Light is the life-blood of photography. Quantity and quality are both important. Quality of light is the core of creative image making, but quantity is required for an image to be exposed correctly, so it is no less important.
The light meter in your camera is a clever device, but it can be tricked in some circumstances, or maybe the “correct” exposure reading may not be to your liking creatively.
The reason the light meter makes mistakes is because it has to be calibrated to a “zero point”, based on some basic assumptions. The designer sets the light meter to read light for a normal exposure in daylight with a scene containing “average” reflected light values. This average is called 18% grey and is the average reflectivity of all of the things that make up our world in normal sunlight.
If the camera encounters a scene made up of too many non average tones (a sunlit white sandy beach, a snow field or lots of deep shadows in a stage performance for example), it will usually* assume that it must compensate to “put things right” by bringing the reflected light values back to it’s normal or “zero point” setting. This results in predominantly white or black scenes becoming grey or average. Not ideal.
Think of a black and white chess board. The camera will have no trouble metering the normal board because it is an even balance of light and dark tones, but if you turn most of the black squares white, the camera will not understand the tones have changed, it will think that the scene is just too light, so it will (over) compensate by darkening it, making the board more grey looking. The same is true if more squares are black. The camera will “fix” the dark scene by making it lighter.
There are two*** ways to avoid this.
The first and least feasible is to take an “incident” light reading using a light meter that reads the amount of light coming from the light source (the sun, a lamp etc), rather that the light reflected from the subject. This cannot be tricked, although it is no guarantee of creative success. A way of doing this with a reflected light camera meter is to take a reading off an area that you believe is an average tonal value in the light you are intending to use (green grass, low angle blue sky or road tarmac is often ok, or ideally a portable grey card), then set the camera to this setting manually, so it will not then change when you point the camera at something else. An example situation for this would be photographing white water rafting. The light meter will be fooled by bright sunlight sparkle off the water, sometimes shaded faces of the competitors and “normal” backgrounds so you would not be able to let the camera decide and compensate constantly.
The second and more common method is to correct the cameras reflected light settings yourself. There are many ways you can do this and with modern digital cameras you can often do it before you make the exposure, without guess work or having fix it by retaking a image again after an error.
Setting the exposure compensation setting on the camera (+/- symbol or sometimes a wheel activated control) will allow corrections of as much as +/- 3 settings or more. Exposure compensation is not available in any automatic modes, but Programme and either Priority mode (aperture or shutter speed) will allow it’s activation. This is especially useful with a mirrorless camera or SLR using live view, because you can see the effect before taking the image.
Manual mode is also ideal for this. Although manual is cumbersome and difficult to use for some subjects, if you have time or want to set the camera so your settings cannot be over-ridden by the camera then it is the best setting. The light meter will offer settings it thinks are right, but it cannot set any for you.
Now the tricky bit is to remember that dark scenes need to be made darker and light ones light, because of the way the light meter will compensate.
In the two examples above, the first image of each set is the camera’s metering of the scene (a happy average). The second of each of the images is taken with -1 exposure compensation applied. In both of the adjusted images, the colours have deepened and there is added drama and mystery. There is no right or wrong here and post processing could change the image quality a fair bit, but the closer you can get to a good exposure at the start of the process the stronger the image and the less harm you will do to the file with processing.
In this set, the camera’s metered image was the middle one. The darker (-1) adjusted one is more accurate to the scene and the last image (+1) is a brighter and friendlier one. The left image has plenty of latitude to salvage detail in the bright area in the front, but lifting detail out of the shadows would likely lead to more noise (noise hides in shadows). The other two have sacrificed highlight detail lifting shadow detail.
Sometimes there is no perfect answer. If an image has too wide a range of contrast you will either have to use a technique like HDR, using multiple exposures at different values and merging them later, or sacrifice either the highlight or shadow end of the exposure. It often is ok to loose detail to solid areas of bright white highlight or inky black shadow, if that is how they really would look.
The original above was taken using -1 1/2 exposure compensation. This retained detail in the highlights at the expense of the shadows. Even thought the highlights were heavily compensated for, they are still a little blown out. The right hand image is the same one processed back up to the exposure the camera wanted to set. In the shadow areas, noise has become apparent, even ugly and the highlights would need to be selectively bought back or protected as they have blown out again. Possibly taking the image a little lighter and trying to salvage the highlights in processing would have resulted in a cleaner file, but highlight recovery can be tricky**. This back and forth is reducing the images quality as all processing does harm.
*Many modern light meters will evaluate the whole scene including distance and colour making them more accurate and harder to trick, but they are never infallible. To test yours, take a series of images of block coloured subjects (a tarmac road, green grass, midday clear sky, then a white sheet of paper making sure you fill the frame with the dominant colour. The camera will likely get most of the images right except the white paper one).
**It is important to remember that most digital camera sensors will loose highlight detail more easily and permanently than shadow detail, but digital noise hides in the shadows.
*** There is a third called the “sunny 16 rule” which used to be in every film box. It goes like this; If you set your ISO and shutter speed to the same settings, then f16 in bright daylight will be correct, f11 in slightly cloudy conditions, f8 in late afternoon light or heavy overcast etc. The only issue with this is it assumes you are ok to either live with the aperture setting you are forced to use or you are ok making a conversion chart so you can use different shutter speed and aperture settings.