How powerful is that flash?

Written by Gary on January 25th, 2009

Following from my last blog on using a bit of math to use your flash in manual mode, I thought it best to look at another confusing topic – comparing flash light output powers.

Electronic flash units:

For electronic flash units without light modifiers, the light output is relatively easily compared with each unit as their light output power is measured as a Guide Number (GN) as outlined in the last blog.

The GN is usually stated in either meters or feet at ISO 100 and for a given zoom setting of the flash and a given room (eg. light colored 10 foot high ceiling).

Most flash units built in digital SLRs have a GN of ~11-13 in m at ISO 100.

The more powerful flash units (strobes or speedlights) have a GN of ~45-54 in m at ISO 100 when used at their telephoto setting (light beam equates to ~100mm focal length lens).

The older Metz 45 series flashes have a GN of 45 in m at ISO 100 but at wide beam coverage equivalent to a 35mm focal length lens.

The most powerful portable battery operated flash units (eg Metz 76MZ-5 digital) have a GN 76 in m at ISO 100 and at their longest telephoto setting.

BUT how do you compare these with studio flash units when these are rated in watt-seconds??

Unfortunately, the short answer is you can’t!

Watt-seconds refers to the ELECTRICAL power the flash capacitor is capable of discharging on full output.

1 watt-second = 1 Joule

Each flash unit will have a different efficiency in converting this electrical energy into light output, and furthermore the actual amount of light hitting the subject depends also upon what flash modifier is used with the studio flash – reflector type vs soft box, etc.

To further complicate matters, many manufacturers specify a value called EFFECTIVE Ws which is usually much higher than the TRUE Ws value and aims to be more reliably comparative between models.

The best way to compare is to actually measure the light output of a given setup using an incident flash meter at a given distance from the subject and use this to determine the GN:

GN in meters = aperture reading given by light meter at ISO 100 x distance from light meter to flash in meters

A low end studio flash tends to be rated at 150 Ws and is generally adequate for portraiture, although a much more versatile studio flash would be a 300 Ws unit.

Professional photographers wanting to turn ambient daylight into night so they can isolate a fashion subject will need a super powerful flash which are rated at 1200Ws-4800Ws.

Bowens rates their Esprit Gemini monobloc strobes at the following GN in m at ISO100:

  • Gemini 125 = 125Ws = GN 41m
  • Gemini 250 = 250Ws = GN 60m
  • Gemini 500 = 500Ws = GN 80m
  • Gemini 750 = 750Ws = GN 104m
  • Gemini 1000 = 1000Ws = GN 120m
  • Gemini 1500 = 1500Ws = GN 160m
  • NB. the digital (DX) versions and those rated 1000Ws or higher are not compatible with their portable Travel-Pak battery

To have some idea of what these high end flash units for fashion photographers can do, check out the Profoto range such as their 2400Ws ring flash for which you need a very expensive generator to provide the power.

A further consideration is the power of the modeling lamp in studio units – 100W is barely enough to view the effect and most prefer 150-200W lamps.

As they say, amateur photographers tend to worry about resolution and sharpness while professional photographers tend to be much more concerned with quality and versatility of their lighting, because at the end of the day, a large part of what makes an image visually appealing is the quality of the light, not the sharpness of the lens or how many megapixels the camera has.

The beam candlepower-second (BCPS) measure:

Some units may be rated in BCPS and although this unit can’t be precisely converted to GN, an approximate conversion which can be used is:

GN in feet = square root (BCPS x ISO / 20)

Other GN conversions:

Obviously to convert GN in feet to GN in meters:

GN in meters = GN in feet / 3.3

Converting GN for a different ISO is a bit more difficult:

GN at new ISO = GN at old ISO x sqrt(new ISO / old ISO)

in other words GN at ISO 200 is 1.4x that at ISO 100,

and, GN at ISO 400 is 2x that at ISO 100.

For tech heads, more information on quantifying light output can be found at:


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