Guide Numbers - Calculating, Charts and OverviewUpdated 24 days ago

This article covers essential information related to Guide Numbers (GN) in strobe and speedlight photography, including an explanation of GNs, factors influencing GN values, and how they apply to the Westcott FJ series. Use the links below to navigate specific topics. 

Topics on this page:

• What is a Guide Number? 
• What factors affect the measured and reported Guide Number? 
• Why don’t Guide Numbers Appear in the Westcott Gear specifications page? 
• How can you determine the GN for your personal setup? 
• What are some ways you can use Guide Numbers in your photography workflow? 

What is a Guide Number (GN)?

In Flash photography, Guide Numbers (GN) are a numerical amount (number) assigned to gear based on the maximum measured output of the unit.   

• GN=metered aperture x distance. 

Standard GN reports use 100 ISO, a shutter speed that falls within a normal sync speed and the maximum power level of the gear.  Guide Numbers may be reported in either meters or feet. Standard measurements will typically be recorded at 2 meters or 10 feet for reports.  

A calibrated flash meter reading of F8 at 2 meters would result in a Guide Number of 16 (8 x 2=16) using meters as a distance measure. The same strobe with the same modifications measured at 4 meters would return a flash meter reading of F4 (4 x 4=16) so the GN always remains the same. 

What factors affect the measured and reported GN?

Some photographers use Guide Number specifications to compare the output level of various gear when considering a purchase. In most cases Watt Seconds (Ws) may be a better indicator of the actual total light output of a strobe or speedlight. GN measurements are influenced not only by the Ws power of the source but also by: 

• Reflective Surfaces

  Westcott records a GN in a testing darkroom to prevent the influence of ambient (existing) light or reflected light on the recorded output reflecting from walls.
  
  

• Modifiers or Modifications

  Any modifiers or modifications used to shape the source will influence GN meter readings and therefore can make it difficult to provide a direct comparison between manufacturers or even various units in the same manufacturer’s ecosystem. One manufacturer uses a shiny, highly efficient, narrow beam reflector which requires an additional optional purchase for gathering and reporting guide numbers. 

• Speed light zoom range

  Speed Lights offer varying zooms with the typical GN reported being the number generated from the highest power level at the maximum zoom.  Speed lights that offer a 200mm zoom setting would therefore report a higher GN than one with a lower maximum zoom like 105mm even though the total Ws power output is similar. 

• Round head Vs Square Head

  Round head Speed lights, while delivering a more pleasing light pattern than a traditional rectangular speed light, will record lower GN results than a standard speedlight of similar Ws since much of the round head light emitting from the head may fall outside the area being recorded.  When used in a round softbox style modifier, the round head speedlight typically regains the edge while providing a more even light across the diffusion front of a round modifier.   

• Diffusion Domes

  The FJ400 has a glass front diffusion dome along with a forward positioned flash tube. Removing the dome will add some recorded output.  The bare bulb no dome and closer to the body flash tube of the FJ200 benefits in output from this arrangement but sacrifices some softening and spread of light. This light spread is noticed particularly when used with large modifiers.    

• Color Consistency

  Strobes across lighting brands that offer tight color consistency (commonly referred to as Pro Models) like the Westcott FJ units, will typically report a GN slightly below what strobes with identical Ws and modifications provide when those strobes ignore color consistency. 

 
 
Why aren’t Guide Numbers Included with Westcott Gear?

Westcott does not provide GN details in its specification’s listings. The wide variations possible for testing and reporting a Guide number are discussed in the previous topic.   

These differences alone are enough to discourage the use of Guide numbers for comparison in the specifications until and unless industry standards are updated to include specific testing guidelines related to how the number is arrived at by the manufacturer.   

Westcott’s first strobe (FJ400) and some future offerings will be delivered with the magnetic reflector 55-Degree Magnetic Reflector (FJ400, Bowens, Godox Mounts) which provides a wider beam than a standard reflector   70-Degree Wide Reflector with Honeycomb Grids (FJ400, Bowens, Godox Mo  or a narrow deep throw reflector.   

The Westcott team decided to include the magnetic wide reflector as it offers more versatility to photographers in the field than a standard reflector. Since most photographers likely expect Guide Numbers that are based on the gear that ships with the initial equipment  offering, reporting the measured Guide Number may have resulted in an inaccurate and misleading comparison of the gear with similar 400Ws units based on this singular specification.   

Using a deeper more focused reflector for gathering GN data as other manufacturers report, would have left Westcott customers confused as to why their results did not match the reported Guide number out-of-the-box.    

How can you determine the GN for your personal setup?

Consider the following before testing; 

• Use a quality light meter  
• Perform tests in a space with minimal to no ambient light sources 
• Avoid using a small space with light or reflective walls and ceilings as this can give you an inflated meter reading which will not likely be accurate. 
• We suggest metering at 2 meters from the light unit as anything closer can be influenced more heavily by the modifications used on the unit 
• Consider modifiers and other variables listed above “What factors affect the measured and reported GN?” section  

After taking all careful considerations and setting up an optimal environment, it's as easy as 1, 2, 3! 

1. Set the meter for ISO 100 and the shutter at 1/160 to 1/250 to reflect your camera’s maximum normal sync speed. 
2. Meter the flash. 
3. Use the F stop reading reported by the meter and multiply it times the distance (2m). This is your guide number in meters for your setup. Guide number=metered aperture x distance. 

Note: If your flash meter reports tenths such as “5.6.7” you would use the corresponding F number designation of F7.1 which is ~7 tenths above 5.6.  7.1 x 2 = GN of 14.2.   

When comparing different manufacturer’s units, using the same or nearly identical reflector or modifier on all units tested is critical.  Best practice for gathering Guide numbers for your own setups would be to run the test with the modifier(s) you intend to use for imaging.  Remember to examine both the output and light quality when assessing the gear.  

What are some ways you can use Guide Numbers in your photography workflow?

(Nerdy Math stuff) 

 
Once you establish a Guide Number for your setup you can use that number to easily calculate the maximum distance range of your light or the effect of your light on an image when changing distances.  

If your setup returns a Guide number of 64 at 2 meters (F32 x 2) at full power, it can translate to any of the following plus several other possibilities.   

• F32 at 2 meters (32 x 2 =64) 
• F16 at 4 meters (16 x 4 =64)    
• F8 at 8 meters (8 x 8 =64).     

Once you determine the F stop at any distance using full power, you can manipulate your F stop further by adjusting the ISO or power levels.  Each time you lower the Decimal power level of an FJ unit you make an equal adjustment to your F-stop.   

• F16 at level 9 (full power)= F11 at level 8 (½ power)=F8 at level 7(¼ power), etc.   

Similarly, each time you double your ISO you gain one F stop.   

• F8 at ISO 100 =F11 at ISO 200= F16 at ISO 400 = F22 
  provided your distance remains the same.   

A real-world example:

My flash has a Guide Number of 32 at the standard 100 ISO.  I need to photograph a large group and to maintain even light over all the subjects and compose them within a normal range lens, my light needs to be at least 4 meters from the group.   

At 4 meters I can set the power at full and know that I have enough light to photograph at F8 to maintain an adequate depth of field to capture front and back rows in focus. (4 x 8 =32) If my flash only had a guide number of 16, I would need to bump my ISO up to maintain my F8 reading.  16 divided by 4 would result in a f4. I would then need to add two stops to arrive at F8 by increasing the ISO by two stops (double it twice -100 to 200 to 400).