Originally posted by biz-engineer What's the f4 in the Sunny f16 rule table? If I understood correctly f16 is , yes , for sunny day, and it says, if cloudy shadow, open up the lens to f4.
Aha!
The sunny 16 rule is somewhat distinct from the tables (roughly based on EV
100 estimates for common lighting situations) that usually accompany articles on "the rule". Unfortunately, there is a dedicated group of users on this site and elsewhere that treat the tables as being the same as the rule and simply proclaim "sunny 16" whenever a noob has a sticky exposure question.
The rule:
"In full mid-day sun at middle latitudes, exposure should be 1/(value of ISO speed) at f/16."
The rule tends to fail above latitude 45°N, below 45°S, in the tropics, in mid-summer/mid-winter, and wherever "full" sun is seldom the norm. In my region, it is closer to "sunny 11" in mid-summer and "sunny 8" at the winter solstice. A person keeping the rule in mind may do a reasonable job of estimating daylight exposure, particularly when shooting to a medium with high dynamic range.
The tables, OTOH, are based upon long-standing conventions (over 100 years?) of EV* based on common subject luminance (full sun, open shade, full moon, etc.) for available emulsions. Most have been applied to ISO 100 to allow easy calculation of reasonable exposure for common lighting situations without relying on a meter. I used to carry a card in my bag with a fairly comprehensive list. They are only tangentially related to the "sunny 16" rule.
Rant over...
Steve
* Exposure Value (EV) is simply a number that represents equivalent combinations exposure time and f-number. When used as an expression of scene luminance, convention is to assign ISO 100 unless explicitly stated otherwise. A full step on the EV scale is equivalent to one "stop" of exposure.