Mastering Flash Exposure: Using a Light Meter with ND Filters (Part 2)

By Jason Bodden

Review of Part 1: Understanding Light Meters for Flash Photography

In Part 1 of this series, we explored the fundamentals of using a light meter for flash photography. Key takeaways included:

• Flash Metering Modes: Modern light meters offer wireless flash metering and corded flash metering (for legacy strobes).
• How Flash Meters Work: Unlike ambient light metering, which can operate in Aperture Priority or Shutter Priority, flash metering only works in Shutter Priority, requiring the user to input ISO and shutter speed while the meter provides the correct aperture.
• Using a Light Meter for Flash Exposure: To achieve accurate results, the light meter should be placed near the subject’s face, pointed toward the key light, and triggered to measure the flash output.
• Flash Power and Aperture Relationship: Since smaller apertures (e.g., f/8) let in less light than wider apertures (e.g., f/4), a higher flash output is needed to maintain exposure at smaller apertures.
• Shutter Speed and Flash: Shutter speed does not affect flash exposure (as long as it remains at or below the camera’s flash sync speed). Instead, aperture and flash power control flash exposure.

With these principles in mind, Part 2 will focus on a more advanced application: using a light meter with neutral density (ND) filters to achieve shallow depth of field in bright conditions without High-Speed Sync (HSS).

Why Use an ND Filter with a Light Meter?

There are situations where a photographer may want to shoot at a wide aperture (e.g., f/2.8) in bright sunlight but cannot use High-Speed Sync (HSS). This could be because:

1. The flash does not support HSS, or
2. The photographer prefers to maintain full flash power efficiency (since HSS significantly reduces flash output).

In such cases, an ND filter is a simple and effective solution. ND filters reduce the amount of light entering the lens, allowing for wider apertures at lower shutter speeds while keeping the camera’s shutter speed at or below its flash sync speed (e.g., 1/250s).

However, using an ND filter introduces a complication for flash metering: since the filter darkens the entire exposure, the light meter reading must be adjusted accordingly.

Example: Balancing Flash with an ND Filter

Let’s break this down with a practical scenario.

Scenario

• Desired Aperture: f/2.8 (for a blurred background)
• Flash Sync Speed: 1/250s (since HSS is not being used)
• ISO: 100
• ND Filter: ND16 (4-stop reduction)

Without an ND filter, the correct exposure at 1/250s and ISO 100 might require an aperture of f/11. However, f/11 would create a deeper depth of field, bringing too much of the background into focus.

By using a 4-stop ND filter, we can reduce the brightness by 4 stops, allowing us to shoot at f/2.8 while maintaining the same overall exposure as f/11 without the ND filter.

Key concept:
✔ f/2.8 + ND16 filter = f/11 exposure without the ND filter

How to Use a Light Meter with an ND Filter

There are two simple methods for adjusting flash power when using a light meter with an ND filter:

Method 1: Adjust Flash Power in Two Steps

1. First, meter for f/2.8 (as if no ND filter were being used).
2. Increase flash power by 4 stops to compensate for the ND filter’s 4-stop reduction.

Method 2: Meter Directly for the Adjusted Aperture

1. Set the light meter to measure flash exposure normally.
2. Adjust flash power until the meter reads f/11 (since f/11 is 4 stops darker than f/2.8).
3. Leave the camera set to f/2.8.

Regardless of the method used, the camera aperture remains at f/2.8, ensuring a shallow depth of field while maintaining proper exposure.

Final Thoughts

Using a light meter with an ND filter is a straightforward technique that allows photographers to balance flash exposure in bright environments while maintaining creative control over depth of field. This method ensures that flash exposure remains accurate, even when ND filters reduce the ambient light.

By understanding how to compensate for ND filter density using a light meter, photographers can confidently shoot in bright conditions without relying on HSS or compromising on aperture choice.

If this is something you’ve ever wondered about, I hope this guide has clarified the process. As always, feel free to reach out with any questions.

Until next time, take care!

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.