The sensor in your camera (just like your eye) can tolerate and record only a certain range of intensity of light. Hence, it is necessary to regulate light and bring it to levels that can be tolerated by the sensor. Not only that, the sensor should be given the right «dosage» of light. This is what we call «Exposure». When the exposure is correct the image will record the maximum number of tones and colors (Picture 1). It should not be overexposed (too light, Picture 2) or underexposed (too dark, Picture 3). Exposure, together with focus, forms one of the the two most critical technical aspects (leaving aside the aesthetics) of a photograph. Exposure depends on three parameters in a digital camera:
1. Aperture 2.Shutter speed 3. ISO
The first two control the amount the light hitting the sensor whereas ISO controls the sensitivity of the camera to light. Wider apertures (smaller f/ numbers) allow more light and narrow apertures do the opposite. Likewise slower shutter speeds allow more light compared to faster shutter speeds.
Making the camera more sensitive to light (higher ISO) means that you need less light to take a photograph. If you keep both shutter speed and aperture constant then you can make the image darker or lighter by decreasing or increasing the ISO. However, ISO is seldom changed to get this effect.
It is more often altered when you want a higher shutter speed (to stop action, for example) or a narrower aperture (for greater depth of field), and the prevailing light is not sufficient to support these. Then, you can achieve either or both of these by increasing the ISO. Conversely, you can decrease the ISO when you want slower shutter speeds and/ or wider apertures.
An interesting principle in photography is called «Reciprocity». This means that you can have several (virtually hundreds of) combinations of shutter speeds, apertures and ISO values for the same exposure. Table 1 illustrates this very well. All these combinations give the same exposure!
While it was once possible to set the exposure using the «Sunny 16 rule» (see box), it is now becoming increasingly difficult to do so due to the enormous ISO range of modern cameras which starts from around 50 and can go all the way up to 200,000! Fortunately, all cameras these days come equipped with light meters (also called exposure meters), and these measure the light and set the exposure. They do this job reasonably well but fail at times — fortunately quite predictably! This article will help you recognize those instances when a meter will give you erroneous readings. It will also tell you how to correct the error. This is one of the classic thumb rules in photography. Though you may not find use for it often (since every camera these days has an exposure meter), it is still a good practice to check occasionally if your meter is really functioning properly. It is defined as follows. On a clear sunny day, for frontal lighting, your exposure should be such that shutter speed = 1/ISO @ an aperture of f/16. For example, if you are using ISO 200, then your exposure should be, 1/200 @ an aperture of f/16. Simple isn’t it?
Exposure modes: A modern D-SLR or an advanced P&S camera offers several exposure modes. These consist of, at the very minimum, the standard modes like Program, Aperture Priority, Shutter Priority and metered Manual. Apart from these, cameras have myriad other modes called «scene modes». These modes go by names like landscape, night scene, night portrait, party, pet (no, that is not a typographical error!), and so on.
However, remember that all modes give you more or less the same exposure. They differ mainly in the flexibility (or ease) they offer in choosing a shutter speed (Shutter Priority) or aperture (Aperture Priority) or both. The former could be useful for action photography, when depth of field (DOF) and hence aperture setting is less important. Similarly, Aperture Priority mode will be useful where you want to control DOF, like in landscape photography (when shutter speed is relatively unimportant due to the static nature of the subject).
The scene modes generally choose a combination of shutter speed and aperture best suited for a given situation. Typically, the Landscape mode will give you greater DOF and the Portrait mode will give you a wider aperture for shallower depth of field for uncluttered backgrounds. In all cases however, the shutter speed and aperture combination (for a given ISO) will be such that the exposure is correct. For example, assuming that the ISO is same, Landscape mode may give you 1/60 at f/11 whereas, under the same lighting conditions, Portrait mode may give you 1/500 and f/4. In both cases the exposure is the same but the combination chosen is more appropriate for the given application, while following the reciprocity principle you just read about.
Barring the metered manual mode, all other modes are «automatic» in the sense that they alter the exposure to suit the changing light. Also, in these modes, if you change the shutter speed for example, the aperture will change in step to keep the exposure same. The same is the case if you change aperture — this time shutter speed will change in step. The same will happen if you change ISO. Thus you cannot change the exposure set by the camera this way, even if you want to. That can only be done by using «exposure compensation» button (Picture 4) on your camera. With this you can reduce the exposure to make the image darker. Alternatively, you can increase the exposure to get the opposite effect. All meters behave the same way:
While all humans don’t behave in the same way, all meters in fact do! What this means is that, when a meter (regardless of the exposure or the metering mode) reads a scene, it thinks that it is seeing a scene that is average in tone, also called the medium tone. This is also sometimes described as meters being calibrated to 18% reflectance, since this corresponds roughly to medium tone.
The result is that, when a meter is not seeing an average scene, it will give an erroneous reading. It is imperative that you recognize these situations and their apply suitable correction in the form of exposure compensation to get the right exposure.
You are likely to come across the following three cases where the overall scene will not present an average tone to the meter, hence causing the meter to err.
1. High-key scenes
2. Low-Key scenes
3. Back-lit scenes
High-key scenes: A scene with predominantly light tones will result in what you call as a «High-Key» photograph. Picture 5 shows a very light toned car model against a white background, making the whole scene predominantly very light. This creates the high-key effect. If you take a meter reading of such a high-key scene, then it will give a reading that will render the scene as medium tone. To get the correct tone in this case, an exposure compensation of +2.0 EV was given. This made the car and white background to come out very light (the way we wanted) rather than in medium tone.
Low-Key Scenes: Opposite of a high key scene is the low-key scene which results in a «Low-Key» photograph. Here, we have a black camera against a black background (Picture 6), making the whole scene predominantly very dark, that is, low-key. If you take a meter reading and expose it as it is, then the scene will be rendered in medium tone once again. To avoid that, an exposure compensation of -1.7 EV was given. Note that this rendered the camera black while at the same time retaining the details of the camera body and lens barrel. Without this minus compensation, the image would have been rendered in medium tone. Why does your meter give an error in these cases? A meter cannot distinguish between a predominantly light (highkey) scene and a predominantly dark (low-key) scene, even though the lighter scene has a much higher reflectance than the darker one. So, it will treat the first case as a medium toned scene in more light, and the latter as a medium toned scene in less light. Hence, it will give less exposure than what is needed in the first case and more exposure in the second case. This will make the high-key scene darker due to underexposure and the low-key scene lighter due to overexposure. The consequence is that both scenes will be recorded as medium toned!
You can easily understand this if you remember that meters are always calibrated to medium tone as they do not take into account the reflectance of the subject. Simply put, meters are «tone blind»!
Since this is an important concept, here is a restatement of the same: Whatever you measure, the meter will give a reading (for that area) such that it will be rendered as medium tone (the calibrated tone) in the captured image. Hence, you need to remember that if you meter a scene whose tone is not medium tone to start with, then you will get results that may surprise you. In case you come across high and low-key scenes, you need to give more and less exposure respectively to get the scenes rendered properly.
Back-lit scenes: This is a very commonly occurring error and occurs most frequently in portrait photographs (Picture 7) that are taken against a bright background. This is really a special case of a high- key scene where the meter gets affected by the bright background and sets an exposure to render the background in medium tone (underexposes). This results in the subject being rendered in a tone that is even darker than the mid-tone. The underexposure increases as the subject size decreases, as the meter will see a greater area of brightness and will underexpose even more. Same is the case if the brightness of the background increases. In extreme cases, this may result in what is called a silhouette. In a silhouette, the subject is rendered totally dark without any details. Thus only the form of the subject is recorded against a bright background. An interesting aspect of silhouettes is that the exposure needed for producing a silhouette is not perhaps technically the correct one, as this would bring out the details of the subject. However, it could be artistically pleasing.
The remedy for back-lit scenes is either to use a fill-flash, or take a spot meter reading of the subject. In the latter case you need to compensate the spot meter reading based on the skin tone of the subject (Picture 8). More on this shortly. High Contrast scenes: These are also troublesome situations but here the problems arise due to the camera’s (or more appropriately the sensor’s) inability to cope up with the brightness range of the scene, rather than the scene not being in average tone. You can easily understand what happens if you know what the two terms brightness range and dynamic range mean. Every scene has a brightness range which is the tonal variation between the brightest and darkest parts of the scene. This is measured in stops (hope you have not forgotten what a ‘stop’ means!).
A scene with a very high brightness range results in what we call a «high contrast» scene. Examples of such scenes are brightly lit landscapes with shadows, night scenes with street lights, interiors of rooms with light spilling from windows, etc.
Generally, modern imaging sensors can be expected to have at least 5-stops of dynamic range though more expensive cameras have a larger range (around 8-10 stops). This means they can record two objects, bright and dark, with proper detail even though the difference in their brightness is about 5 stops. This is called the dynamic range of the camera.
You will have no problem if the brightness range of the scene is less than the dynamic range of the sensor. This is what you would call as a low-contrast scene (you still need to compensate if the scene is low in contrast but low- or high-key).
If it is a high contrast scene where the brightness range exceeds the dynamic range, then due to the limitation of your camera you will not be able to record the full brightness range present. Hence you need to settle for a compromise (Picture 10) and decide what you want to record and what you want to discard — that is, to record without detail.
If you set a positive compensation, then you will expose for shadows thus preserving the details in the dark areas, while highlights will be overexposed (that is, blown out). The opposite will happen if you expose for highlights (negative compensation in this case), resulting in blocked shadows. Alternatively, you can set an exposure optimized for mid-tones, in which case the image may have both blocked shadows and blown highlights, but the mid-tones will be recorded correctly. You will have to decide the direction (positive or negative) and the quantity of compensation depending on which details (shadows or highlights or mid-tones) you want to preserve! A spot meter is not always spot on! A common mistake many photographers make (it is written like this all over the Internet as well as in some magazines too) is that whenever your subject is very heavily contrasted (like a back-lit portrait), you should take a spot reading of the subject and then take the picture. While this is not wrong it is only half correct. What happens is that when you take a spot reading, the meter sets the exposure so that the area you have measured is recorded as medium tone. This is the fundamental characteristic of any meter as you have just read. If this happens to be what you want, then you are home. If not, you have to compensate it properly by following Table 3. In fact you need not even bother what the original tone of the subject is, as it will be recorded always in medium tone without compensation! The compensation you give is what determines the final tonality of the image.
That is why a spot meter is called a «thinking photographer’s» meter!
This is also the reason why (assuming that a spot meter is working properly) it can never get «fooled» like an evaluative meter! However, what compensation to give depends on you, and if you err in the value of compensation you will get a wrong tone.
Now, let us revisit the back-lit portrait case. As an example, if you are photographing a very light skinned person, then you need to take a spot reading and then give +1.0 to +1.5 stops compensation. If the subject is dark skinned then you need to give -1.0 to -1.5 stops compensation. Without compensation the subject will appear medium toned in both cases — that is, subject will appear darker in the first case and lighter in the second. Of course, if the subject is medium toned then you need not give any compensation.
Exposure compensation is very important when you use a spot meter and here’s the reason why. While all meters, including spot meters, look at the scene as medium toned, this issue is very critical in the case of spot meters. Meters other than spot meters measure the whole scene, or at least a large part of the scene. Since large areas will have different tones, they tend to compensate each other and average out to medium tone. This does not happen with a spot meter with its narrow coverage, and thus more often than not a spot meter reading will require exposure compensation.