# foot candles

**URL:** <https://boards.straightdope.com/t/foot-candles/291522>\
**Category:** Factual Questions\
**Created:** [February 23, 2005, 2:12pm UTC](https://boards.straightdope.com/t/foot-candles/291522 "2005-02-23T14:12:53Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![drhess](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@drhess](https://boards.straightdope.com/u/drhess)\
**Post date:** [February 23, 2005, 2:12pm UTC](https://boards.straightdope.com/t/foot-candles/291522/1 "2005-02-23T14:12:53Z")

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What does knowing the footcandle number for a light tell me in terms of photographing a person? Assuming the room is pitch black, is there a certain number of fc needed for shooting at a certain ISO and aperture?

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**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\
**Post date:** [February 23, 2005, 3:36pm UTC](https://boards.straightdope.com/t/foot-candles/291522/2 "2005-02-23T15:36:38Z")

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It tells you the intensity of light in number of candles at once foot distance. It can be used to determine exposure but I’m far too lazy to look up any conversion factor since all my cameras have light meters plus I have an excellent external meter.

Manual flash often used a guide number which is easy to use. Since shutter speed usually doesn’t matter the guide number for a specified ISO is dvided by distance from flash to subject to give shooting aperture. You could very easily take a test shot have a sort of guide number for a given shutter speed.