guide number

Since the light from the flash is more concentrated, this increases the guide number.

Comparing guide numbers isn't easy, especially when camera makers decide not to provide them.

Guide numbers were used to calculate what aperture to use at a given subject distance.

The guide number represents an exposure constant for a flash unit.

A higher guide number indicates a more powerful flash.

But guide numbers remain important numbers, for they tell us just how far away we can be and still have our subject fully exposed.

As far as we remember, this is the first time that the company has guided numbers higher in three years."

The distance in the guide number calculation is the distance from the flash to the subject.

Each monolight has a guide number specifying its range at certain film speeds.

Manufacturers typically advertise the guide number for their flashes at the narrowest setting.

Calculating realistic flash limits from manufacturer-supplied guide numbers can be difficult, as the example above suggests.

A primary concern is guide number.

Therefore it had a guide number selector.

Those of us ancient enough to remember what photography was like before auto flash have "guide numbers" engraved in our consciousnesses.

The strength of a flash device is often indicated in terms of a guide number designed to simplify exposure setting.

A guide number is practical; given in feet or meters, and for a particular film speed, it is used for calculating exposures.

Guide numbers can be given in feet or metres, and are usually given for ISO 100 sensitivity.

That leaves guide numbers, sometimes abbreviated GN.

Of course, if your flash pictures are consistently underexposed, you can always switch to a higher-speed film in effect "uprating" the guide number.

In practice, it's normal to know the guide number and the distance and need the aperture, so the equation is rearranged to give:

The flash's guide number is set on the lens, and the aperture is accordingly coupled to the lens's focus ring for correct exposure.

Doubling the guide number requires a quadrupling of the flash's power, as the area to be illuminated increases as the square of the distance.

This leaves Guide Number calculation as a very rough guide to exposure at best, and in some practical situations it can give grossly inaccurate results.

The guide number of 80 feet in the previous example corresponds to a guide number of approximately 24 metres.

The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor sensitivity and angle of view.