Lesson plan
Guide number
When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", "speedlights", and "speedlites") is a measure photographers can use to calculate either the required f-stop for any given flash-to-subject distance, or the required distance for any given f-stop. To solve for either of these two variables, one merely divides a device's guide number by the other. Though guide numbers are influenced by a variety of variables, their values are presented as the product of only two factors as follows: This simple inverse relationship holds true because the brightness of a flash declines with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. Accordingly, as illustrated at right, a guide number can be factored to a just as readily as a . Guide numbers take into account the amount of luminous energy of the flash, the camera's ISO setting (film speed), flash coverage angle, and filters. Studio strobes in particular are often rated in watt⋅seconds, which is an absolute measure of illuminating power but is not particularly useful for calculating exposure settings.…
Article summary
When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", "speedlights", and "speedlites") is a measure photographers can use to calculate either the required f-stop for any given flash-to-subject distance, or the required distance for any given f-stop. To solve for either of these two variables, one merely divides a device's guide number by the other. Though guide numbers are influenced by a variety of variables, their values are presented as the product of only two factors as follows: This simple inverse relationship holds true because the brightness of a flash declines with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. Accordingly, as illustrated at right, a guide number can be factored to a just as readily as a . Guide numbers take into account the amount of luminous energy of the flash, the camera's ISO setting (film speed), flash coverage angle, and filters. Studio strobes in particular are often rated in watt⋅seconds, which is an absolute measure of illuminating power but is not particularly useful for calculating exposure settings.…
Key claims
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.