One of the key tools in your photographic artist’s palette is the light meter. This essential item provides you with exact lighting information so you can properly set your exposures (which could be over-exposed by +1.5 stops or under exposed by -2 stops; it's all a creative choice that must be strictly controlled to get the artistic and aesthetic quality that you're seeking). Since you want to precisely measure the brightness of a particular scene, you need to be aware of how to do it properly. The computerized meter inside your camera can and will be tricked in a variety of lighting situations (too much black or too much white). Hand-held Light Meters will alleviate problems associated with the camera’s meter and give you better control of your exposures.
What is a Hand-Held Light Meter?
A hand-held light meter is a device that measures incidental (or ambient light) as well as reflective light (the same type of light measuring that your camera's on-board meter does) through a photo cell that reacts to the intensity of the light. All you do is set the ISO (and in some cases the shutter speed), and you get a reading in f-stops or foot candles (in some older models) so that you can adjust your camera for a precise and ideal exposure.
How it Works
A hand-held light meter works differently than a TTL meter in your camera or a spot meter, though both types of light meters measure reflected light. The H-H LM can measure the light falling on a subject (the incidental light) or the light being reflected off an object. The H-H LM can have an extremely narrow field of view (sometimes as little as 1 degree), which will enable you to have the most accurate light readings. You can't get that narrow a reading on a TTL light meter, even when it's set to Spot (between 3 to 5 degrees). On some types of H-H LM, holding the button down will give a continuous exposure, so you can see how the lighting changes from place to place -- even within a few inches. You might need to be this accurate when envisioning a complex light scheme in the studio, or trying to determine the best EV for your brightest highlights or darkest shadows.
How to use Hand-Held Light Meters
There are two attachments that go on your hand-held light meter, one is a Flat Disc and the other is a half-sphere. The Flat Disc is used to read reflected light. To do so, you just slip the disc on and hold the metering cell directly toward the object in which you want a reading. The hemisphere is used to measure incidental light. You slip the hemisphere on the metering cell and hold the meter in front of the subject, so the light is falling on the sphere in the same way it is hitting the subject. If you can, make sure that the hemisphere is pointing at the camera lens... this way you get a better idea of what the camera's image sensor is going to register, not just a general light reading.
Incident vs Reflected Light Meters
Incident metering with the hand-held light meter measures the amount of light falling on to a subject, whereas reflected metering measures the light that's reflecting off an object's surface. Holding the incident light meter directly in the light that's falling on the subject is important. As an example, if the subject is standing in the noontime sun, the incidental light under the chin is much different than the light on the bridge of the nose. In contrast, a reflected meter is pointed at the subject, and the light reflected off the subject is measured. The main difference in these two measurements is that the reflected light picks up the reflective qualities of the surface you are measuring (so a purple sweater might read with a lower exposure value even with a powerful light is hitting it. In this case, reflected metering isn't exactly accurate and you'd overexpose other segments of your image if you used that reading to set your f-stop). Whereas the incident meter measures the exact power of the light source, providing a better reading.
Dynamic Range is the range (in f-stops) between the whitest white and the blackest black of the scene that you're photographing. The middle point of the dynamic range will always be the 18% grey that every light meter uses as a benchmark for optimal exposure. The dynamic range is referred to as the latitude. Most digital cameras have 7 or 8 f-stops of latitude, so that there are about 4 stops above and 4 stops below the neutral 18% grey. This is important to understand with digital photography as with slide/transparency photography because there are appreciable limits to the dynamic range of the image sensor (or piece of film). What you'll discover is that the "best" shot might be over-exposed 1-1/3 stops or underexposed 3 stops. If you blindly followed the meter you'd miss that perfectly exposed shot. This example demonstrates the value of bracketing your exposures.
18% Gray Cards
You can purchase 18% grey cards in order to determine the exact dynamic range of your camera. There's a complicated process pioneered by Ansel Adams, called The Zone System, which enables you to more effectively understand how the 18% Grey Card operates as dead center of the dynamic range. But essentially, 18% is tonally neutral such that you'll have to most shadow and highlight detail if 18% is your «set» exposure value.
Regardless of the 9- or 12-point matrix metering systems that grace the latest and greatest DSLR, effective use of a H-H LM will provide the most accurate light readings (and exposure settings) to fit your creative vision. All light meters seek to find the benchmark 18% grey, but this isn't necessarily the "best" exposure for your purposes. Remember an incorrectly exposed shot can lose subtle shadow and highlight details, and it's virtually impossible to recover lost information in the highlights with digital photography (even with Photoshop). So the trick with digital photography is to expose for the highlights (much like you do with slide film), and you'll have greater exposure control. When accurate exposure is paramount in your photographs, you'll want to invest in and master the use of a Hand-Held Light Meter.