Concepts and terms are often used without really understanding their meaning, and photography is no different. My knowledge and understanding of photography have come through experience and practice, not necessarily by studying or research, though that has increased since I began writing these posts three years ago. One term I have used a lot in the past two posts is one that, if asked to explain it before a couple of weeks ago, I probably couldn't.
A "stop" is a standard measurement of exposure and refers to the doubling or halving of light. The camera's shutter speed and aperture diameter are both designed on this principle, as is film speed (ISO), which was discussed last week.
Shutter speed is the amount of time, measured in seconds, that the the shutter remains open during a shot. Doubling or halving the shutter speed results in an increase or decrease of 1 stop of exposure. For example, changing the speed from 1/125 to 1/60 lets in twice as much light, increasing the exposure by one stop. Going in the other direction from 1/30 to 1/60 cuts the amount of light by half, decreasing the exposureby one stop.
Aperture, or f-stop, is a ratio that describes the diameter of the shutter when it opens. The smaller the f-stop, the wider it opens, letting in more light (the phrase "wide open" usually refers to the lowest f-stop available, usually f/1.4 or f/1.2). Understanding the f-stop scale involves a little math and is based on the powers of the square root of 2, which is 1.41, and a little rounding. For example, going from f/1.4 to f/2 decreases by one stop because 1.4 x 1.41 = 1.9, which rounds to 2. Going in the other direction, changing from f/16 to f/22 increases by one stop since 22 ÷ 1.41 = 15.6, which rounds to 16. The rounding is there to help simplify the scale, and I appreciate that!
With film speed and digital sensors, the ISO values represent the sensitivity of the substrate to light. Higher ISO values are more light-sensitive and require less time for proper exposure while lower ISO values require much more time under the same conditions. The numbers in this scale are conveniently simple with each doubling or halving equal to one stop. ISO 200 is twice as sensitive than ISO 100 and half as sensitive as ISO 400.
I hope this helps in your understanding of exposure stops. In a few weeks, we'll take a look at how they all work together.