Mastering Flash Exposure: A Brief Guide to Using Light Meters (Part 1)

By Jason Bodden

Light meters have long been a valuable tool in photography, yet their use remains a topic of debate among photographers. While some professionals swear by them, others dismiss them as unnecessary, relying instead on their camera’s built-in metering system and experience. This two-part series aims to provide a deeper understanding of light meters, their functionality, and how they can be used to achieve precise exposure, particularly when working with flash.

This first article focuses on flash metering—how light meters measure and interpret flash output to help photographers achieve consistent and accurate results.

Flash Metering Modes: Wireless and Corded

Modern light meters typically offer two flash metering modes:

• Wireless Flash Mode (indicated by a lightning bolt icon) allows the meter to detect and measure the light emitted by a flash without any physical connection.
• Corded Flash Mode (indicated by a lightning bolt with a small "c" next to it) is used when working with older strobes that require a sync cable for metering. While most photographers today rely on wireless flash metering, it is useful to be aware of the corded option for legacy equipment.

How Flash Meters Differ from Ambient Light Meters

A key distinction between ambient and flash metering is the way settings are determined:

• Ambient Light Metering: Light meters can measure ambient light in both Shutter Priority and Aperture Priority modes. This means the meter can provide either the correct aperture for a given shutter speed or vice versa.
• Flash Metering: Flash meters, however, function only in Shutter Priority Mode. The photographer sets the shutter speed and ISO, and the light meter provides the corresponding aperture needed for correct flash exposure.

Using a Light Meter for Flash

To accurately measure flash output:

1. Position the Light Meter – Stand at your subject’s position and place the meter near their face, typically under or in front of the chin.
2. Point Toward the Light Source – Aim the meter at the key light to measure the light falling on the subject.
3. Press the Measure Button – The meter will wait for the flash to fire and then display the recommended aperture setting for proper exposure.

Since flash metering relies solely on incident metering (measuring light falling on the subject rather than light reflecting off it), using a light meter with TTL flash is impractical. TTL adjusts flash power dynamically for each shot, whereas a light meter assumes consistent manual flash settings.

Understanding Light Meter Readings for Flash Exposure

Let’s consider a common scenario:

• Your camera’s flash sync speed is 1/250s
• You set your ISO to 100
• You want an aperture of f/4

After inputting 1/250s and ISO 100 into the light meter, you fire the flash and adjust its power until the meter reads f/4.

Some photographers initially find it counterintuitive that a wider aperture (e.g., f/4) corresponds to a lower flash power reading, while a smaller aperture (e.g., f/8) requires more flash power. However, this aligns with fundamental exposure principles:

• A wider aperture (f/4) allows more light into the camera, requiring less flash output to achieve proper exposure.
• A smaller aperture (f/8) lets in less light, necessitating higher flash power to compensate.

For instance, if you expose an image at f/4 and then stop down to f/8, you must increase the flash power by two stops to maintain equivalent exposure. This reciprocal relationship between aperture and flash output is at the core of flash metering.

Shutter Speed and Flash Exposure

It is also important to note that shutter speed does not affect flash exposure, as long as it remains at or below the camera’s flash sync speed. Flash exposure is controlled by aperture and flash power, whereas shutter speed influences ambient light exposure.

The Limitations of Standard Light Meters

This discussion focuses on conventional light meters, which do not measure High-Speed Sync (HSS) flash. Only a few specialized light meters are capable of HSS metering, and they are prohibitively expensive—typically priced around $900 or more. For most photographers, standard light meters remain the most practical choice for measuring flash in manual mode.

Conclusion

Understanding how to use a light meter for flash metering can provide greater consistency and control over exposure, especially in studio settings or when using manual flash. In Part 2 of this series, we will explore advanced metering techniques and how to use a light meter to balance flash with ambient light effectively.

For any questions or further discussion, feel free to reach out. Stay tuned for the next article in this series!

About the Author

Jason Bodden is a Florida-based portrait and general assignment photographer who hails from the Caribbean’s Trinidad & Tobago. He has extensive expertise in both studio and location lighting.