How to get a correct exposure in digital photography - Part I

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.