Education, Tutorials

Perfecting Exposure: How and When to Use a Light Meter

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If you’re using a camera that was made this century, then it likely has a built-in reflective light meter. That means there’s no need for a handheld light meter, right? Not quite.

An in-camera reflective meter is a great tool. It’s especially good at measuring light coming from a distance, a backlight, a highly reflective subject, or for when an incident light meter can’t be used, such as during a live event. In almost all situations, however, an incident light meter will be more accurate. Many handheld light meters measure both incident and reflective light.
 

Reflective Meters vs. Incident Meters

As mentioned, there are times when an incident meter is just not feasible, which is why handheld meters often have the ability to measure both incident and reflective light. To understand when to use one measurement over the other, you need to know how each kind of light meter works, and why.

Reflective Meters

A reflective light meter, such as the one in your camera, measures the intensity of light reflecting off of a subject. The light hits your subject, bounces off of, and then is measured as it hits the reflective light meter. The measurement is taken from the position of your camera.

The problem is, a reflective light meter isn’t actually measuring the light falling on the subject, which requires it to interpret what it’s measuring. Reflective light meters assume that all subjects are of 18-percent reflectance, or have a mid-tone of neutral gray. As a result, the measurement can be affected by variations of color or tone in a scene.

Wait, what? I know, it all sounds so technical. Let me try and simplify. Let’s say you have a 1,000-watt light. An incident light meter, which we’ll discuss next, will measure the intensity of light that the 1,000-watt light emits. It doesn’t matter what the subject is, the location, or the time of day. An incident light meter measures the light falling on the subject, and will be the same no matter what, unless you change the intensity of the actual light.

Generic camera viewfinder view looking through lens with camera settings and focus area

Camera Viewfinder by THPStock

A reflective light meter, on the other hand, measures the intensity from that 1,000-watt light after it’s been reflected off of the subject. So, if the subject were holding a mirror, or wearing a highly reflective running jacket, the intensity of the light being measured would be much greater than if the subject was dressed as a cat-burglar wearing all black.

If a subject is more reflective, it will obviously reflect more light, making the light hitting the meter more intense. A reflective light meter will assume it’s a very bright scene, even if it isn’t. So, if you adjust your camera settings based off of the measurement from the reflective light meter, your image may be underexposed, because the meter “thought” the scene was much brighter than it actually was. In the same way, a subject that’s less reflective could result in an overexposed image.

The reflective meter has to make assumptions about what it’s looking at, and while it usually does a good enough job, the results are not always accurate. Most reflective meters have a few different modes to choose from, each measuring the reflected light slightly differently.

Spot Metering: A spot meter measures only the light in a small area of the frame, usually in the very center. The rest of the image isn’t accounted for and the readings are a reflection of the light intensity in that single, small area. You may want to use a spot meter when the subject is the brightest part of your image, such as a swan on a bright day, or a moon in the night sky.

Evaluative (or Matrix) Metering: Evaluative meters separate a frame into different sections and then perform a super nerdy algorithm to decide the correct exposure. Evaluative metering is a good go-to for a nice full-frame exposure, such as in landscape photography.

Center Weighted Metering: Center-weighted meters gather readings of light reflected throughout the whole frame, but put more importance on the center of the image. If you’re taking portraits, I’d suggest trying center-weighted metering. While it focuses on the center of the frame, it still accounts for the rest of the image.

A camera’s built-in light meter usually offers the ability to switch between the different modes, which can be optimized depending on the shooting situation and end goal.
 

Incident Meters

Hi end light meter isolated on white background.

High-End Light Meter by frenky362

An incident light meter has a little white ball on it called a lumisphere. The meter measures the amount of light that falls on the lumisphere, and is not influenced in any way by a subject’s reflectance. The measurement is taken where the subject is and measures the intensity of light falling on the subject.

This is a much more accurate way of measuring light, because it’s the actual light that is being measured. Subjects that are darker than neutral gray will be darker, while subjects that are lighter than neutral gray will be lighter. Incident light meters offer a consistent measurement that is not affected by the tone or color of a scene.
 

How to Use a Handheld Light Meter

After you turn your unit on, there will be a number of icons that represent the different measuring modes.

  • Sun Icon: Ambient, non-strobe (flash) metering
  • Lightning Bolt Icons: Strobe (flash) metering
  • T Icon: Shutter speed
  • F Icon: F-stop, or aperture
  • ISO: What used to be referred to as film speed
  • To navigate between the different modes, hold down the mode button and rotate the jog wheel. Once you release the mode button, the jog wheel will cycle through the specific setting you’ve selected.

    To adjust the ISO, click the ISO button and rotate the jog wheel. The lumisphere can be raised or lowered by rotating the dial around it.

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    Measuring Ambient Light

    Ambient light is any form of continuous light, such as the sun or hot lights.

    Aperture Priority Mode

    1. Make sure the lumisphere (the little white dome) is fully extended.
    2. Select the F-stop mode and adjust to your desired setting.
    3. Hold the light meter in front of your subject so that the lumisphere is saturated by the light you want to measure.
    4. Click the measure button on the side of your light meter, and the correct shutter speed will be displayed.
    5. Set the aperture, shutter speed, and ISO on your camera to match the settings on your light meter.
    6. Take your perfectly exposed photo or video!

    Shutter Priority Mode

    1. Make sure the lumisphere is fully extended.
    2. Select the Shutter mode and adjust to your desired setting.
    3. Hold the light meter in front of your subject so that the lumisphere is saturated by the light you want to measure.
    4. Click the measure button on the side of your light meter, and the correct shutter speed will be displayed.
    6. Transfer the settings to your camera and shoot.
     

    Measuring in Strobe Mode

    Photography Studio Lighting by THPStock

    There are usually different strobe modes to choose from.

    Cordless Strobe Mode: This mode works just like ambient mode, only the meter will detect when a flash has been triggered and take the selected measurement at that time, so you don’t have to click the meter button. While in this mode, the meter will take a new measurement each time a flash is triggered.

    Corded Strobe Mode: This mode requires a sync cable to be connected from your camera or trigger to your light meter. When the photo is taken, a signal is sent through the cable telling the light meter to take a measurement. Again, there is no need to manually click the meter button.

    Radio Mode: Radio mode allows you to use different channels for specific strobes or groups of strobes. This way, you can measure the different lights or groups individually.
     

    Getting the Shot

    Your camera’s built-in meter is great, and will often do the job on the fly. However, when you have a controlled situation and are looking for the best possible exposure, I would always suggest using a handheld light meter. It will always be more accurate.

    Whichever meter you choose to use, it’s best to take multiple readings from your scene to get an average. This will help you avoid blowing out your highlights or losing too much detail in the shadows.

    Top image: Photographers Taking Pictures in Studio by bluejeanimages