Exposure part one

I’ve enjoyed working on these “tutorial” posts, so here’s another one.

One of the most important factors in creating the image you visualize in your mind’s eye is knowing how your camera works. That seems kinda obvious, but I’m quite sure many people never bother to learn their tool. Since your camera is your light-capturing tool, learning how it works is quite beneficial.

For digital cameras, three different controls affect your exposure (or in very basic terms, how bright or dark your shot is): shutter speed, aperture, and ISO (film speed or ASA in yesteryear). Let’s take a look at each of those specifically and see how they work and their other implications.

Shutter speed

Shutter speed is the period of time during which the sensor is exposed to light. In many compact cameras, this is a “leaf shutter” while in most system cameras it is a dual curtain shutter. A leaf shutter works much like an aperture (discussed below) that closes all the way to fully disrupt light from reaching the sensor. In a dual curtain arrangement, the first curtain reveals the sensor and the second curtain conceals the sensor after a specific period of time.

For either system, the shutter speed controls the time or duration of exposure. Apart from controlling the exposure, this fact implies that shutter speed also controls the camera’s perception of anything which takes place over time, such as movement. There are also some interesting implications here about shooting with flash, but we’ll get to that in part two.

One needs to be careful when using a very long shutter speed, however. The longer the shutter is open, the more time the sensor will introduce noise into the image (more on that in the ISO section). Fortunately for long-exposure generated noise, the pattern is predictable and can be effectively removed.

Aperture

Aperture, in very simple terms, is the size of the hole through which light passes. It controls how much light gets through the lens at any one time.

An aperture “diaphragm” is pretty universally constructed of “blades” mounted in the lens which close together like an iris to reduce the amount of light that can pass through. In fact, an alternate name for a camera aperture is an iris, and it functions identically in purpose to the iris of a human eye.

Depth of field is implicitly controlled in part by the aperture. One can think of it in a similar way one would to a light source: the larger the light source, the softer the light and similarly the larger the aperture, the softer the out-of-focus areas. When the aperture is wide open, light can pass through the lens in a more diffuse way (from more angles at once) and when the aperture is smaller, light must be more directional in order to align to pass through the smaller opening. Something to remember here though is that, since aperture numbers are actually fractions, the smaller the number, the larger the aperture opening.

Speaking of fractions, Aperture is actually expressed as f (the focal length of the lens) divided by a number (the aperture number) resulting in f/x. Usually this is simply written as f2 or f8, but it is helpful to remember that it is actually f/2 or f/8. When we get down to something like f/0.95, we’re dividing by a decimal, which means the aperture is actually wider than the lens’ focal length!

Aperture functions independent of shutter speed, which has some very useful implications. If a shot would look better with a shallower depth of field (less area in focus), then one can open the aperture more and compensate by reducing the shutter speed to achieve an equivalent exposure. Alternatively, if one wanted to illustrate motion in a shot, one could increase the shutter speed to allow the subject to blur and close down the aperture to compensate.

ISO

ISO is our third variable. It essentially works like a volume knob for the camera’s sensor: the higher the ISO, the more the camera amplifies the signal from the sensor. Unfortunately, as with a volume knob, more amplification also produces more unwanted noise. Unlike in an audio signal where noise sounds like a hiss, camera noise is false information in either luminance (lightness) or chrominance (color). High ISO noise often shows up as red specks.

As a general rule, the lower the ISO the better, because lower ISOs keep the image much cleaner and free of noise. Higher ISO capabilities really come in handy in lower light shooting when you can’t open your aperture any more or you can’t lower your shutter speed any more for fear of blur. Usually high ISO noise is less distracting than motion blur or poor focus.

I hope that was all helpful. Those are the basics, but the basics are important and review never hurts. Check back for part two where I’ll talk about exposure for flash and some of the implications there.

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