Editing Sunny 16 rule (section)

==Using the rule==
The basic rule is, "On a sunny day set [[aperture]] to {{f/|16|link=yes}} and [[shutter speed]] to the [reciprocal of the] ISO [[film speed]] [or ISO setting] for a subject in direct sunlight."<ref name="suess">{{cite book | title = Mastering Black-and-White Photography | author = Bernhard J. Suess | publisher = Allworth Communications | year = 2003 | isbn = 1-58115-306-6 | url = https://books.google.com/books?id=7LaRPNINH_YC&dq=%22sunny+16+rule%22&pg=PT112 }}</ref> In simplest terms, bright sun = f:16 @ 1/[[Film speed|film-speed]]-number (aperture and shutter speed, respectively).

For example:

* On a sunny day at ISO 100 ("100 speed film"), the aperture is set to {{f/|16}} and the shutter speed (i.e. exposure time) to {{sfrac|1|100}} or {{sfrac|1|125}}<ref>{{cite book|last1=Shaw|first1=John|title=John Shaws's Nature Photography Field Guide|date=2000|publisher=Amphoto Bookds|isbn=0-8174-4059-3|page=16}}</ref> seconds (on some cameras {{sfrac|1|125}} second is the closest available setting to {{sfrac|1|100}} second).
* On a sunny day at ISO 200 and aperture at {{f/|16}}, set shutter speed to {{sfrac|1|200}} or {{sfrac|1|250}}.
* On a sunny day at ISO 400 and aperture at {{f/|16}}, set shutter speed to {{sfrac|1|400}} or {{sfrac|1|500}}.

Shutter speeds can be changed as long as the f-number is adjusted accordingly, e.g. {{sfrac|1|250}} second at {{f/|11}} gives equivalent exposure to {{sfrac|1|125}} second at {{f/|16}}. Exposure adjustments are done in a manner that retains the [[Exposure value|EV]]. As the aperture is opened (f:11, f:5.6, f:4, etc.) the shutter-speed/exposure-time is reduced by a factor of approximately one-half (1/250, 1/500, 1/1000, etc.) This follows the mathematical relationship between aperture and shutter speed where exposure is inversely proportional to the square of the aperture ratio and proportional to exposure time; thus, to maintain a constant level of exposure, a change in aperture by a factor ''c'' requires a change in exposure time by a factor {{sfrac|1|''c''<sup>2</sup>}} and vice versa. A change in the aperture of 1 stop always corresponds to a factor close to the [[square root of 2]], thus the above rule.