Mastering Light In Photography: Measuring Light

There are many tools, techniques and vocabularies that help when adjusting your exposure. In this article we take a look at the most common ones and how you can use them to measure light in photography.

Photo by Annie Spratt on Unsplash

This is part four of our four-part article series on understanding light. You can find the rest of the series here:


  1. Measuring Light
    • A. Light Meters
    • B. Histogram
    • C. Stops of Light
    • D. The Inverse Square Law
  2. Getting Started with Light

Measuring Light

Luckily, you do not need to rely on your eye in order to properly expose your images using the exposure triangle. There are many tools, techniques and vocabularies that help when adjusting your exposure. Here we’ll look at the most common ones:

  1. Light meters
  2. Histogram
  3. Stops of light
  4. The inverse square law

Light Meters

Light meters are quite simply devices that measure light. Your camera relies on this built-in tool anytime you use an automatic exposure mode.

Photo by Ritchie Rodas on Unsplash

There are two types of light meters: incident and reflective.

  1. Incident light meters: These directly measure the light’s intensity. It does this before the light is reflected off of the subject which results in a more accurate measurement.
  2. Reflective light meters: These measure the light that’s being reflected.

Your camera’s built-in light meter is a reflective light meter. This is usually sufficient but can sometimes cause problems in tricky light scenarios such as high contrast light. This is a big reason why it’s important to understand how to manually control your exposure triangle.


The histogram is a graphical representation of the tones within an image. This is an extremely useful tool that allows you to check on your exposure and make sure that you are not losing any detail.

  • The bottom axis of the histogram represents the tones of an image from Blacks to Whites with Shadows, Midtones and Highlights in between.
  • The y-axis represents how much light there is for any given tone in your image. The higher the spike, the more light there is for that tone.

When detail is lost in the bright or dark areas of your image, the histogram will show this with spikes hitting the left or right borders. In most cases, you should strive to keep all of your tones within the histogram.

This tool is especially useful when editing your images and when shooting in bright conditions that make it hard to see your camera’s LCD screen.

Photo by Maxim Medvedev on Unsplash

Stops of Light

“Stops” are a helpful way to measure the increase or decrease of light exposure. Each stop refers to either a doubling or halving of light.

Increasing your light by 1 stop means you will let in twice as much light. Decreasing your light by 1 stop means you will let in half as much light.

Stops of light are calculated in a slightly different way between ISO, shutter speed and aperture.

Calculating Stops of Light Using ISO

  • Doubling the ISO value increases light by 1 stop. ISO 100 -> ISO 200 = 2x brighter.
  • Halving the ISO value decreases light by 1 stop. ISO 1600 -> ISO 800 = 2x darker.

Calculating Stops of Light Using Shutter Speed

  • Doubling the shutter speed decreases light by 1 stop. 1/500 -> 1/1000 = 2x darker.
  • Halving the shutter speed increases light by 1 stop. 1/60 -> 1/30 = 2x brighter.

Calculating Stops of Light Using Aperture

Unlike ISO and shutter speed, aperture increases or decreases in multiples of 1.4.

  • Multiplying the aperture by 1.4x decreases light by 1 stop. F/8 -> f/11 = 2x darker.
  • Dividing the aperture by 1.4x increases light by 1 stop. F/2.8 -> f/2 = 2x brighter.

Rather than doing these calculations, it’s much easier to learn the sequence of aperture values (f/1, f/1.4, f/2, f2.8, f/4, f/5.6, f/8, f/11, f/16, etc.) and adjust accordingly.

Understanding The Inverse Square Law

The inverse square law is often dismissed because of its complexity at first glance. It feels more like math than it does art and because of this, it’s easy to walk away from it. However, understanding this concept is one of the most important things you can do for developing your lighting skills.

The inverse square law, by definition, is a law stating that the intensity of an effect such as illumination is inversely proportional to the square of the distance. Is your head spinning yet? Let’s simplify that and consider how it affects you as a photographer.

The inverse square law is especially useful to photographers who are using artificial lighting like flash or studio lighting. It’s helpful to photographers for two main reasons:

  1. It describes how fast light will “fall off” or lose intensity, and;
  2. How wide the light spreads.

Light intensity diminishes rapidly over its initial distances

It’s fairly easy to calculate exactly how much this light is diminishing using the inverse square law. Simply take the distance from the light to the subject and inverse the square of it.

  • At one foot away, the light will be 1/1 or 100% of its output.
  • At two feet away, the light will be 1/4 or 25% of its output.
  • At three feet away, the light will be 1/9 or 11% of its output.

Every time you double your distance, you lose 75% of light intensity.

This is useful to know for many reasons. Knowing this, you can place objects closer together in relation to the light so that they receive the same amount of light output. Or, you can do the opposite and place the light much closer to one object so that it is significantly brighter than the rest.

A common way to use this principle is with portrait lighting in front of a white background. You can make a white background appear to be any tone just by shifting the distance of your light and subject.

Photo by Angelina Litvin on Unsplash

Distance affects the characteristics of light on a single subject

When you place your light closer to your subject, you have:

  • more light fall off (light gets darker quicker)
  • darker shadows
  • smoother gradation between tones
Photo by Belén Martín on Unsplash

When you place your light further away from your subject, you have:

  • less light fall off (light gets darker more gradually)
  • lighter shadows
  • harsher gradation between tones producing “sharp shadows”
Photo by Alex Suprun on Unsplash

Light spreads as it gets further away from the source

Light covers more area as it travels. In the same way as light intensity, this coverage can be calculated using the square of the distance. Each time the distance is doubled, the light will cover 4x more surface area.

This principle is especially useful when photographing groups of people. In order to make sure that your light is covering everyone being photographed, you must increase the distance of the light to your subjects. This ensures that the light is spread enough to cover your subjects. Too close and certain subjects may appear brighter than others.

Getting Started with Light

Light is a complicated topic that deserves a thorough study as a photographer. It is, after all, our primary resource when creating images.

As you move forward in your photography, consider how important light is to the photographic process. Try to notice light in your everyday life. Look for how it changes throughout the day and interacts with the different surfaces around you. You can even start to classify the light that you’re seeing based on the characteristics of light we discussed earlier. This practice will strengthen your ability to use light without even using a camera.

This article marks the end of our four-part series on light in photography. You can find the rest of the series here:

This series introduced you to the key concepts of light in photography and how you can begin to control light as a tool. Come back to this series throughout your photographic practice—you might understand some of the more complex topics easier with time and practice.

Is there anything here that surprised you about light? Do you see light any differently now? Let me know in the comments below.

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