Manual Flash Exposure Calculator
A simple yet powerful tool for calculating exposure using manual flash based on the Guide Number (GN) system.
Enter the Guide Number (GN) of your flash, usually found in its manual.
Select the unit system that matches your flash’s Guide Number.
Enter the distance from your flash to the subject in the selected unit.
Enter your camera’s ISO setting. The standard GN is based on ISO 100.
Aperture vs. Distance Chart
Exposure Table
| Distance | Required Aperture (f-stop) |
|---|
What is Calculating Exposure Using Manual Flash?
Calculating exposure using manual flash is a fundamental photography technique that allows for precise and consistent control over lighting. Unlike TTL (Through-The-Lens) systems that automate exposure, manual flash requires the photographer to set the flash power and camera settings. The core of this calculation relies on the Guide Number (GN) system. A flash’s Guide Number is a rating of its maximum light output at a specific ISO (usually ISO 100) and flash zoom setting. By knowing the GN and the distance to the subject, you can accurately determine the correct aperture (f-stop) needed for a perfect exposure. This method provides predictability and repeatability, which is essential in studio and controlled lighting environments.
The Manual Flash Exposure Formula
The relationship between Guide Number, distance, and aperture is governed by a simple formula. This formula is a practical application of the inverse square law of light.
Primary Formula: Aperture (f-stop) = Guide Number / Flash-to-Subject Distance
This formula works when the camera’s ISO is set to the base ISO of the Guide Number (typically ISO 100). If you use a different ISO, you must first calculate the “Effective Guide Number” for that ISO.
ISO Adjustment Formula: Effective GN = Base GN * sqrt(New ISO / Base ISO)
For example, doubling your ISO from 100 to 200 increases the effective GN by a factor of approximately 1.4. This calculator handles these adjustments automatically for you.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Guide Number (GN) | The power output of the flash. | Meters (m) or Feet (ft) | 20 – 60 (for speedlights) |
| Distance | The distance from the flash to the subject. | Meters (m) or Feet (ft) | 1 – 20 |
| ISO | The camera sensor’s sensitivity to light. | Unitless | 100 – 6400 |
| Aperture (f-stop) | The size of the lens opening that controls light entry. | f-number (e.g., f/4, f/8) | f/1.4 – f/22 |
Practical Examples
Example 1: Portrait Photography
Imagine you are shooting a portrait with a flash that has a Guide Number of 40m at ISO 100. Your subject is 5 meters away, and your camera is set to ISO 100.
- Inputs: GN = 40m, Distance = 5m, ISO = 100
- Calculation: Aperture = 40 / 5
- Result: You should set your lens aperture to f/8 for a correct exposure.
Example 2: Event Photography with Higher ISO
Now, consider you are at an indoor event. You need more flexibility, so you increase your camera’s ISO to 400. You are using the same flash (GN 40m at ISO 100) and your subject is now 10 meters away.
- Inputs: Base GN = 40m, Distance = 10m, ISO = 400
- ISO Adjustment: Effective GN = 40 * sqrt(400 / 100) = 40 * sqrt(4) = 40 * 2 = 80m.
- Calculation: Aperture = 80 / 10
- Result: You should set your lens aperture to f/8. Notice how increasing the ISO allowed you to maintain the same aperture at double the distance. For more on this, you might read about {related_keywords}.
How to Use This Manual Flash Calculator
- Enter Guide Number: Find your flash’s Guide Number in its manual and enter it into the first field. Make sure you know if it’s rated in feet or meters.
- Select Units: Choose “Meters (m)” or “Feet (ft)” to match your Guide Number’s specification. The calculator will use this unit for distance as well.
- Enter Distance: Measure or estimate the distance from your flash to the subject and input it. Remember, it’s the flash’s distance, not the camera’s, that matters.
- Set Camera ISO: Enter the ISO you plan to use on your camera.
- Read the Result: The calculator instantly shows the required aperture (f-stop) for a correct exposure. The “Effective Guide Number” is also shown, which is useful for your own understanding.
- Analyze Chart and Table: Use the dynamic chart and table to see how aperture changes with distance, helping you anticipate adjustments as you or your subject moves. To better understand this relationship, check out our guide at {internal_links}.
Key Factors That Affect Manual Flash Exposure
While the Guide Number formula is a powerful starting point, several real-world factors can influence the final exposure. For a deeper dive, consider this article about {related_keywords}.
- Flash Zoom Head Position: Most modern flashes have a zoom head. A narrow zoom (e.g., 105mm) concentrates light, increasing the effective GN, while a wide zoom (e.g., 24mm) spreads light, reducing it.
- Light Modifiers: Using a softbox, umbrella, or diffuser will absorb some light, effectively reducing the GN. You typically need to open up your aperture by 1-2 stops to compensate.
- Bounce Flash: Bouncing the flash off a ceiling or wall creates softer light but increases the light-travel distance and absorbs light. This significantly reduces the amount of light reaching the subject, requiring a much wider aperture.
- Ambient Light: The formula assumes the flash is the primary light source. In bright conditions, ambient light will also contribute to the exposure, which may require you to adjust your shutter speed or use a smaller aperture.
- Subject and Environment Color: Light-colored subjects and walls reflect more light than dark ones. You may need to slightly adjust your exposure based on the reflectivity of your scene.
- Battery Power: As flash batteries deplete, the power output and recycle time can become inconsistent, slightly affecting exposure from shot to shot.
Frequently Asked Questions (FAQ)
1. What is a Guide Number (GN)?
A Guide Number is a rating used by flash manufacturers to measure the power output of a flash. A higher number means a more powerful flash. It’s calculated as GN = Distance × f-number at ISO 100.
2. Why use manual flash instead of TTL?
Manual flash provides consistency. Once you set it, the exposure remains the same for every shot, regardless of changes in composition or background reflectivity. TTL can be fooled by very bright or dark backgrounds, leading to inconsistent exposures. If you are new to this, we have a great post on {related_keywords} that can help.
3. Does shutter speed affect manual flash exposure?
The brightness of the part of the scene lit by the flash is *not* affected by shutter speed (as long as it’s at or below your camera’s sync speed). However, shutter speed *does* affect the brightness of the ambient light in the scene. A slower shutter speed will make the background brighter.
4. How do I convert a Guide Number from meters to feet?
To convert a GN in meters to feet, multiply by 3.28. For example, a GN of 36m is approximately equal to a GN of 118ft (36 * 3.28). This calculator handles the conversion automatically when you switch units.
5. What happens if I bounce my flash?
Bouncing the flash increases the distance the light must travel (from flash to ceiling to subject) and the surface absorbs some light. You will need to open your aperture significantly to compensate. There’s no exact formula; it requires experience or a light meter.
6. How do I use this calculation for off-camera flash?
The principle is exactly the same. The “Distance” is always measured from the flash unit to the subject, not from the camera to the subject. Explore more at {internal_links}.
7. What if my flash has different power levels (e.g., 1/2, 1/4 power)?
Reducing flash power reduces the Guide Number. Halving the power (e.g., from 1/1 to 1/2) reduces the GN by a factor of 1.4 (the square root of 2). Quartering the power reduces the GN by a factor of 2. You would need to input this new, lower GN into the calculator.
8. Why does my result show f/17.9? My lens doesn’t have that setting.
The calculation provides a precise mathematical result. You should choose the nearest standard f-stop on your lens (e.g., f/16 or f/18). Minor differences are usually not noticeable. Photography techniques like this are covered in our resource about {related_keywords} here: {internal_links}.