Camera exposure in bright direct sun is often seen to meter near EV 15 at ISO 100 (one of those EV 15 equivalent exposure choices is 1/125 second at f/16).
Sunny 16 is another general rule of thumb we should all know, an old guideline for determining outdoor daytime camera exposure without a light meter. I've mentioned it a few times, as an automatically assumed notion of proper exposure under bright sun. The idea is that since the bright sun is a constant, it always provides the same exposure. The Sunny 16 rule says that the exposure for frontal lighting in bright sun is f/16 at 1/ISO seconds (all Equivalent Exposures work too of course). So, Sunny 16 in bright direct sun with ISO 100 is 1/100 second at f/16, or 1/200 second at f/11, or 1/400 second at f/8, etc. (within about 1/3 stop anyway. EV 15 in bright sun is ISO 100, 1/125 at f/16, or 1/3 stop different than Sunny 16. But close.)
The f/16 is only important because it matches the 1/ISO shutter speed at ISO 100 in bright sun. But f/16 ISO 100 is equivalent to f/22 ISO 50, or f/11 ISO 200, etc. Use the combination of best use for you. One of the oldest rules used for news photographers for depth of field is "f/8 and be there". Landscape and macro photographers may tend towards equivalent exposures with f/22, and there are times for f/2.8 or f/4, but if the light is adequate, f/8 is really hard to fault.
Sunny 16 exposure charts also include approximations of lighting conditions which cause steps of one stop changes of exposure. You would open additional stops for degrees of cloudy, overcast or shade, which are judged by noticing the shadows. This will vary, because our eye cannot judge exactly, so we should of course learn to use our light meter, it is normally better, and that's what it's for. But we should also always notice the shadows, they give good information. The direct sun casts sharp shadows under people or things. We can notice those shadows. Sunny16 can help when we don't have the light meter.
Shadows in Sunny 16 are technically not precise, and which did work better for negative film (negative film had more latitude than digital, plus the processing guy corrected the prints too). Light meters do of course have an advantage, more important for slides and digital today. But if no light meter, Sunny 16 and the shadows can be a big help for estimating exposures in outdoor daylight.
Back in the day, every roll of film came with a data sheet that included a Sunny 16 chart (for the film speed). In the 1940's and 1950's, before cameras had built-in light meters, it was used. This seemed normal, but photography was harder work than today, with no light meter, and we couldn't see the result until later (when no longer at the scene). Bracketing helped difficult cases, but we also learned to think about what we were doing. And the B&W negative film had greater latitude which allowed it. Dark room work could fix it (as can shooting raw today). But my notion of the significance of Sunny 16 for digital today is this: If you have access to a light meter, go with it, learn to use it, that's what it's for. If no metering capability is available, Sunny 16 may not be precise, but it can get close to the ballpark. And when there is doubt about your metered value, knowing Sunny 16 can be a confirmation of reasonableness.
In bright sun, Sunny 16 implies any Equivalent Exposure of f/16 at shutter speed at 1/ISO seconds (i.e., ISO 100 uses f/16 at 1/100 second, or equivalents, like f/8 at 1/400 second). This assumes frontal lighting, from two hours after sunrise to two hours before sunset. Back lighting probably needs another stop or two of more exposure.
|Sunny 16 Rule||Shutter speed 1/ISO seconds, or Equivalent Exposure|
|F/stop||Lighting conditions||Shadow detail|
|f/22||Snow/sand/water||Dark with sharp edges||Much bright reflection adding to the direct light|
|f/16||Sunny||Very distinct shadows||Normal bright sun|
|f/11||Slight overcast||Soft around edges||Softer shadows - Or maybe 1/2 stop for Hazy|
|f/5.6||Heavy overcast||No shadows|
|f/4||Open shade/sunset||No shadows||Open shade is illuminated by clear sky overhead|
Sunny 16 is not affected by the subjects reflected colors. It is not metered, and instead approximates the daylight light intensity by judging the shadows. Of course not precise like an incident light meter, but the same concept of trying to judge the actual light level. Whereas a reflected meter only sees reflected light affected by subject colors, and makes exposure of all scenes average out to be middle gray (see metering), so white scenes will be gray (underexposure), and black scenes will be gray (overexposure). If using the reflected camera meter, white snow needs additional compensation to be white instead of gray (probably +1 or +2 EV compensation, perhaps more). An incident meter will get it right (it is aimed at the camera, away from subject, metering the actual light).
Note the unmetered Sunny 16 f/22 effect for bright sun on snow or beach goes the opposite direction from reflected meters, for the opposite effect of greater overall illumination total due to brighter reflections adding to the accumulated light - Sunny 16 is not about the reflected color. This distinction is saying:
In general, bright sun Sunny 16 is a good safety check to consider if your metered exposure is plausibly reasonable. Remembering that can help us to rethink abnormal metering results. However, lower light levels can get quite vague.
If bright sun EV 15 is 1/125 second at f/16, and Sunny 16 is 1/100 second at f/16... then, that's close, within 1/3 stop, but which is it? I loved Sunny 16 back in the day, but now my bet is on the metered EV. Of course, we do encounter situations agreeing either way. It does vary slightly, with the clarity of the sky that day. Here is some background:
But overexposure was a plus for negative film, it added detail in shadow areas. Until 1960, Kodak B&W film speed (called ASA then, American Standards Association, before ISO) was intentionally specified as half of what it actually was (an intentional safety factor against underexposure). The first selenium cell light meters were in the 1930s (not very sensitive to low light), but light meters in cameras did not happen until the transistor era, about 1960. Light meters in cameras were becoming popular about then, and in 1960, Kodak doubled the ASA speed number for their negative films (effectively reducing the speed safety factor). Kodak simply doubled the printed number, the film did not change (everyone already knew to double it). ISO (International Organization for Standards) adopted the same ASA numbers in 1974, no difference in the numbering.
But today, digital images are more demanding, due to less latitude, and digital clipping, etc, so clipping due to overexposure becomes a Real Bad Thing now. Back in the negative film days, the standard saying was "Expose for the Shadows", suggesting intentional ample exposure of shadows, and overexposure of highlights (which did not hurt negatives much). However, that applied only to negative film (which will be inverted). "Expose for the shadows" was a bad plan for slide film (and digital), because of the highlights. The rule for positive film was "Expose for the Highlights" (don't burn out the highlights), still applicable to our digital cameras.
But no one changed the Sunny 16 charts then, or the 1/film speed concept when we modified the film speed. :) But that popular Weston meter system shift could explain the slight 1/3 stop discrepancy between EV 15 and Sunny 16.
Still, Sunny 16 definitely is an important and honored fundamental to know, and it ought to be the first thought remembered when wondering if a metered exposure is reasonable or plausible.