• Guide number comparisons can be particularly enlightening (refer to the Speedlites Comparison Table);
  
  • For 24-28mm and 80-105mm coverage, the 540EZ provides more light than the 480EG.
• Guide numbers relate linearly to the distance the flash can illuminate; but the quanity of light is proportional to the square of the guide number. Compare various speedlites at 35mm coverage:
  • GN=20 (200E) is 156% of the light compared to GN=16 (160E)
  • GN=25 (300EZ) is 244% of the light compared to GN=16
  • GN=28 (380EX) is 306% of the light compared to GN=16
  • GN=30 (430EZ) is 351% of the light compared to GN=16
  • GN=36 (540EZ) is 406% of the light compared to GN=16
  • GN=48 (480EG) is 900% of the light compared to GN=16
• Although a flash with a guide number of 48 has 9 times the light output as a flash with a guide number of 16, for a given f-stop it can illuminate a distance only 3 times further.
• You can use a flash with coverage wider than the lens you are using with no change to the guide number. Light falling outside the lens coverage is "wasted".
• For guide number calulations and flash power, flash-to-subject distance is important, camera to subject distance is not.
  • If you hold the flast-to-subject distance constant (with off-camera cords), and move the camera away from the subject (and hold the focal length constant), the subject will be smaller, but illuminated at the same intensity.

Guide Number (GN): A guide number is a reference number used to calculate flash exposure. The guide number (GN) relates film speed to flash output. These factors, plus flash-to-subject distance and effective lens aperture (f/stop), determine flash exposure.

The guide number for a given flash unit is different for each film speed. When film speed doubles, the guide number changes by a factor of 1.4X; when film speed is halved, the guide number changes by a factor of 0.7X. Some Canon Speedlites come with a guide number chart sticker for their housings as well as a chart in their instruction books.

Guide numbers differ according to whether the distance is to be measured in feet or in meters. Conversions between a guide number for feet and a guide number for meters are:

GN(ft) = GN(m) x 3.3
GN(m) = GN(ft) x 0.3

A guide number test is made by placing the flash unit a convenient distance from the subject and making a series of exposures at various f-stops. When the best exposure is selected from the processed film, the guide number can be calculated from the distance and the f/stop used:

GN = f/number x Flash-to-subject distance

Using color reversal (slide or transparency) film provides results that are the easiest to evaluate.

There are several common formulas for flash exposure that involve guide numbers. Here are two of the most useful ones:

f-number = GN / Flash-to-subject distance
Required flash-to-subject distance = GN / f-number