Understanding Equivalent Exposure in Photography and Its Relationship to Off-Camera Flash
When diving into photography, one of the most important concepts to master is equivalent exposure. This principle allows you to make adjustments to different settings in the exposure triangle—aperture, shutter speed, and ISO—while maintaining the same overall exposure. For photographers new to off-camera lighting, it’s crucial to understand how these settings not only affect the brightness of the image but also interact with flash. In this article, we’ll break down equivalent exposure and how changes to the exposure triangle impact both natural and artificial light in your photos.
The Exposure Triangle: A Quick Review
Before we dive into equivalent exposure, let’s quickly review the three components of the exposure triangle:
1. Aperture: The size of the lens opening, which controls how much light enters the camera. A lower f-stop number (e.g., f/2.
means a wider aperture, allowing more light in, while a higher number (e.g., f/16) means a smaller aperture, letting in less light. Aperture also affects depth of field—wider apertures produce a shallow depth of field (blurry background), while narrower apertures keep more of the image in focus.
2. Shutter Speed: The amount of time the camera’s sensor is exposed to light. Faster shutter speeds (e.g., 1/1000) freeze motion but let in less light, while slower shutter speeds (e.g., 1/30) allow more light but can introduce motion blur.
3. ISO: The sensitivity of the camera’s sensor to light. A low ISO (e.g., ISO 100) is less sensitive, resulting in a cleaner image, while a high ISO (e.g., ISO 3200) increases sensitivity, brightening the image but introducing noise.
These three elements work together to create a balanced exposure. However, they don’t work in isolation—changing one element will often require adjusting another to maintain the same overall exposure.
What Is Equivalent Exposure?
Equivalent exposure refers to adjusting one or more settings in the exposure triangle while keeping the total exposure constant. For example, if you need to use a faster shutter speed to freeze motion but don’t want the image to become too dark, you could open up the aperture or raise the ISO to compensate. Here’s an example:
Original exposure: 1/125 sec, f/5.6, ISO 200
New exposure: 1/250 sec, f/4, ISO 200
By increasing the shutter speed by one stop (from 1/125 to 1/250), we reduced the amount of light coming in. To compensate, we opened up the aperture by one stop (from f/5.6 to f/4) to let in more light. The exposure is equivalent, but the depth of field is now shallower because of the wider aperture.
This principle is key for adjusting exposure in varying light conditions while maintaining creative control over how the image looks. However, when you introduce off-camera flash into the mix, the dynamics change slightly.
Equivalent Exposure and Off-Camera Flash
When using off-camera flash, understanding how changes in the exposure triangle affect both the ambient light and flash exposure becomes critical. Flash and ambient light are controlled by different settings:
Shutter speed affects ambient light but has little effect on flash exposure (up to the camera’s sync speed—usually around 1/200 sec or 1/250 sec). A faster shutter speed reduces ambient light, while a slower one increases it. The flash, however, remains constant in its output unless you adjust the flash power or distance.
Aperture affects both ambient light and flash. A wider aperture (e.g., f/2. lets in more of both, while a narrower aperture (e.g., f/11) reduces both flash and ambient light.
ISO affects both ambient light and flash, similar to aperture. Increasing the ISO makes the camera more sensitive to light from both sources, while lowering it decreases the sensitivity.
Balancing Flash and Ambient Light
Let’s say you’re shooting an outdoor portrait with off-camera flash. You want the background (ambient light) to be properly exposed while using the flash to light your subject. Here’s how the exposure triangle and equivalent exposure come into play:
Step 1: Set the ambient exposure. First, set your camera settings to properly expose the background. Since the flash doesn’t affect ambient light, use the combination of shutter speed, aperture, and ISO that gives you the background exposure you want. For example:
1/200 sec (sync speed), f/8, ISO 100
Step 2: Adjust for flash exposure. Now, introduce the flash to light your subject. The flash exposure is primarily affected by your aperture, ISO, and the power output of your flash. In this case, if f/8 is too dark for the flash exposure, you can either increase the flash power, bring the flash closer, or open up the aperture (or increase the ISO if you want to maintain the depth of field).
Now, suppose you open the aperture to f/5.6 to let in more light from the flash. This will also brighten the background (ambient light) because the aperture affects both flash and ambient light. To maintain equivalent exposure for the background, you might need to adjust your shutter speed to 1/400 sec (if your camera supports high-speed sync) or lower the ISO to 50.
How Flash Power Fits Into Equivalent Exposure
The power of your flash is another variable you can adjust, and it works independently of your camera’s exposure triangle. When you change flash power, you’re altering how much light the flash emits. This allows for flexibility in maintaining equivalent exposure while still controlling the creative aspects of the image.
For instance, if you want a darker background but need to maintain the same flash exposure, you could:
Lower the ISO or close the aperture to reduce the ambient light, but then increase the flash power to maintain the proper exposure on your subject.
Conversely, if you want to lower the flash power to create a softer effect on your subject, you might need to open the aperture or raise the ISO to maintain the same exposure.
Summary of Flash and Equivalent Exposure Adjustments
Shutter Speed: Controls ambient light. Flash exposure remains unaffected as long as you stay below the sync speed.
Aperture: Controls both ambient light and flash exposure. Wider apertures brighten both; narrower apertures darken both.
ISO: Controls both ambient and flash exposure. Higher ISO brightens the image; lower ISO darkens it.
Flash Power: Independent of ambient exposure, adjusting the power changes how bright the flash-lit subject appears.
Practical Application for New Photographers
Now that you understand how equivalent exposure works in both ambient and flash photography, it’s time to practice. Start by:
1. Setting up your camera without flash to expose for the ambient light.
2. Introducing flash and adjusting your aperture, ISO, or flash power to get the desired exposure on your subject.
3. Experiment with balancing flash and ambient light by adjusting the settings in the exposure triangle while maintaining the overall exposure.
By understanding these principles, you’ll gain more control over your lighting and be able to craft images that have just the right balance of flash and ambient light, all while preserving the creative aspects of your photography.