Camera determines the shutter speed and aperture, based on its built in Metering. All in-camera light meters only measure reflected light. The problem with all reflective light meters, is that, the reading they suggest is influenced by two variables. The amount of light reflected from any scene or subject, which is determined by how much ambient light is present and the inherent reflectance characteristics of the object being metered.
The meter does its best to determine how much light is present to properly expose the subject and attempts to determine how much is out there so you’ll get a well exposed image. But, there is another variable that influences how much light reflects from a subject. This second variable is subject reflectance. Given the same amount of ambient light, white subjects reflect more light to the sensor than dark subjects, Subjects vary greatly in their reflectance.
Now we have two variables, subject reflectance and ambient light. The camera meter attempts to solve the exposure equation with its sophisticated metering system. Even such a sophistication is no match to the complexity of solving an equation with two unknowns. So camera manufacturers decided to say one variable, subject reflectance, doesn’t vary and is a known value of 18% reflectance. Now the meter only need to solve the equation for the ambient light variable. Therefore, camera meters are programmed to believe everything they “see” is middle tone which is known as 18% reflectance. Notice middle tone is not 50% reflectance. As it turns out, 18% reflectance is the middle tonality between pure white and solid black. Since the meter “knows” everything reflects 18% of the light, it can set the aperture and shutter speed automatically to yield great exposures. Many people say cameras are calibrated for 18% middle tone grey. Actually, the colour grey has nothing to do with this. Middle tone could be 18% reflectance blue, red, green, brown, or even orange.
Let’s take a theoretical example – a black Crow, Egret (which is whiter) and an 18% grey toned Grey heron are standing in clean white background. Everything is illuminated by the same amount of ambient light. On these birds if I use spot meter which meters from a very specific and small area (2-3% area) and meter, here is how meter will interpret the exposure. If you meter the middle tone Grey heron, it will be perfectly exposed. If you meter the black crow, it will be overexposed. If you meter the Egret, it will be underexposed. The camera makes it 18% reflectance so it lightens up the crow way too much and egret too dark.
Your in-camera light meter is not smart. The meter has been programmed to believe that everything it “sees” is 18% reflectance. Since most things are not 18% reflectance, the meter knows one thing which is wrong most of the time. The meter only knows how to expose middle-tone scenes well. The meter tries to make whatever it “sees” to be middle tone in reflectance which sometimes is not the best exposure. We’ll learn how to compensate for subject reflectance shortly to get the best exposure.
In order to accurately expose a greater range of subject lighting and reflectance combinations, most cameras have several metering options. Each option works by assigning a relative weighting to different light regions; regions with a higher weighting are considered more reliable, and thus contribute more to the final exposure calculation. Metering options often include partial, evaluative zone or matrix, centre-weighted and spot metering.
Evaluative or Color Matrix Metering Mode
This metering mode meters the entire scene, divide up the image into many sectors which are analysed by the camera using complicated algorithms to determine the exposure. The camera compares the brightness of the subject and the background, compensates for backlighting, and other variables. If you are going to use automatic exposure, this sophisticated mode is an excellent choice.
This metering mode meters the entire scene, but puts emphasis on the middle of the image because that is most likely where the subject is. It works well when bright sky is near the top of the frame because the sky doesn’t influence the meter much. Centre-weighted metering was once a very common default setting in cameras because it coped well with a bright sky above a darker landscape. Nowadays, it has more or less been surpassed in flexibility by evaluative and matrix, and in specificity by partial and spot metering. On the other hand, the results produced by centre-weighted metering are very predictable, whereas matrix and evaluative metering modes have complicated algorithms which are harder to predict. For this reason some still prefer to use centre-weighted as the default metering mode if you are from old school.
This mode uses the centre of the frame to determine exposure. It is most effective when bright areas surround the frame. Since the meter only considers the centre of the frame, it ignores white clouds at the top of the frame that might cause the camera to underexpose the main subject. Area it meters is larger than the spot meter (approximately 6.5%).
This metering mode measures the light only from a small area in the middle of the frame which makes up about 2–3% of the viewfinder. It is the most precise way to meter, but requires a good deal of practice and study to really master. You must meter carefully, learn to judge reflectance values, and know how to compensate the exposure.
Partial and spot metering give the photographer far more control over the exposure than any of the other settings, but this also means that these are more difficult to use — at least initially. They are useful when there is a relatively small object within your scene which you either need to be perfectly exposed, or know will provide the closest match to middle grey.
One of the most common applications of partial metering is a portrait of someone who is back-lit. Metering off their face can help avoid an exposure that makes the subject appear as an under-exposed silhouette against the bright background. On the other hand, care should be taken as the shade of a person’s skin may lead to inaccurate exposure if this shade is far from neutral grey reflectance.
Any of the above metering modes can use a feature called exposure compensation (EC). When this is activated, the metering calculation still works as normal, but the final exposure target gets compensated by the EC value (EV) ; 1 EV is equal to one exposure step (or stop), corresponding to a doubling of exposure both in positive or negative direction. This allows for manual corrections if you observe a metering mode to be consistently under or over-exposing. Most cameras allow up to 2 to 3 stops of exposure compensation, where each stop provides either a doubling or halving of light compared to what the metering mode would have done otherwise. A setting of zero means no compensation will be applied (which is the default).
Exposure compensation is ideal for correcting in-camera metering errors caused by the subject’s reflectivity. No matter what metering mode is used, an in-camera light meter will always mistakenly under-expose a subject such as a white subject in a white background. Photographs in the snow will therefore always require around +1EV, whereas a dark subject on a dark background may require negative compensation like -1EV.
You may be wondering what that bird, juvenile Oriental Magpie Robin, pictured here got to do with metering. I have pictured it using Canon EOS 5D Mark III on a Canon EF 500mm f/4L IS II USM Lens + Canon EF 1.4x III Extender in a pretty low light. If I use a spot meter and measure very small area on the head and eye region, I will get a wrong meter reading leading to over exposed picture as the region measured is darker than 18% reflectance. I metered from the breast region of the bird which is closer to the 18% reflectance. I could have corrected , if I had metered the darker head region to give negative exposure compensation (-1/2 or 2/3 EV) to the meter reading.