Part III Using the 18% gray card to get a correct exposure.
Reflected light meters are calibrated to give an accurate exposure when pointed at subject with reflectivity somewhere near 18%; the exact value varies and the details are complex. By placing a gray card in the scene to be photographed and taking a reading off of it with a reflected light meter, the photographer can be assured of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject.
Using the gray card to determine exposure
For artificial light place the card to and in front of the subject, aimed halfway between the main light and camera.
When using daylight place the card in front of the subject pointing at the camera.
You can take your your readings with the card near your camera if you place the card so it is in the same relationship to the light source as the subject.
Exposure compesation:
- if the subject is very dark -> increase your calculated exposure by 1/2 or 1 stop
- if the subject is very light -> decrease your calculated exposure by 1/2 or 1 stop
- if you are unsure of the best exposure -> use bracketing technique to be sure you have the appropiate exposure.
Using the gray card to determine lighting ratios
Lighting ratios are very important in studio portraiture and product photography because they reflect how much contrast there will be between important light and dark areas in a picture.
Some contrast is essential to give a three-dimensional quality to a photograph, however too great a difference will exceed the ability of the film to capture both the highlights and shadow detail. For best quality prints, the lighting ratio should not exceed 3:1 for color film and 5:1 for black and white film.
In calculating a lighting ratio, the combined illumination from the main light and the fill light are compared against the illumination provided by the fill light alone. This ratio is expressed as two numbers, as in 2:1. The first number represents the combined strength of the main light and fill light, while the second number represents the fill light alone.
Lighting Ratio Technique
You can use an incident light meter to determine the lighting ratio, or a gray card and your camera light meter can do the same. Let's see how this is done.
Placing the gray card close to the subject, first read the main light plus the fill light, positioning the card for the highest reading on your light meter. Usually this means aiming the gray card at the main light. (Position 1 in the diagram.) Note the reading. Turn off the main light, aim the gray card toward the camera lens (position 2) and read the fill light illumination. Note the reading and calculate the difference in f. stops.
Part II Get exposure readings with camera-in lightmeter
Digital cameras have a build in lightmeter. This lightmeter measure the amount of light (luminance) reflected from subject to the camera. Read the Reflected-light readings from Part I. With other words camera in lightmeter is very similary with reflected lightmeter, but in this case exposure readings depends by metering modes.
Spot metering
With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area). This will typically be the very centre of the scene.Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot very high contrast scenes. For example, if the subject's back is being hit by the rising sun and the face is a lot darker than the bright halo around the subject's back and hairline (the subject is "backlit"), spot metering allows the photographer to measure the light bouncing off the subject's face and expose properly for that, instead of the much brighter light around the hairline. The area around the back and hairline will then become over-exposed. Spot metering is a method upon which the zone system depends.
Another example of spot metering usage would be when photographing the moon. Due to the very dark nature of the scene, other metering methods tend to overexpose the moon. Spot metering will allow for more detail to be brought out in the moon while underexposing the rest of the scene.
Partial metering
This mode meters a larger area than spot metering (around 10-15% of the entire frame), and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly. Like spot metering, some cameras can use variable points to take readings from, (in general autofocus points), or have a fixed point in the centre of the viewfinder. Partial metering is found mostly on Canon cameras.
Center-weighted average metering
In this system, the meter concentrates between 60 to 80 percent of the sensitivity towards the central part of the viewfinder. The balance is then "feathered" out towards the edges. Some cameras will allow the user to adjust the weight/balance of the central portion to the peripheral one. One advantage of this method is that it is less influenced by small areas that vary greatly in brightness at the edges of the viewfinder; as many subjects are in the central part of the frame, consistent results can be obtained.
Average metering
In this metering mode the camera will use the light information coming from the entire scene and averages for the final exposure setting, giving no weighting to any particular portion of the metered area.
Multi-zone metering
This mode is also called matrix, evaluative, honeycomb, segment metering, or esp — (electro selective pattern) metering on some cameras.
This metering mode was first introduced by the Nikon FA, where it was called Automatic Multi-Pattern metering. On a number of cameras this is the default/standard metering setting. Here the camera measures the light intensity in several points in the scene, and then combines the results to find the settings for the best exposure. How they are combined/calculated deviates from camera to camera. The actual number of zones used varies wildly, from several to over a thousand. However performance should not be concluded on the number of zones alone, or the layout. In general, the most advanced metering is found on single-lens reflex cameras.
Many manufacturers are less than open about the exact calculations used to determine the exposure. A number of factors are taken into consideration, including the following: Autofocus (AF) point, distance to subject, areas in focus or out of focus, colours/hues of the scene, and backlighting. Multi-zone tends to bias its exposure towards the autofocus point being used (whilst taking into account other areas of the frame too), thus ensuring that the point of interest has been exposed for properly. A database of many thousands of exposures is pre-stored in the camera, and the processor can use a selective pattern to determine what is being photographed.
Some cameras allow the user to link or unlink the autofocus and metering, and allow the option of locking exposure once AF confirmation is achieved, AEL, (auto-exposure lock). Using manual focus, and on many compacts/bridge cameras, the AF point is not used as part of the exposure calculation, in such instances it is common for the metering to default to a central point in the viewfinder, using a pattern based on that area. There is considerable variation from different manufacturers as to how multi-zone metering is implemented, and even from the same maker in their model range, and how much "priority" is given to the AF point itself. Some "Scene" modes, such as sunset, sports, night exposures etc, also often affect the calculations of this metering pattern.
However, some photographers may be uncomfortable with multi-zone metering. This tends to stem from a lack of clarity about "how" the camera reacts in certain situations. The design concept behind multi-zone is to reduce the need to use exposure compensation.
Some users have problems in wide-angle shots with high contrast, due to the large area which can vary greatly in brightness. It is important to understand that even in this situation, the focus point can be critical to the overall exposure.
Reference:
Light metering
Understanding metering
Part I Take exposure readings with handheld lightmeter
Exposure can be measured in two basic ways. One way is to measure the light incident on the subject, i.e. the brightness of light illuminating the subject (illuminance) (see Fig1.); the second way is to measure the light reflected by subject, i.e. the intensity of the light reflected from the subject in the direction of the camera (luminance)(see Fig.2).
Before selecting the most suitable measuring method, you need to fully understand the different sources of light you are working with, as well as the influence of the positions and directions of receptors during measurement.
Incident-light readings
In general photography, light from the illuminating light source reflects off the subject and passes through the lens to form an image on the film or digital sensor for exposure.
To accurately calculate exposure in incident-light readings, you need to know how much of illuminating light is actually reflected from the object to the camera. To do this, you need to know how light or how dark the subject is, i.e. the reflectance of the subject.
Since a typical value of reflectance for many scenes is 18%, this value is used to calculate the light intensity reflected from the subject towards the camera. The exposure reading ( f-number and shutter speed) are then calculated to reproduce the metered area as a midtone with 18% reflectance.
Thus, incident-light readings are based on this standard value of 18% reflectance.This means that areas of subjects having a reflectance higher than 18% will turn out brighter (e.g. white), while areas of reflectance lower than 18% will turn out darker (e.g. black).This will produce a clear contrast in the picture of the subject.From this we can see that this measuring method provides for natural tonal range over the entire composition.
When make incident-light readings with a handheld lightmeter, you must use spherical diffuser and flat diffuser creatively. When photographing three-dimensional objects such as people, the highlights and shadow areas of scene depend on the direction of main illuminating light source. Exposure is also influenced by any light reflected towards the camera from the sides or rear of the subject (Fig.3.a) 
In these situations, the Spherical Diffuser captures the illuminating light coming from different directions at the position of the subject, so that the exposure readings takes into account the contribution of this light on illuminating the subject.
On the other hand, with flat subject such as pictures or documents, light from the sides or rear of the subject generally make little or no contribution to illuminating the subject (Fig.3.b).So, for these situations, accurate exposure readings are made using a Flat Diffuser to capture only the illuminating light from the front of the subject.
Incident light readings is a proper method to take correct exposure for portrait, still life photography or studio, i.e. subjects can be separate from background. This method is not indicate for landscape photography.
Reflected-light readings
Reflected-light exposure readings directly measure the amount of light (luminance) reflected from subject to the camera.Unlike the case of incident light reading, this method does not rely on the assumption of standard subject reflectance of 18%.Based on the measured amount of light falling on the subject, the meter calculates the appropriate exposure value for reproducing the subject on film at a suitable medium density (midtone). This means that in reflected-light readings, all subjects, regardless of their reflectance, i.e. regardless of whether they are bright or dark (white or black), will be reproduced at the same tonal density (midtone).For this reason, when making reflected-light exposure readings, it is important to decide which area of the subject to measure, since the reflectance will generally vary quite widely over composition under different conditions.
There are two basic methods for making reflected-light readings with a handheld lightmeter.These methods use different distances and angles between exposure meter and subject. One method is “averaging metering” where the entire scene is measured for calculate exposure readings (Fig.4).
The other is “selective metering”, where illumination is measured only for a specific part of the composition (Fig.5).
In averaging metering, all the reflected light from the entire subject that fits within the field of view of meter is measured using a receptor with a relative wide acceptance angle.If the average reflectance of subject is close to 18%, the exposure reading will be close to the value produced by an incident-light reading. For this kind of situation, this is the fastest and easiest kind of reflective-light reading to make.
However, if there is a large bright or dark area within the camera’s field of view, or if there is any backlighting, these factors may cause the exposure reading to be too high or to low.So caution is needed in these situation.
In selective metering, measurement are taken so that only light reflected from specific, selected part of the subject falls within the acceptance angle of the meter. This is achieved by using a receptor with a narrow acceptance angle, or by placing the receptor sufficiently close to the subject.Since the meter is not affected by light from other parts of the scene, the selected part of the subject is reproduced at a suitable tonal density.When the measured part has a normal reflectance, e.g. human skin, then, as in the case of averaging metering, the exposure reading will give a very similar result to incident-light reading. Thus, for photographing subjects of normal reflectance, such as people, this method offers highly accurate exposure readings. However, this method can cause problems if you selectively measure very bright (high reflectance) or very dark (low reflectance) subjects, because the meter assumes that the whole image should have the same tonal density as the selected area. So take special care about which part of a subject you selected area.So take special care about which part of a subject you select to measure when using selective metering.
Some examples of tricky exposures
-Point your camera at a scene with a lot of snow, sand, water, or sky - the camera doesn't know what it's looking at. Take pictures without adjusting the exposure and you'll get badly underexposed photographs - snow will look dirty and gray, landscapes with big expanses of sky or water will be too dark.
-Take a picture of a person standing in the shade with a bright background or with the sun behind them, backlighting their head. This can be very flattering portrait lighting if you know how to use your light meter correctly, but if you don't adjust your exposure to compensate for the bright background, you'll end up with near silhouettes.
