view is 1 degree or less and the meter is equipped with a viewfinder to allow accurate placing of the measured spot, then they are called Spot meters. Reflected light meters are influenced by the tones of the subject and so need to be used more thoughtfully than an incident meter. For example, the reflected light readings for a man wearing a dark suit and the same man in the same position and light wearing a white shirt, will be considerably different since the shirt will reflect much more light than the suit. The exposure needed to record his skin tones, the most important tones, hasn't changed though. Because the meter is calibrated to give what the manufacture considers to be the correct exposure for a notional grey tone (often thought to be a Kodak 18% grey card, but different manufacturers use different tones) it will indicate too much exposure for the dark suit portrait and too little for the white shirt portrait. It's easy to show this effect. Take a light reading from a white card and use the exposure indicated to photograph it filling the frame (use either B&W or colour trannie film - not colour neg or chromogenic (C41 process) B&W), then do the same for a black card. Process the film and compare the negs or trannies, you may be surprised that they are so similar (the colour trannie may show a colour difference). So what use is a reflected light meter when it can be fooled so easily? This is where the photographer's intentions and knowledge comes in. By taking reflected readings from the lightest and darkest tones that we want detail to record in, we can measure the brightness range (contrast) of the subject to see if it falls within the latitude of the film and development we are using and either adjust the development if necessary or, if the subject allows, either reflect light back into the darker tones or use fill-in flash. It also lets us pick a particular tone and make it the basis for the exposure. For example, imagine a forest scene where the sun is breaking through the trees towards the camera, the trees themselves and the other foliage will be in shadow. A reflected light reading will be influenced by the bright sun and show an exposure that would underexpose the trees and foliage. However, if a reading is taken by going up to the trees and foliage, the meter will indicate much more exposure. We could use this exposure, but it would give too much detail in the darker areas, spoiling the effect. If we reduce this exposure by a stop or two, then we effectively darken down the trees and foliage in the shadows, but still retain enough detail to allow them to show in the print or trannie. The actual amount we reduce the exposure will depend on the effect we want and how dark the colour of the foliage is. So the best thing to do would be to bracket the exposures around what we think would be the best exposure. Film and processing is relatively cheap and it's always worthwhile to bracket when in doubt. This adjustment of exposure, combined with the adjustment of development, forms the basis of the Zone System, a method of determining exposure based on the tones of the subject and the photographer's previsualisation of the finished print.
This brings us to the Spot meter, since it is normally recommended to use one when using the Zone System to make measuring the tones more easy. It is really just a telephoto reflected light meter and saves you having to walk up to each area you wish to measure or have to go without measuring any tones you can't walk up to. What it is not is magical. Just because you spot meter part of the subject, it doesn't mean that the exposure will be correct without putting some thought into it.
No matter how sophisticated, through the lens (TTL) meters in cameras are reflected light meters. The light is reflected off the subject, passes through the lens and is then measured by some arrangement of sensors. Various methods are used. Some SLR cameras have them built into the viewfinder prism, measuring the light coming off the screen in the viewfinder. Others use a semi-transparent viewfinder mirror to let some of the light through then either reflect it down onto sensors in the base of the camera, or if the camera has a focal plane shutter, then allow it to reflect off a light area printed on the shutter blind. Rangefinder cameras with focal plane shutters can also use this method. Enhancements to the basic principle have been made over the years. The sensors can be arranged so that they bias the exposure towards a certain part of the subject, usually the centre to give so-called centre-weighted metering or spot metering. More than one sensor can be used to allow the subject to be evaluated in some way, usually by processing the information. For example, if two sensors are used, one measuring the top half of the subject and the other the bottom half, then the metering system could base the exposure equally on both if they are within certain limits, but bias it towards the bottom if the top is a lot brighter. This would provide a certain amount of compensation when the sky is in the top of the picture. This simple method falls down when the camera is turned. The more sensors, or segments, that the system has, the more it can try to eliminate errors, by biasing away from very bright or dark segments, even using information from the autofocus system and referring to a database of subject patterns to try to refine this biasing. These systems can give very reliable exposures over the range of subjects and conditions that they are designed for, but by doing the white card/black card test, they can be shown not to be faultless.
No exposure system can ever be perfect, because it can't read our thoughts and adjust the exposure based on how we previsualise the final print or trannie. Going back to the example in the forest, it can't know that we wish to keep detail in the shadows, even though there is strong sunlight coming through the trees or that we wish to allow the same shadows to go inky black, but capture the subtle tones in the mist on the ground.
We can only measure the amount of light. Correct exposure then comes from the application of that measurement based on our intentions and experience of how the film will behave. If we want to make great photographs, we can't leave something as important as exposure up to automation.
© Barry Leighton FRPS