The flash guide number (GN) is a crucial specification for understanding the power of a flash unit. It represents the relationship between the flash-to-subject distance, aperture, and ISO to achieve a properly exposed image. Understanding and utilizing guide numbers can unlock precise flash control, especially in manual mode.
Understanding the Guide Number Formula
The guide number formula reveals the connection between these elements:
Guide Number (GN) = Subject Distance × f-stop
This formula shows that the guide number is equal to the product of the subject distance and the aperture required for correct exposure at a specific ISO (typically ISO 100).
Practical Example: Calculating Flash Settings
Let’s illustrate this with a practical example:
- Flash: Canon 600EX II-RT
- Focal Length: 80mm
- Flash Zoom: 80mm (Guide Number 49 at 80mm, ISO 100)
- Subject Distance: 3 meters
Applying the formula:
49 = 3m × f-stop
f-stop = 49 / 3 = 16.3
Rounding down, we get f/16. This means that at full flash power, with a flash zoom of 80mm, ISO 100, and a subject distance of 3 meters, an aperture of f/16 will yield a well-exposed image.
Adjusting Aperture and Flash Power
What if you want a shallow depth of field with a wider aperture? Let’s say you want to shoot at f/2.8 for a blurry background. That’s a change of 5 stops from f/16, meaning f/2.8 lets in 5 stops more light than f/16. You need to reduce the flash power by 5 stops to maintain correct exposure. The following table shows the relationship between aperture and corresponding flash power:
| Aperture | Flash Power |
|---|---|
| f/16 | 1/1 (Full Power) |
| f/11 | 1/2 |
| f/8.0 | 1/4 |
| f/5.6 | 1/8 |
| f/4.0 | 1/16 |
| f/2.8 | 1/32 |
| f/2.0 | 1/64 |
| f/1.4 | 1/128 |
Therefore, to shoot at f/2.8, you would need to set your flash power to 1/32.
Challenges with Guide Numbers
While helpful, relying solely on guide numbers presents several challenges:
- Inconsistent Information: Manufacturers rarely provide comprehensive GN tables. Often, only one GN is given, and the corresponding focal length may not be specified.
- Inconsistent Standards: Guide numbers aren’t standardized across different flash models or brands. Comparing GNs from different manufacturers can be misleading.
- Limited Practicality: Calculating flash settings using guide numbers can be time-consuming, especially in fast-paced shooting environments.
Zoom and Guide Number
The guide number changes depending on the flash zoom. As the flash zoom increases (narrowing the beam), the light becomes more concentrated and intense, resulting in a higher guide number.
Practical Alternatives: TTL and Manual Adjustments
In the digital age, alternatives like TTL (Through-The-Lens) metering and manual adjustments offer more efficient ways to control flash exposure.
- TTL Metering: TTL allows the camera to automatically measure the light reflecting off the subject through the lens and adjust the flash power accordingly.
- Manual Adjustments: Start with a manual flash setting and take a test shot. Evaluate the image and adjust the flash power up or down until you achieve the desired exposure.
Comparing Flash Units
If you’re comparing flash units based on guide numbers, try to find complete GN tables from the manufacturer’s specifications. Compare the values for a consistent set of flash zoom settings to make a fair assessment. If you cannot find consistent, comparable data, the comparison may be unreliable.
Conclusion
While guide numbers can seem complex, understanding the concept allows for precise flash control in manual mode. While TTL and manual adjustments are more commonly used today, a grasp of guide numbers provides valuable insight into the relationship between flash power, distance, aperture, and ISO. This understanding empowers photographers to make informed decisions and achieve optimal flash exposure in various shooting scenarios.
