ISO is a scale that indicates how sensitive the sensor or film is to light. The more sensitive it is, the less light required to make an image. ISO 200 is the ideal or default ISO for most digital cameras. Whenever possible, one should use this setting. ISO 400 is twice as sensitive as ISO 100 so half the light is required to make a correct exposure. ISO 800 would require half as much light as ISO 400 and so on. The cost to increasing the ISO is noise. Noise appears as small spots in the image. As the ISO rises, so does the amount of noise, each step making obscuring more detail and making the image less clear. Depending on the camera and the scene, the noise may be nearly imperceptible or it may completely ruin the image.

The aperture controls the amount of light that passes through the lens and falls on the sensor. It is essentially a variable size hole. A large hole allows more light get through. A small hole lets less light get through. The aperture settings are commonly called F-stops and have an associated number like f/4 or f/16. Here’s the confusing part, larger numbers indicate a smaller aperture opening and smaller numbers indicate a larger opening. So f/4 would be a wide aperture (big hole) and f/16 would be a much narrower aperture (small hole)

Beyond the amount of light that is allowed to pass, aperture has another affect on the image. Aperture controls the Depth of Field (DoF). Depth of Field is how much of the scene is in sharp focus. Using a f-stop like f/16 (small hole) will render more of the scene in focus and an f-stop like f/4 (large hole) will render less of the scene in sharp focus.

There is no right or wrong Depth of Field, it’s a matter of choice. However, landscape’s are often shot with a narrow aperture so that the foreground, middle and background are all in focus. A portrait, however, might be shot with a wide aperture so that the subject is in sharp focus but the background  is unfocused and indistinct. The purpose is to make the subject more prominent.

Some lenses are capable of very wide apertures like f/1.4. The Depth of Field at f/1.4 can be so narrow that the subjects eyes might be in sharp focus while the ears and the tip of the nose are soft.

Shutter speed controls how long light, gathered by the lens, falls on the digital sensor (or film). Going back to our water analogy, it would be how long the water flows. Shutter speed is typically expressed in fractions of a second like 1/250 but can be much longer.

It would be impossible for someone to set completely still throughout a 5 second shutter opening. Moreover, few photographers can hold a camera still for more than 1/15th of a second. Even tiny movements by subject or photographer , will create blurriness on the final image. Often, a photographer wants a shutter speed of 1/60th of a second of faster when taking a portrait to prevent blurriness from tiny movements. With action shots like sports or busy toddlers and/or long telephoto shots, One needs a faster and faster shutter speed to stop the action. A shutter speed of 1/500 – 1/1000 is a common for sports.

Assume one wishes to capture a young child at play and, as a result, wants a shutter speed of 1/500th to freeze the action. Outside, on a bright day, the sun provides enough light that a narrow aperture and a small ISO setting can easily be used. But at night under lamp light, one would get a seriously underexposed image. It would be very dark or possibly totally black. In order to compensate for the lower ambient light in the scene, a wider aperture (larger hole) and / or increased ISO is required to make an image.

Exposure is a photography term that has several meanings. For our purposes here, it refers to the Shutter Speed, Aperture and ISO sensitivity camera settings used to create an image. Photography is a process of capturing light to create an image. The three settings that form the exposure triangle control light gathered by the lens in different ways. A correct exposure is any combination of settings that results in an image that is neither too light nor too dark. Ideally, this image would retain details in the brightest and darkest tones with no area being featureless white or black.

A simple way to visualize how it works think of light as if it were water. Creating a correct exposure is like filling  a container with water until it’s exactly full. Too much water and the container overflows; the image is over exposed (too bright). Too little and the image is underexposed (too dark). The length of time that the water is on, represents the shutter speed. The aperture is like controlling the flow of the water. The ISO is akin to the size of the container.

Filling a small container (high ISO) with the faucet completely open (wide aperture) will take very little time (shutter speed).  If the container is made to be twice as large, the water must flow twice as fast or for twice as long to fill it. This is how a camera works as well. Each increment of a given setting is called a stop. If one increases a particular setting by a stop, one must decrease one of the other settings by an equivalent stop to maintain a correct exposure. Each stop allows twice as much light as the previous and half as much as the next.

Any combination that results in an image that has exactly enough light is a “correct exposure” each combination will create an image with different qualities and require different trade-offs.  Those will be discussed in upcoming  articles.

Snapping the shutter is not the end for me, it’s just the beginning. The post processing work I do in Lightroom and Photoshop is just as important as the original. This is a picture I took a few years ago. I always liked the composition but the original image that came out of the camera failed to capture the mood I wanted. Much f it was too dark and many of the details were indistinct or buried in shadows. Truth be told, the image was a failure. Click to continue »