Tuesday, January 17, 2012

Tech Talk: Candles and Calculations

Greetings, volks. Welcome to Hohenfels Volks, THE place for our place!

Today we’re going to go a little bit into what makes up your exposure numbers and how metering works. It sounds familiar, but we’ll be dealing with more math and other advanced parts of your exposure settings.

Let’s start with your ISO. Back in the old days there were lots of ways to label film sensitivity. There was by DIN number, ASA, Weston, and EV, as well as various others fallen by the wayside. Today we use ISO, which is for the most part ASA. Some folks use DIN, that’s why film still carries the DIN number. Under today’s scheme, 3 degrees DIN are equal to 1 full stop. ISO 100/21 has half the sensitivity of ISO 200/24. The Weston number, another popular standard used to gauge a film’s sensitivity, referencing Weston’s meters, is usually .8 times the ISO. Since we’re shooting today, we’ll use ISO, but there may be times we reference others in our future.

Moving on we come to light and light levels. Without the fancy gadgets we have to meter our exposure built into cameras, folks used external meters. Ansel Adams broke it down in his book The Negative. Meters used to measure light in candles, or candela, per square foot. That was the basis for your shutter speed. This will become important in a couple paragraphs.

Next we have aperture, the size of the opening through which light enters the camera. Each ISO had its native f/stop. To find the native stop for any ISO take its square root. For ISO 100 the native f/stop is f/10, for ISO 125 it’s 11, for 200 it’s about 14, and for 400 the native f/stop is 20. Apertures are measured in f/stop, which is a ratio based on the square root of 2, as each step is doubling or halving the light.

The last part is shutter speed. For a native aperture, the shutter speed should be 1/ the light measured in candles per square foot, or c/ft². If you read 125 c/ft², your shutter speed for a native aperture should be 1/125 second. It sounds easy doesn’t it? Well, don’t give up hope. Things are about to get tricky.

Since most light meters today don’t measure in c/ft², finding that speed may be difficult. Automation has taken away many of the tools used by those who came before us in every arena. You can buy a cheap ambient light meter and use it for your photography. The readings are usually in LUX. LUX can be converted, however, by dividing by 10.764. Just round to 10, and there you have it. 1000 LUX is 100 c/ft². At your native aperture, shoot 1/100 for zone 5 placement of what you just read and you’re cooking with gas. Using you digital camera’s metering, set your aperture to the ISO’s native aperture in AV mode. Then seeing the required shutter speed tells you there are that many c/ft² or 10 times that much in LUX.