From about two hours after sunrise until two hours before sunset on a bright, clear, pollution free day it is far easier to photograph using an estimated exposure than to meter the light. In a bright sunny situation, the correct exposure anywhere in the world for a middle-toned, front-lit subject larger than a breadbox is expressed in the sunny f/16 rule.

A middle-toned subject is defined as being photographically average in tonality. It is neither light nor dark, but halfway in between. In terms of black and white, it is medium gray. According to the sunny f/16 rule, the correct exposure for this tonality is f/16 at the shutter speed with a number closest to the film's ISO number.

In other words, a sunny f/16 exposure is 1/ISO at f/16, or any equivalent exposure. Let's assume you are shooting Kodachrome 25 with an ISO of 25. The sunny f/16 rule says that the correct exposure is 1/30 second (30 is the number closest to ISO 25) with an aperture of f/16. Or it could be any equivalent. An exposure of 1/30 sec. at f/16 is exactly the same as 1/60 sec. at f/11, 1/125 sec. at f/8, 1/250 sec. at f/5.6, 1/500 sec. at f/4. The sunny f/16 rule for Fujichrome 50 starts at 1/60 sec. at f/16; for Ektachrome 200 at 1/125 sec. at f/16; for Ektachrome 400 at 1/500 sec. at f/16.

When you use Kodachrome 64 (K64) at ISO 64, you must modify the sunny f/16 rule just a little. We know that there is one and one third stops between the ISO film speeds of Kodachrome 25 and Kodachrome 64. If Kodachrome 25 is properly exposed at a sunny f/16 exposure of 1/30 sec. at f/16, then K64 should be one and one third stops faster. But the sunny f/16 rule for K64 suggests an exposure of 1/60 sec. at f/16, which is only one stop different. So with K64 when you're estimating the light and shooting non-metered sunny f/16 exposures, you must stop down one third stop more.

The way to calibrate a meter is based upon the one correct exposure you already know, the sunny f/16 exposure. Just go outside on a clear, sunny day and meter off something middle-toned with a normal or longer lens set at the infinity focusing mark. Adjust the ISO dial until you get the right sunny f/16 reading for whatever film you're using. Whatever the setting you come up with is now the number you will use when shooting that film in that camera body. All you have done is correct the camera's film speed dial. Now you can go out and photograph as usual.

When the day isn't clear and sunny, you then have to adjust the exposure. Bob Harris was one of the speakers at the NECCC conference at Amherst last year speaking on this very topic. His Harris Photoguide for Xisting Light ($11.95 from Harris Photoguides, 83 Rock Beach Rd., Rochester, NY 14617) shows that with hazy sun you must open up one stop from the sunny 16 exposure, two stops on cloudy bright days, three stops on heavy overcast days, etc.

 

The Sunny 16 Rule is also useful to those who have abandoned film and are using digital cameras. Let's face it, most of today's cameras just won't work if the meter is broken. However, not all meters are calibrated correctly. Using the Sunny 16 Rule, you can check your meter's accuracy and calibrate it if necessary.

As described above, set your camera's ISO at 100. Set a normal or telephoto lens at infinity focus (be sure you're in the manual focus mode). Make sure the sun is behind you, coming over your shoulder and hitting a neutral-toned object. I always carry a gray card - a cardboard rectangle colored 18% reflective gray, in my camera bag. This card represents mid-tone...the reading that your meter uses to determine exposure. (That's why, when you shoot in a snowy field, your images will be underexposed - dark. The meter thinks that the snow is a mid-tone and closes down the aperture accordingly.) 18% tone doesn't have to come from a gray card. There are substitutes you can use, such as a field of green grass or stone-washed denim.

So, once you've got an 18% tone subject, set your exposure system to Aperture Priority. Set the aperture to f/16, then point the camera at the 18% tone subject and press your shutter part way to engage the camera's metering system. If you're camera ISO is set at 100 and your camera's meter calls for 1/125 of a second, then you're all set. If the camera tells you the shutter speed should be 1/250 or 1/60, then you MIGHT have problems. Before shipping the camera back to the manufacturer, check to make sure it's a bright, sunny day; you have the ISO set at 100; the sun is behind you and shining directly on your subject; that your aperture is set at f/16; and that your subject is 18% tone.

For those of you who might have limited experience with adjusting aperture and shutter speed while keeping the same exposure rating, we thought this little table might be helpful.