Ambient light is the existing or available light surrounding a subject. Incident light is the light falling on a subject. Think of them this way: the light all around you right now – from any source – is ambient light; but if you are the subject being photographed, the light actually striking your body is incident light.
An incident light meter placed close to the subject reads the ambient light that is falling on the subject. That is why the meter is sometimes called an incident light meter and sometimes called an ambient light meter. In either case, it reads the light that is actually there, striking your subject.
Reflected light is the light reflected by your subject. The light meters in most popular cameras read reflected light, but with a little addition they can also be made to read incident light.
A light-weight, white styrofoam cup and your camera can make a good substitute ambient light meter.
Converting your camera’s reflected light meter into an ambient (or incident) light meter involves placing a white diffuser over the front of the lens. This simulates an incident light meter’s translucent cap, enabling your camera’s meter to read the incident light. A plastic deli container, a pure white cap from an aerosol can that fits snugly on the lens or a sheet of translucent white plastic will all do the trick. For convenience and general availability, however, it’s hard to beat an empty white styrofoam coffee cup.
Here’s how to figure out which makeshift diffuser works best. First, it must completely fit over the lens. Styrofoam cups come in different thicknesses. The greater the thickness, the more light it absorbs (blocks). So, to use this technique, you have to calibrate your camera’s meter with the type of cup. Calibrate it once, using one common make of styrofoam cup, and you will never have to do it again, so long as you always use the same type of cup.
How to calibrate? Here’s a rough but fairly reliable way. With the cup in place over your lens on a clear sunny day, take an ambient light meter reading – i.e. bright sunlight must be striking the cup. Compare the reading to the sunny 16 rule. If your camera meter reading shows a similar shutter speed as the sunny 16 rule, the cup is the right thickness. For example, with your meter set for ISO 100 film, your reading should be 1/125 at ƒ16. If not, try another type of cup that absorbs more or less light until you find the one that does the job. For real accuracy, borrow an actual ambient light meter or take a TTL (through the lens) meter reading of a gray card in bright light to compare your diffused reading with.
One final point: because the ambient light meter reads the light falling on the subject, not the light reflected off of it, you need to direct the camera away from the subject, towards the light source, when taking your reading.
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