The Sunny 16 Rule (Google) dates from the days before light meters, when we used B&W negative film. Negative B&W film thrived with mild overexposure, and was easy corrected by adjusting printing exposure in the dark room. But exposure is more critical for digital or for positive film slides (and which today are often not printed). However, for any case dimmer than direct brightest sun, Sunny 16 involves human guesses about shadow intensity, which can be surprisingly close with practice, but usually is not as accurate as actually metering the light.
Cameras today all have light meters and probably exposure automation for point-and-shoot. But still, if your use actually is concerned with selecting proper camera settings for the difficult situations (still true for many of us), then Sunny 16 is a general rule of thumb you should know, a very old guideline for determining reasonable outdoor daytime camera exposure without a light meter. If you reach an exposure in far disagreement with Sunny 16, you might be correct, but maybe give it more thought. I've mentioned Sunny 16 a few times elsewhere, used as an automatically assumed notion of reasonably proper exposure under bright sun. The idea is that since the bright sun is a constant, it always provides the same exposure (but atmospheric conditions like haze will vary it). 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). 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. Metered EV 15 in bright sun is ISO 100, 1/125 at f/16, or 1/3 stop less than Sunny 16. But in fact, humidity might meter to read 1/3 EV less than EV 15 even in bright sun in clear sky, but it’s close.
The f/16 in the rule is only important because then it matches the 1/ISO used as shutter speed in bright sun. But if the same shutter speed, f/16 ISO 100 is also equivalent to f/22 ISO 50, or f/11 ISO 200, etc. Use the equivalent combination of best use for you. Landscape and macro photographers may tend towards stopped down equivalent exposures with f/22 for maximum depth of field. And there are times for f/1.4 or f/2.8, but if the light is adequate, f/5.6 or f/8 are really hard to fault in DSLR type cameras. Automation in cellphones with tiny camera sensors will often use f/1.4 or thereabouts and will still have impressive depth of field.
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 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 if 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, and saw advantage in overexposure of shadows, plus the film processing guy corrected the prints too). Light meters do have an advantage, 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 similar exposure chart, used before cameras had built-in light meters. This seemed normal, but photography was harder work than today, with no light meter, and no automation, and we couldn't see the result until much later (when no longer at the scene). Bracketing helped difficult cases, but we also learned to think about what we were doing. Actually consciously looking at the scene to judge what it needs is a skill that photographers should learn. And the B&W negative film had greater latitude which allowed more leeway. 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 for many outdoor scenes. And when there is doubt about your metered value, knowing Sunny 16 can be a confirmation of reasonableness.
Camera exposure in direct bright direct sun in clear sky (with low haze or humidity) is often seen to meter near EV 15 at ISO 100. One of the EV 15 equivalent exposure choices is 1/125 second at f/16. And Sunny 16 uses shutter speed of 1/ISO, which if ISO 100 second at f/16, is 1/3 EV more exposure than EV 15.
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. Side or back lighting will need another stop or two of more exposure.
Confession: I (like many others) have a bad habit of absentmindedly mentioning ISO 100 when relating to Sunny 16 or to the EV Chart. But neither system is in any way based on ISO 100, which is a commonly heard mistake. Sunny 16 does not say ISO 100. It specifies a shutter speed of 1/ISO, which is about any ISO. Similarly, the standard EV Chart applies to whatever ISO you might be using, any ISO, even ISO 100. But ISO 100 is in no way special. It might be commonly popular, but it is just one of many numbers.
|Sunny 16 Rule||Shutter speed 1/ISO seconds, or Equivalent Exposures|
|F/stop||Lighting conditions||Detail of Shadows|
|f/22||Water/sand/snow||Added Reflections||Much bright reflection adding to the direct light|
|f/16||Sunny||Distinct dark shadows||Normal bright sun in clear sky|
|f/11||Slight overcast||Soft around edges||Softer shadows - Or maybe only 1/2 stop if just Hazy|
|f/8||Overcast||Barely visible||Vague trace of shadows|
|f/5.6||Heavy overcast||No visible shadows|
|f/4||Open shade/sunset||No shadows||Open shade is illuminated by clear sky overhead|
Sunny 16 is an incident method, describing the Suns light, and is not affected by the subjects reflected colors. The method was a big deal in the days before light meters were common. It is not metered, and instead approximates the daylight light intensity by judging the shadows. It’s not precise like an incident light meter, but is 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). So 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 direct light instead of what is reflected from colored surfaces).
If NOT using ISO 100 or 1/100 second shutter, then use Equivalent Exposures to compensate. For example, if using ISO 400 (2 EV faster than ISO 100), or if using f/8 (2 EV faster than f/16), then adjust to 2 EV lower ISO, OR to 2 EV faster shutter speed.
Note the f/22 Sand, Snow, Water is a different concept here. The Sun is no brighter then, but these are highly reflective beach scenes, or snow fields. The f/22 idea is more about the sum of added reflections from water, sand or snow, brighter and increasing exposure, but this is affected by the direction of sun and shadows. This second source onto the face may make the Sun's shadows on the face be actually somewhat diffused, as you'd expect. Some sources may describe it's shadows as "Dark with sharp edges", but I'm not sure they get it. It is the same sun, but with possible added accumulated intensity from reflections.
Sunny 16 is NOT point & shoot. Instead, we need to actually think a second about what we see first (which is a wonderful skill for any photographer to learn). Observing the shadows is great for brighter levels, however lower light levels in shade can get quite vague, and it's of no use indoors or at night.
In general, 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.
For bright sun and ISO 100, Sunny 16 is 1/100 second at f/16 and EV 15 is 1/125 second at f/16. That's close, within 1/3 stop, but which is it? But in fact, it might meter either way.
Sunny 16 was all that we had at first, originating before light meters were common, and also before camera settings even offered third or half stops. Technically, 1/128 second is the full stop (the actual even power of 2), and 1/100 second was a third stop. But the older cameras did not do third stops, so we had Sunny 16 calling shutter speed to be 1/ISO. ISO in those days (actually called ASA then but same numbers) was typically ASA 80 or 125 and multiples instead of 100 like digital today, and print processing could easily tweak exposure of negatives. Later in the 1960's when cameras began building automatic light meters into cameras, the EV nomenclature was invented, and ISO was recomputed so that ISO 100 will in fact be called a full stop (instead of ISO 128). This surely sounds mixed up and confused, but there have been changes. And note, Sunny 16 does not say ISO 100, it specifies shutter speed of 1/ISO. So now 1/100 f/16 ISO 100 is EV 14,67, and EV 15 for bright direct sun is 1/125 second f/16 ISO 100 (the math of this, near page bottom). Either one might be encountered in apparent bright sunlight. I loved Sunny 16 back in the day of negative film, and exposure was generally easily correctable during darkroom printing, but today my bet is on the metered EV. Both will vary a bit.
The concept is that the Sun gives a constant exposure, however the sky clarity does vary. Weather, haze, humidity, pollution, hour of day, season of year, elevation, latitude, old uncoated lenses, side or rear lighting, and clouds, all can affect it somewhat. Depending on haze and humidity, bright sun at ISO 100 does not always meter as much as EV 15, nor does Sunny 16 always reach its peak. Due to the angle though the atmosphere, the sun's light does vary slightly with latitude on the earth, and with the seasons, brighter at equator or in summer. Just slightly, but Kodak in Rochester NY was near 45 degrees. And sunlight varies at early and late time of daylight, and at altitude (mountains). An even larger factor is that side and rear lighting needs increased exposure. Sunny 16 applies to frontal lighting, from mid morning to mid afternoon (at least two hours from sunrise or sunset).
A third of a stop should not be a surprise when guessing, which was a very minor problem in negative film days, which had considerable latitude, and the print exposure required adjustment in the darkroom anyway (when printing). If shooting B&W negative film, substantially more exposure may be desirable anyway to enhance shadow detail. The old rule of "Expose for the Shadows" was advice for negative film, but that DOES NOT apply to color slides or digital, when preventing overexposure of highlights becomes more important.
Here is some background:
EV 15 is speaking of an incident meter reading, directly metering the actual light incident on the subject. Whereas a reflected meter (camera meter) reading varies with the reflectivity of the subject colors, when observing the subject is another skill we do need to master.
Sunny 16 is just ballpark. We cannot judge the shadows precisely to within 1/3 stop, but with practice, we can get better at it. And we ought to always notice the shadows, they are a strong clue of the lighting.
Sunny 16 is just a general rule of thumb, applicable to a few standard situations outdoors, an approximation used in the early days of negative film. Sunny 16 was the only guide we had before light meters were common, and it worked pretty well for negative film. Because negative film had wide latitude, was easily corrected in the dark room, and the photo finishers in fact helped us a lot, correcting exposure, and later correcting white balance and color too. We could use either the clear or the blue flash bulbs indoors, and then they fixed it for us, and we might never realize there was any issue. Sunny 16 is harder for digital cameras now (no dark room), so we also have to do our own digital corrections now too (no one else is going to fix it now).
But a little overexposure was a plus for negative film, it added detail in shadow areas. Until 1960, Kodak B&W film speed (which used ASA since 1946, American Standards Association, before ISO) was intentionally specified as half of what the speed 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. This was Not a change, the film did not change. Kodak simply doubled the printed number, eliminating the previous speed safety factor. Again, film negatives enjoyed a little overexposure, but when cameras started actually metering, this became noticeable. This was a Big Event, but everyone already knew to double the boxes film speed. This double rating in 1960 affected B&W European DIN film speed ratings too, which is a logarithmic scale, adding 3 doubles the speed, starting at ASA 1 = DIN 1. 10 × log10(2) = approx 3, so ASA 100 is DIN 21, and ASA 200 is DIN 24.
ISO (International Organization for Standards) adopted the same ASA numbers in 1974, no difference in the numbering (format often written as ISO 100/21).
Bright sun usually meters (up to) EV 15 (at ISO 100). Specifically, EV 15 is 1/125 second f/16, 1/3 stop faster exposure (less exposure) than Sunny 16 if ISO 100. Some instructions said to always increase Sunny 16 shutter speed to the next Full stop (because cameras only had full stops back then), which is 1/125 second (Both 1/125 second and EV 15 are full stops). Film charts often said 1/125 second at f/16 (or equivalent 1/250 at f/11). Because again, full stops were normally the only choice on older cameras. More precision was mechanically difficult, and precision really wasn't critical for B&W negative film (the result was handled in the darkroom).
I doubt we know the first use of the term Sunny 16, but from the beginning, there were always similar daylight exposure charts (with values for bright, cloudy, overcast) since Daguerreotype days in 1840 (which was about six minutes in bright sun initially, but later was less than one minute). Emulsion speed improved fairly rapidly, but a very special case was that Muybridge developed high speed emulsions and shutters for his 1878 galloping horse showing all four feet came off the ground (said to be a 1/2000 second shutter, with flashes along the background so the horse was in silhouette. It had never been done before.) Kodak's first 1888 box camera was 1/25 second f/9, which for bright sunshine would be ISO 6.
Actual literal Sunny 16 could come only after we had film speed ratings. However, there were several different early film speed rating systems, and therefore, early light meters in the 1930s specified their own film speeds for various films (before standards). The first Weston light meter was in 1932. Hollywood and Ansel Adams had them, but it was the Depression, and not so many others had one yet. General Electric also made light meters starting 1935. Both meter brands independently created empirical film speed ratings for use with their meters. Here's a 1942 Kodak data sheet that gives Kodak, Weston, and GE film speeds (before ASA, 1943), and the ever-present exposure chart. The GE rating approximates the later ASA rating. Even later, ASA speeds of most Kodak B&W films were doubled in 1960, which was not a change in the film, but just that a safety factor was removed (meters were getting more popular). General Electric meters switched to use ASA when the standard was defined in 1946. Weston meters did not switch to ASA until the mid-1950s, after which ASA 125 corresponded to older Weston 100. This difference is 1/3 stop. The EV system did not appear until the late 1950s. ISO film speed specification adopted the ASA numbers in 1974 (called ISO since then, with same speed numbers).
Every box of consumer film included a data sheet with a similar exposure chart, which was used outdoors without a light meter. The charts were not named Sunny 16, but it was the about same thing, at least the same concept, including watch the shadows, and open a stop or two for cloud cover. However even after ASA ratings, they were not all exactly 1/ASA seconds at f/16. Back in the day (through the 1960s), our cameras only had full stops. Mostly these charts were close to Sunny 16. Kodak data sheets always specified only full stops. Kodak ASA 64 sheets said bright sun was 1/125 at f/11. ASA 125 and ASA 160 mostly said 1/250 at f/11. ASA 400 said 1/500 at f/16. These were all full stop shutter speeds, and back then, full stops was all our cameras could do. Half stops could be attempted by setting the aperture between stops.
Times were different back then, and here are sample modern data sheets for those who have never used film, at least not without today's automation. Kodak Plus-X Pan speed was ISO 125, which matched a full stop shutter speed, so recommendations were Sunny 16. But Kodak Tri-X Pan ISO 400 speed was not a full stop, and it specified 1/3 stop faster shutter speed than Sunny 16 (1/3 stop less exposure, like metered EV). That seems no doubt because cameras only had full stop settings then. But film speed was not especially critical for the wide latitude of negative film. In fact, a little intentional overexposure was considered a plus for B&W negative film shadows. But digital needs to be closer today.
So Sunny 16 (and judging the shadows) is a little vague, just a ballpark rule of thumb, requiring a bit of thinking about the light. Lighting conditions are always vague unless we meter it. I was a big fan of Sunny 16, but times change, and I use the light meters today. Sunny 16 indeed was a good general rule for a simpler day (before light meters, no alternatives then). We used B&W negative film back then, which had much more exposure latitude, which made a big difference. And for exposure, the darkroom work offered the same attention and benefit as the tweaking of Raw image exposure today. Sunny 16 is still the same that it always was, but for digital work today, it is less precise than actually metering the light.
Still, Sunny 16 definitely is an important and honored fundamental to know. The Sun is a constant (but weather is not). Actually seeing and recognizing the shadows is quite important to photography. When taking a picture of someone outdoors, do you notice what kind of shadow they are making on the ground? That shadow might advise you about moving into shade first. And Sunny 16 ought to be the first thought remembered when wondering if a metered exposure is reasonable or plausible.
Reflective meters certainly do have their own problems, see how light meters work. Sunny 16 does not meter anything but the constant Sun idea is more like incident meters, or like reflective metering on a gray card, which are attempting to judge the light directly, instead of the variable reflection from the subjects colors.