The guide number (GN) of a flash is a crucial specification that helps photographers understand the flash’s power and range. Understanding guide numbers allows for more precise control over lighting in various shooting conditions.
Understanding Guide Number Basics
The guide number essentially represents the flash’s power. It is used to calculate either the distance a flash can reach or the aperture needed for proper exposure at a given distance. The higher the guide number, the more powerful the flash.
The maximum reach of the flash can be calculated using the formula:
Distance = Guide Number / Aperture (f-stop)
For example, if a flash has a guide number of 44 meters and you’re shooting at an aperture of f/2.0, the maximum reach would be 22 meters.
Alternative Text: A visual representation of the guide number formula, showing how distance, guide number, and aperture are related.
Factors Affecting Guide Number
It’s important to note that guide numbers are not standardized across the industry, and several factors can influence their actual performance:
- ISO Setting: Guide numbers are typically rated at ISO 100. Increasing the ISO will effectively increase the flash’s reach, while decreasing the ISO will reduce it.
- Focal Length: Some flashes specify guide numbers at a particular focal length (e.g., 50mm, 105mm). A flash might have different guide numbers at different focal lengths, especially if it has a zoom function. For example, a flash might have a GN of 44 meters at 70mm but a GN of 52 meters at 105mm.
Here’s how a flash might perform at various focal lengths:
- GN 52m with 105mm focal length
- GN 44m with 75mm lens
- GN 37m with 50mm lens
- GN 32m with 35mm lens
Guide Number and Flash Power Settings
Most flashes allow you to adjust the power output. Reducing the flash power affects its reach. A general rule of thumb is that halving the power reduces the distance by a factor of approximately 1.4 (the square root of 2).
Here’s an example using a flash with GN 44m:
- Full power: GN 44m, reach 22m at f/2
- 1/2 power: GN 32m, reach 16m at f/2
- 1/4 power: GN 22m, reach 11m at f/2
- 1/8 power: GN 16m, reach 8m at f/2
- 1/16 power: GN 11m, reach 5.6m at f/2
- 1/32 power: GN 7.8m, reach 4m at f/2
- 1/64 power: GN 5.6m, reach 2.8m at f/2
- 1/128 power: GN 4m, reach 2m at f/2
- 1/256 power: GN 2.8m, reach 1.4m at f/2
Alternative Text: Chart showing the relationship between flash power, guide number and distance.
Balancing Flash with Ambient Light
To avoid an overexposed subject and a dark background, photographers often use techniques like slow sync. This involves adjusting the shutter speed to properly expose the background, allowing the flash to fill in the shadows on the subject. The balance between ambient light and flash light can be controlled by adjusting the ISO, aperture, and shutter speed.
For example, using a flash with GN 44m at 1/16 power and f/2:
- Subjects closer than 5.5m will be overexposed.
- Subjects near 5.5m will be properly exposed by the flash.
- Subjects beyond 5.5m will not be lit enough by the flash.
- Backgrounds beyond 10m will mostly be lit by ambient light, controlled by shutter speed.
Flash Sync Speed
Flash sync speed is the fastest shutter speed at which the entire sensor is exposed when the flash fires. Exceeding this speed can result in a portion of the image being underexposed or blacked out.
Typical APS-C cameras have a flash sync speed around 1/160 second to 1/250 second. This limitation exists because at faster shutter speeds, the camera uses a slit between the first and second curtain rather than fully exposing the sensor.
Alternative Text: Diagram showing how the shutter curtain works at high speeds, explaining the flash sync speed limitation.
In one millisecond (1/1000 sec) increments:
- 0ms – all closed
- 1ms – 1st curtain is 1/3 open
- 2ms – 1st curtain is 2/3 open
- 3ms – 1st curtain is fully open
- 3.01ms – flash fires
- 4ms – flash is either finished or finishing at a much reduced brightness
- 5ms – flash is either finished or finishing as shutter remains open
- 6ms – 2nd curtain starts closing
- 7ms – 2nd curtain is 1/3 closed
- 8ms – 2nd curtain is 2/3 closed
- 9ms – 2nd curtain closed exposure finished
The 1/160 sec (6.25ms) exposure took almost 1/100th sec (10ms) to fully occur. The 2nd curtain followed the first curtain by a delay of 1/160th sec (6.25ms).
Choosing the Right Guide Number
Selecting an appropriate guide number depends largely on the types of photography you do, your typical shooting environments, and your preferred style. For indoor portraits or close-up work, a lower guide number might suffice. For larger spaces or outdoor fill flash, a higher guide number is generally more desirable.
Conclusion
Understanding the guide number of a flash and how it interacts with other camera settings is crucial for achieving well-exposed and balanced images. By considering factors like ISO, focal length, power settings, and flash sync speed, photographers can leverage their flash equipment more effectively and creatively.
