Spot metering. Ambient metering. What are they? How are they different? How do you use them?
Before we dive into these light measuring and exposure techniques, you'll first need to understand the two ways of measuring light for exposure and how they're different:
Reflective Metering measures the light that bounces off of or reflects from your subject. These are most commonly found inside your camera or as an app for your phone. This is often called spot metering.
Incident Metering measures the light that falls on the meter, often directly from the source. These are always handheld and have a bulb-shaped sensor. This is often called ambient metering.
HOW A REFLECTIVE METER WORKS:
A reflective meter measures the intensity of light bouncing off the subject you’re photographing. Because the meter measures the light after hitting the subject, how reflective the subject is or isn’t will affect what the meter “thinks” is the proper exposure setting.
Reflective meters, like the one in your camera, are set to balance everything to mid-gray, also known as “Zone V” in the Zone System of Exposure. So what does that mean in everyday, practical terms?
It means if you’re photographing a medium gray dog against a medium gray fence on a medium gray day, your in-camera metering will be perfect. But how often does that happen?
It also means if you try to photograph a white dog against a white fence, your in-camera meter will underexpose the image. And the opposite would be true with a black dog and black fence, leading to overexposure.
White dog. Black dog. It doesn’t matter to the meter. It wants it all to be mid-gray and will provide different settings in the same light to get you there. Luckily, you're way smarter than your meter!
HOW TO TAKE A PRECISE REFLECTIVE METER READING:
The rule of thumb in a really white, bright scenario, like a fresh-fallen snowfield or a white sand beach, would be to add two stops of light to the given reading from a reflective meter. If you’re in a very dark setting, i.e., shooting in a coal field or dark, deep woods, you can subtract two stops of exposure.
A sure-fire way to obtain perfect exposure using a reflective or spot meter is to take a reading on a middle-gray card (also known as 18% gray card).
If you’re doing a portrait session on a bright, sandy beach and set your camera settings based on the exposure of the middle-gray card you can use those same setting in the same light and ensure perfect exposure and tonality. If you use your in-camera meter, be sure to set it to “spot metering mode” while using the gray card.
ADVANTAGES OF REFLECTIVE/SPOT METERING:
Though you can undoubtedly use spot metering in any situation to achieve an accurate reading, reflective meters excel in difficult or extreme lighting conditions, or when your subject is far away. If your subject is backlit, you can take an accurate reading of the subject without the light coming from behind affecting your reading. If you’re shooting a concert in a dark venue, you can take an accurate reading of the light on the performer and ensure proper exposure. And if you’re shooting landscapes and want to expose for shadows in the distance, you can easily do so.
No matter which meter you are using, each can provide an accurate reading as long as you take the proper variables into account.
HOW AN INCIDENT METER WORKS:
To take an incident meter reading, you'll need a hand-held light meter. (Many hand-held meters have the additional capability of taking a reflective measurement.)
An incident meter (sometimes called a bulb-meter) works by reading the intensity of the light that falls onto the subject, as opposed to reading the light reflected off of the subject as the reflective meter does. Usually, you take the reading from the position of the subject.
HOW TO TAKE A PRECISE INCIDENT METER READING:
Most hand-held meters have multiple modes, including the ability to be used with a flash or strobes. In this case, we’re talking about ambient light. Make sure the meter is in incident mode. A little sun icon is usually used to represent this.
Input the ISO you’re shooting into the meter and place the meter in front of your subject, with the bulb facing towards the source of light. The meter will give you a reading of the suggested shutter speed and aperture settings, which you can also adjust.
Where you place the meter can also affect the reading. For instance, in a portrait, you can meter for shadows by angling the meter 45º downward. The angle effectively gives you the reading for the shadow beneath your subjects chin and will add about a half a stop of light to the reading.
For an extra touch of overexposure leave the bulb in. It will give you a half of a stop of light.
ADVANTAGES OF INCIDENT METERING:
Because incident meters are not as affected by reflections and color variance, they will record the light with high precision. In scenarios where you can be close to your subject, they are often the ideal choice because of their accuracy without having to consider other variables.