Does the Sunny 16 Rule Really Work?

I'm starting the new year going back to the basics of photography -- the "sunny 16 rule" to be specific. Does the sunny 16 rule still work across modern digital cameras?

About Sunny 16 If you don't know the sunny 16 rule, you're missing what might be the easiest and fastest way to get exact exposure on a sunlit subject without metering anything. The rule states that on a sunny day, you should be able to set the shutter speed to 1/ISO, choose f/16 and get a perfectly exposed negative. For example, you could set the camera to ISO 200, shutter speed 1/200 and aperture at f/16; or ISO 400, 1/400 and f/16 -- any lens, any camera. Photographer Tony Corbell recently sparked new interest in this rule during a CreativeLive workshop in late 2013 when he stated that you could calibrate a hand-held meter by using the sunny 16 rule and pointing the meter at the sun and seeing if it read f/16. If it were off, you could adjust it until it read 16.0. So, is it true? Does this old rule still work with new technology? Of course it does, but to satisfy the non-believers, I went out and tested it for myself so I could show you the results.

Testing Sunny 16 First I set my Nikon D300 to ISO 200, 1/200 and f/16 and took some pictures. There happened to be snow during my test, which would have been a challenging scene to meter through the camera, but my exposure was perfect. Using the law of reciprocity, you can alter the sunny 16 settings to get whatever exposure you want, so ISO 200, 1/200 at f/16 is the same as ISO 100, 1/100 at f/16. I then set my Sekonic L-758 meter to ISO 200, 1/200 and pointed the dome at the sun. Sure enough, the meter read f/16.1. When I tested it again, it bounced a bit between 16.1 and 16.0, which I chalked up to user error in pointing it exactly. So I concluded indeed the rule is true and my meter is calibrated correctly. As further evidence, I wanted to show you exactly how the images appeared in Lightroom so you can see the settings and histograms. Due to these being screen shots of the files, and differences in monitors, the color and exposure may not appear to be accurate, but look at the settings and trust the histogram. They're accurate. Also, I used auto white balance unintentionally because I forgot to change it to daylight, which I would normally use in full sun. Nevertheless, the data shows I used the sunny 16 rule and the histograms (the graphs in the upper right) show that the negatives are exposed properly and contain all of the detail in the scene, without any exposure adjustments in software. In the last image you'll notice some of the shadows are showing clipped in the histogram. That tells me the scene might have been outside of the dynamic range of the camera. You'll also note that the histogram trails almost to the right edge, meaning there's still plenty of detail in the highlights. I could have bumped the exposure up a third in camera to push the histogram to the right, and I often practice shooting to the right, but the sunny 16 rule still works, and if I were to bump exposure as a rule, I would have overexposed the other scenes by not trusting sunny 16, especially dangerous if you shoot JPEG.

Sunny 16 test results For your enjoyment, here are the files from camera with just basic color and lens corrections, no exposure adjustments. This rule is incredibly handy and it's not a matter of guessing. It works because the sun is the same brightness most of the day, and the sun is almost exactly the same distance from the Earth most of the time. If brightness and distance don't change, neither will exposure, and the light falloff between the sun and Earth is so great that any changes in distance due to orbit don't affect sunny 16. No matter where you go or where you point the camera, as long as the subject is lit in full sun you can use this rule. It's like having an incident meter in your head! Just remember that filters will cut light so you'll need to compensate. When the sun is 20 degrees above the horizon and full, not clouded or overcast, it will work, and any discrepancy is the result of equipment or human error. With knowledge of the exposure triangle and law of reciprocity, you can manipulate the rule to suit your exact shutter speed and aperture needs. For example, when I'm making portraits of a bride and groom outdoors on a sunny day, I'll know that 1/1600 and f/4 at ISO 100 will give me the same exposure as 1/100 @ f/16. It's simply opening up the aperture four stops and speeding up the shutter four stops, so the exposure doesn't change. What changes is my depth of field, which is my primary storytelling tool. I generally use f/4 or wider when photographing a wedding. Let's say I don't want f/4 and I want 5.6 instead. That's one stop. So if I close the aperture one stop, I must slow the shutter one stop to 1/800 so the light recorded is the same, so the correct exposure becomes ISO 100, 1/800 at f/5.6. It doesn't matter if it's a wedding, vacation, sports game or family portrait; sunny 16 gives you the correct exposure for a sunny day. Why fiddle around with matrix guessing, reflective metering and exposure compensation when you can just dial in the exact correct exposure instantly and make every shot under the same light consistent? You can, technically, rely on the rule with matrix metering, but using the camera's meter with the sunny 16 rule defeats the purpose and adds extra steps to making a picture. Under a given light source, the correct exposure is constant and doesn't change. The trick is knowing the correct exposure for that light. Once you do, you can lock it in and there's no need to re-meter. The sunny 16 rule gives you that exposure for daylight. Matrix metering is meant for situations in which you don't know the exposure and need a quick way to find it. If, for example, the camera's meter tells you the correct exposure with an aperture of f/16 @ ISO 200 is 1/800, you know to open up two stops with exposure compensation, but that's a calculation you'd have to make and an unnecessary step since the rule tells you that 1/200 is the correct shutter speed to start with.

Sunny 16 and auto exposure While you can use the sunny 16 rule in automated exposure modes if you understand exposure compensation -- which defeats the purpose -- you cannot verify the sunny 16 rule with the camera's meter alone. Some people have tried to test sunny 16 by using matrix metering and a programmed exposure mode. They go out to a sunny scene, expecting the camera to come up with settings that satisfied the rule. That won't work because the camera is metering luminance, or the light reflected off of objects and ignores illuminance, or the light falling on objects -- two different things. The light that reflects off of objects is different depending on the object, so it causes the camera's meter to constantly change. Sunny 16 is an exposure for the light that falls on objects and anything under that light will render with appropriate tonality with the correct exposure. If you want to verify the sunny 16 rule with a meter, you must use an incident meter, not a reflective meter. There's one way to verify it with your camera, other than make an exposure at sunny 16 settings. It involves a gray card. Set the ISO and then pick a shutter speed of 1/ISO; fill the frame with a gray card illuminated in full sun, and see if the camera reads f/16. If it doesn't, it's possible the camera meter or gray card is slightly off, or the card is lit improperly. Of course sunny 16 isn't for everything and there are creative reasons for breaking away from it. If your subject is in shade, obviously the rule will render the subject darker than daylight, but that doesn't work creatively if you want the subject to look "normal." In that case you would use an incident meter in the shade or meter off of the face to expose the subject to appear normal. The rule is for outdoors on a sunny day only. Photography rules do work! Feel free to use this rule to calibrate your incident meter. Whatever the meter is reading, dial in the opposite until it reads f/16 exactly. Photography is most fun when you go out and do it. In fact, what I've been doing is outlining a guide for how you can try this yourself. Also, using the sunny 16 rule will help you understand light, the exposure triangle and manual exposure. Go out and experiment with sunny 16, and have fun.