Understanding Digital Photography

A camera has to make a lot of decisions when you press the shutter button. It needs to determine the brightness of the scene and the distance of subjects in order to set the camera's ISO, aperture, shutter and focus. This all happens within milliseconds. It is these settings that I'll explain in order to give you a better understanding of how they affect your final captured image.

The main function of a camera lens is to collect light. The aperture of a lens is the diameter of the lens opening and is usually controlled by an iris, or in other words aperture refers to the opening of a lens's diaphragm through which light passes. It is calibrated in f/stops and is generally written as numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11 and 16. The lower f/stops give more exposure because they represent the larger apertures, while the higher f/stops give less exposure because they represent smaller apertures. This may seem a little contradictory at first but will become clearer as you take pictures at varying f/stops. Be sure to check your manual first to learn how to set your camera for Aperture Priority, then try experimenting to get comfortable with changing the aperture and recognizing the effects different apertures will have on the end-result image.

Depth of field is defined as "the zone of acceptable sharpness in front of and behind the subject on which the lens is focused." Simply put: how sharp or blurry is the area behind your subject. Here's the equation:

The lower the f/stop—the larger the opening in the lens—the less depth of field—the blurrier the background.

The higher the f/stop—the smaller the opening in the lens—the greater the depth of field—the sharper the background.

The f/stop also affects shutter speed. Using a low f/stop means more light is entering the lens and therefore the shutter doesn't need to stay open as long to make a correct exposure which translates into a faster shutter speed. Again, the reverse is true: using a high f/stop means that less light is entering the lens and therefore the shutter will need to stay open a little longer which translates into a slower shutter speed.

Now that we know how to control depth of field, what determines the choices we make in selecting the aperture? We use focus and depth of field to direct attention to what is important in the photograph, and we use lack of focus to minimize distractions that cannot be eliminated from the composition. While there are no rules, there are some guidelines.

For classic portraiture we separate our subject from the surroundings by using "selective focus." Choosing a large aperture (lower f/stop, like f2.8) creates very shallow depth of field with only the subject, or just a portion of the subject, in focus. This helps direct the viewer's attention to the subject.

In a landscape or scenic photograph we usually want to see as much detail as possible from foreground to background; we want to achieve the maximum depth of field by choosing a small aperture (higher f/stop, like f/8 or f/11).

While we can get the maximum or minimum depth of field by working at each end of the aperture range, sometimes we want a more intermediate level of depth of field, limiting focus to a specific range of distances within the overall photograph. One way to do this is to choose a mid-range f/stop, like f/5.6, and shoot a test frame. In image playback, use the magnifying function of the LCD to zoom in and check the depth of field; make adjustments if necessary and reshoot.

A faster shutter speed, similar to blinking your eye more quickly, helps to reduce the amount of motion the cameras image sensor will capture. If the shutter stays open longer, it will allow more potential motion in the captured frame. Any movement during the "capture" process of a scene will end up as a blur. If it's camera movement then the entire photo will be blurred. If only the subject moves then only the subject will be blurred.

In order to capture blur-free "action" photographs (Sports is one example), we'll need to make sure the camera is using a high shutter speed (check your camera manual to learn how to set your camera for Shuter Priority). Typically a shutter speed setting of 1/125th of a second (shutter speeds are measured in fractions) is considered a "high" shutter speed, although the setting will depend on the speed of the action that we want to "freeze".

Example (to "freeze"): To capture a golf swing we might need to use a 1/500th speed shutter, while trying to capture kids playing basketball might only require a 1/125 or 1/250 shutter speed.

Example for opposite situation (to blur):  Suppose we want to take a picture of flowing water. To depict flowing water, we usually want to use a slow shutter speed so that the water blurs. It is this blurring that makes the picture so effective in depicting water motion.

Well, if you decide to use a slower shutter speed (say, 1/12 sec.), this means that you have to compensate by closing down the aperture to allow less light in.

Important: Less light gets through to the imager (or film) as the shutter speed is increased, thus it's very difficult to use higher shutter speeds in lower light situations. There are ways to improve the speed of the shutter in these situations. One is to allow more light to pass through the lens (aperture setting), the other is to increase the ISO sensitivity of the imager.

Traditionally, the International Standards Organization (ISO) has provided a benchmark rating of the relative sensitivity of film. The higher the ISO rating, the more light sensitive a particular film is. Higher ISO films require less light to record an image. The ISO rating has been redefined for digital cameras, indicating the image sensor’s sensitivity to light.

So, a camera's ISO function sets the light sensitivity of the camera's image sensor. ISO settings are often rated at 100, 200, or 400 but go as high as 800, 1600, and even 3200 on some advanced models. A lower ISO setting is used when capturing overly bright scenes, since it reduces the light sensitivity of the image sensor. This is ideal when shooting at the beach, on a ski slope, or under the midday sun. A higher ISO settings is often used when shooting under dimmer conditions (cloudy days, indoors, etc.) since it increases the light sensitivity of the image sensor. As brightness in a scene is decreased the camera tries to compensate by slowing the shutter speed which in turn lets in more light but increases the risk of motion blur. To prevent this, you can increase the ISO or sensitivity of the camera, which allows the camera to select a higher shutter speed, thus reducing motion blur. In other words, if ISO sensitivity is raised from ISO 100 to ISO 200 while aperture is left unchanged, the same exposure can be achieved with a shutter speed twice as fast.

Why not just use a higher ISO all the time? While using a higher ISO setting is often needed to capture images with reduced blur in lower light, it also increases the noise level of the image. Noise, when present, can be seen throughout a photo but usually appears most in dark areas.The amount and degree of noise varies from camera to camera.

AUTO ISO: digital camera automatically sets the ISO speed according the the brightness of the scene, increasing or decreasing the sensitivity. User has no control over which ISO number is used.

ISO 100: Great for bright sunny days, at the beach or on the snow, excellent for close-ups, landscape, and portraits. Produces fine detail and clean images that are great for enlargements.

ISO 200: Great for cloudy and overcast days (noise levels may increase, but in most cases not noticeably).

ISO 400: good for indoor photography whether or not a flash is used. Useful for “stop-action” and sports photographs. Most compact digital cameras produce high to very high image noise.

ISO 800, 1000 and above: useful for taking photos in very low light, or outside in good light when increased shutter speeds are required. Results can be disappointing when shooting at these high numbers with compact digital cameras, so take test photos before photographing an important event.

High Auto ISO mode : Some digital cameras have a High ISO scene mode, which uses a very high ISO number such as ISO 3200. Use High Auto ISO mode if it is a choice between missing a picture and being able to capture an image.

This is where technique varies between film and digital, making digital a little more tricky yet more powerful and versatile than film. With film, you simply put the canister in your camera and shoot. Film is a physical material and captures light based upon its physical make-up. As a photographer, you have no control how the film will record light because the manufacturer decided that for you when they made the film. Digital is very different. Unlike film the sensor uses one more pieces of information when capturing light. That information is what kind of light are you capturing? This question is defined as White Balance, and the sensor needs this information in order to expose the image properly.

Let me try and explain this in an easy way. Lets pretend you take a piece of white paper outside on a clear day at high noon. Now when you look at the paper your brain is smart enough to convert all the colors and say yes that is white. Lets pretend you take the same piece of white paper outside during a cloudy day, your brain will still register it as white. If you did the same thing with film you will see the first white paper is actually blue and the second will be a slightly orange color. With a digital camera, white balance simply tells the sensor what color the white paper is. The first scenario you would say it is cool and the second warm. This setting insures that white is truly white, not blue!

We can simply set auto white balance and the camera will read the scene's color temperature (basically the hue and intensity of a particular light source, measured in degrees Kelvin) and choose a setting from its collection of pre-programmed adjustments. That's a good solution when a scene's lighting is pretty much all of one type and there's a prominent white or neutral subject in the scene.

Auto White Balance (AWB) setting tells the camera to set the white balance for you automatically. This is a good place to start; if the photo turns out well in your preview, then there is probably no need to further adjust the white balance. But this setting can be hit or miss, so you may have to try another option.

Incandescent/Tungsten setting is best for traditional household bulbs. So this is a good setting to use indoors when photographing under incandescent lights.

Fluorescent will prevent the green cast common to photos taken in fluorescent light. Use this setting indoors under fluorescent lights.

Flash adds a more aggressive touch of warmth to take the edge off the bright light of a flash. Without setting your camera to the Flash mode, the entire photo will turn blue when using an external flash. Your regular built-in flash does not require you to use this setting; the camera will automatically adjust accordingly.

Cloudy will add a bit of warmth to the light. Use this setting when photographing outdoors in cloudy or overcast situations.

Shade adds a slight pink tone to eliminate the blue cast that shadows take on in open shade.

Sunny sets the color temperature to 5000 degrees Kelvin, which is typical of mid-day sun. Use this setting when shooting outdoors in a sunny situation.

Custom/ Preset Manual setting it's a do-it-yourself choice, in which you actually set the degrees Kelvin to affect how the camera will see and render the hue and intensity of colors in the scene. Using the Custom setting is an ideal way to handle a scene that presents mixed lighting - fluorescent lights in the ceiling and daylight streaming through a window, for example - because it takes into account all the lighting in the scene. You can also fine tune those colors to your definition of "accurate," or skew the colors to reflect your choice of the scene's mood and feeling.

No matter how you set your white balance, check the results of your choices on the camera's LCD and adjust according to your preference. Often your choice will depend on the look, feel and mood you want for your picture. And remember, Live View will show you the effect of your choice before you take the photo.