Exposure

We were supposed to start discussing exposure this week in an effort to get everyone on the same page. However, at the last minute, I surprised my wife with a trip to Barrow, Alaska. Within the span of a few days, it was our 15th wedding anniversary, my wife's 39th (plus 1) birthday, my daughter turned 13 (man, I'm feeling old), and St. Valentine's day. Why Barrow, you ask? Well mostly because not many people can say that they have been 350 miles north of the Arctic circle in the winter. Can you? Well, now my wife, kids, and I can. We were also fortunate enough to see polar bears and about 10,000 eskimos who had gathered in Barrow for their annual Kigviq festival. The location changes annually. Unfortunately, we saw very minimal northern lights (aurora borealis) -- it has apparently been a bad year all over for the lights. I apologize that our discussion was delayed.

Exposure is really just about one main thing: recording the "correct" amount of light. Correctness is really subjective. It depends on what you consider to be the main subject in the frame and how you want to portray that subject. Dark or light? Sharp or blurry? Those are decisions you as the photographer make.

Exposure is controlled by three variables: aperture, shutter speed, and sensitivity (ASA or ISO). By varying one or more of these, you control how much light is recorded. If you change any one, you can change how much light is recorded. If you don't want to change how much light is recorded but want to change one of the factors for effect (e.g., sharpness, blurriness, grain, etc.), you can vary two or more of the variables. We'll discuss all of those coming up. First, let's discuss what each one is.

Aperture controls the lens opening. It is indicated in "f-stops." On your lens or camera, it may be indicated as f/2.8, f2.8, or 2.8 -- all mean the same thing. By convention, it is expressed fractionally. Thus, larger numbers mean smaller openings. For example, f/4 is a smaller opening than f/2.8 but larger than f/5.6.

Shutter speed controls the duration that light is allowed to enter the camera body and is usually measured in seconds or fraction of a second. On a sunny day using ISO 200 and an aperture of f/16, you will probably use a shutter speed of around 1/250 second.

ISO (ASA in the old days) is a measure of the sensitivity of film or sensor. Larger numbers mean higher sensitivity. In most current DSLRs, the higher the ISO, the more noise you introduce.

We'll talk about each of these in the next few postings.