Wednesday, August 25, 2010

Who Says Sunny 16 Can't Lie

The title is a spoof on this 1959 Popular Science article Who Says a Light Meter Can't Lie I doubt if this was the first article on the subject it certainly isn't the last. Personally I use a good light meter mostly in incident mode. If it lies it's because of operator error usually because I didn't make up my mind before I took the shot on what the subject should be.

One of the proposed solutions is not to use a light meter. If you shoot film you have most likely heard of Sunny 16 Fred Parker's The Ultimate Exposure Computer is the best reference I can find on the application. Sunny 16 seems to be a good method in normal outdoor nature type photography that is if you have some experience with the light in the location. Normally scattered light makes up only 10% of the incident light but this can change because of reflected surfaces and atmospheric conditions. How hazy, diffused or how soft can be a difficult measurement to make with only the human eye. As it was not a survival requirement us humans are not good at making absolute light intensity measurements, you need experience and references to adequately judge light in many conditions.

Most of my shooting is done in a city environment where tall building often shade out the sun, where  scattering and reflections make up a higher proportion of the light then in the open air. Even here a friend of mine has learned to judge the light mostly correctly but sometimes a couple of stops off. He uses years of experience not sunny 16. In the golden hour shooting into the sun and away from the sun is one of the best times to get dramatic city scenes. But sometimes he is fooled and is several stops wrong.

When the sun is at 10 degrees to the horizon on a early fall afternoon in Toronto because of the extra amount of atmosphere or air mass (AM) it has to pass through there is about 1 stop less light then at noon. The science is well developed and you can go here to see the math and use their calculator. The factor depends mainly on global position, time and date.

Shooting into the sun you have to contend with a super bright background (the sun and sky) and reflections of surfaces like pavement, choices like highlight or silhouette. How much reflected light is getting back on the subject and so on. Shooting away from the sun both the reflected light and the angle of the subject to the sunlight can make up to 4 stops of difference. The amount of light falling on the subject varies around 3 stops depending on the angle of the sun to the subject. At  10 degrees to the horizon there is about 2 stops lower illumination between the horizontal surface for example a road and a vertical standing structure such as a person. Once again this interactive graphically illustrated calculator comes to the rescue to save the reader from having to view my hen scratching.

Tip: I learned to use a incident/reflective light meter by carrying the meter around with my digital camera. After about a month I could set my digital camera using the meter and get better exposure first time then using the digital: shoot, chimp the histogram and repeat method. At that point I knew I could rely on on the meter for all my film camera. Even with digital I still prefer the light meter in studio and critical lighting conditions.

Don't forget that film is usually more sensitive to under exposurer while clipping because of overexposure is the bane of digital cameras.