Exposure Part 3 - Sensitivity

Sensitivity is the last setting that controls exposure.

Sensitivity is also called "ISO" or "film speed", which are much more commonly used, but are somewhat less descriptive. There is also the now-antiquated "ASA" just in case you needed more jargon rattling around in your brain.

Just to set the record straight, "ISO" refers to the International Standards Organization. Its not a unit per se, but just an agreed upon convention about how sensitive a photographic medium is (such as film or a digital image sensor). ASA is the old school American Standards Organization, which actually uses the exact same scale as ISO, it's just different letters. So ignore that crap.

As far as film speed goes, when you are dealing with film, its sensitivity is a fixed property engineered into the film. You can't change it, you can just change which film you are using. Since most of us are shooting digital, at least most of the time, it's important to know that you CAN change the sensitivity of a digital sensor with just the press of a button.

So what is sensitivity? Do we have to watch LifeTime movies now? Bravo? No, fortunately we don't.

Sensitivity is how easily a digital sensor (or film) reacts to light. If the sensitivity is high, it takes less light to get a correct exposure. If the sensitivity is low, it takes more light. As mentioned above, sensitivity is most commonly given as an ISO setting. Here's a progression of ISO settings, all one full stop apart.

100, 200, 400, 800, 1600, 3200

Notice something familiar? The number doubles or half when moving from one setting to the next. Just like aperture and shutter speed, sensitivity operates in "stops", but this time we aren't changing the amount of light allowed in the camera, just how the image sensor reacts to it.

Let's go back to the ever-exciting water analogy. If aperture is the width of the hose, and shutter speed is how long its turned on, sensitivity is the size of the bucket we are trying to fill. ISO 100 is a kiddie pool, ISO 1600 is a coffee cup.

Another familiar example of sensitivity is sun burn. Some people are fair-skinned and burn to a crisp in 30 minutes. They are ISO 1600. Other people are dark as night and take all day to get burned. They are ISO 100. Get it?

ISO, just like aperture and shutter speed, has other important affects on your photography, especially regarding image quality and noise. We'll circle around for that shortly when we get into the creative uses of ISO, but first we'll start applying what we've learned about exposure.

Exposure Part 2 - Shutter Speed

Let's move on to our next exposure setting: shutter speed.

Your brain can have a break on this one, because shutter speed is much simpler to understand than aperture. A shutter is just a light-proof curtain that covers and uncovers the film or digital sensor inside your camera for a specific period of time. Normally, the shutter curtain is closed, protecting the image sensor from light until you are ready to take a picture. When you press the shutter button, the shutter uncovers the sensor for a certain amount of time and then immediately recovers it. The amount of time that the sensor is uncovered and exposed to light is called the "shutter speed".

Fortunately, shutter speed uses normal, familiar units: the second. Shutter speeds are generally given in fractions of a second rather than decimals (i.e. "1/8", not ".125"). This doesn't mean all shutter speeds are less than a second long, in fact some exposures can take several minutes or even multiple hours in special situations, but most of the time you will be dealing with fractions of a second.

Like aperture, the difference between shutter speed settings is talked about using "stops". A difference of one full "stop" (whether we are talking about aperture or shutter speed) is either double, or half, the amount of light. With shutter speeds, this corresponds directly to either doubling the shutter speed or halving it.

For example, here's a progression of shutter speeds, all one stop apart:

1/15, 1/30, 1/60, 1/125, 1/250, 1/500

The longer the shutter is open, the more light makes it into the camera. The shorter the shutter speed, the less light admitted.

Let's revisit the garden hose analogy. If I turn on the hose for 15 minutes, I'm going to get a certain amount of water. If I leave it on for twice as long, 30 minutes, I'm going to get twice as much water. This may be too much water, it may be too little, but the point is I can control how much water I get by changing how long I leave the hose turned on. The goal is to get the correct amount of water for the container I'm trying to fill, whether that's an Olympic swimming pool or a Dixie cup.

The same is true for photography. You want to get the "right" amount of light to create a correct exposure. Next we'll talk about the final piece of the puzzle, sensitivity (or ISO, or film speed).

PS - shutter speed, like aperture, affects other things besides exposure, such as sharpness and whether moving objects appeared blurred or frozen. We'll get to that later on when we discuss how to use shutter speed creatively.

Exposure Part 1 - Aperture

In this post, we'll talk more in depth about one of the three settings that controls exposure: aperture.

"Aperture" refers to the size of the opening inside the lens that admits light into the camera. More specifically, the aperture is a hole in a piece of opaque material that "stops" light. This piece of light-stopping material is called the "diaphragm"; the hole itself is the aperture. If you're getting too caught up in the terms, just think of your eye. Your pupil is the hole in your eye that allows light in, and this is just like the aperture in a camera. Your iris is like the diaphragm, it controls the size of your pupil and can get bigger to let more light in, or smaller to admit less light.

To use another example, think of a fire hose compared to a garden hose. The bigger opening in the fire hose allows more water to pass through in a given amount of time. If I turn on a firehouse for 10 seconds, I'm going to get a lot more water than if I turned on a garden hose for that long. Although this isn't a perfect metaphor (you can change the speed of water, while the speed of light is always the same), the basic principle is the same for the aperture in a camera. Big hole, more light. Little hole, less light. Nothing to it.

When we talk about aperture settings in photography, we use a weird unit of measure: "f-stops". When written, an aperture setting appears as the letter "f" followed by a number. Sometimes there is a slash between the "f" and the number, which I tend to use but is totally optional (example: f/11 and f11 are both fine). If you read this out loud, you'd say "f eleven".

The only weird part about aperture settings is how random the numbers seem. Here's a progression of  aperture settings in "full" stops, then I'll explain a bit more below:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16

The lower the number, the wider the aperture, that is, the bigger the hole. f/1.4 is a very large aperture, f/16 is a small aperture. So what's the difference between these settings? If I change the aperture setting on my camera from f/1.4 to f/2, this halves the size of the aperture, letting in only half as much light as before. If I go back to f/1.4, I am now letting in twice as much light as I was at f/2. This halving or doubling is a "full stop".

To wrap things up, let's discuss how aperture ties in to exposure. In order to get a proper exposure, we want to let the "correct" amount of light into the camera, depending on how dark or bright our environment is. One way to control this is to adjust the aperture. Outdoors in direct sunlight, I may select a very small aperture such as f/16. Inside my house I would probably use a larger aperture like f/1.4.

At this point in the discussion, there doesn't seem to be much reason to choose one aperture over another. However, aperture affects more than exposure. Later in the blog we'll talk about how aperture affects sharpness and depth-of-field (i.e. how much of an image is in focus at one time). Also, your choice of aperture will affect the other exposure settings you choose when you set your shutter speed and ISO.

For now, here's the big things to remember:

1. Big, wide aperture is a large hole that admits more light. A small, narrow aperture is a tiny hole that only admits a small amount of light.
2. Low f-number = big hole. f/1.4 is a very wide aperture. Conversely, a high f-number means a small hole. f/16 is a tiny pinprick.
3. Aperture will affect the appearance of your images and impact other camera settings in important ways which we will talk about in-depth later on. Selecting an aperture setting can be either a creative decision or a practical one, and this will be a constant consideration as you begin exploring photography.