Digital Photography 101 (Part 1)
Ever since the advent of the first digital camera, photography has experienced an explosion of growth. Nowadays, its hard to even find a cell phone without a camera. Unfortunately, the ability to take limitless shots without wasting any film and the ubiquitousness of the digital camera has not done anything to improve the quality of the pictures we take. I'd like to change that.
Taking good photos is not difficult. You don't need an expensive equipment, and once you get it you may find photography to be a fun and rewarding hobby.
Cameras work by allowing light to come through the lense which focuses it onto some sort of sensor (or film for traditional cameras). The light is only exposed to the sensor for a very brief amount of time. That light will cause the sensor to create all sorts of electrical signals representing the image that the camera sees at that point in time. The camera's internal computer will then convert those signals into a digital image file on your memory stick. I've glossed over several details here, but this is the general idea and it will serve as the basis for the rest of the article.
The Basics of Exposure
Exposure is all about how we get the light from the scene you are photographing, to the camera's sensor. Most digital cameras allow you to modify 3 basic components for controlling your exposure. Understanding how these components affect your exposure will help you keep most of your photos out of the recycle bin.
When light enters the lens, the light gets focused onto our camera sensor. Just like how your eye glasses can have different strengths, camera lenses have different strengths too. To measure this we use the term focal length. Focal length is measured in millimeters (mm). If you want to shoot something far away, you will want a lense with a large focal length (ex. 800mm). These are called telephoto lenses. If you want to shoot something thats really huge and you don't want to step back so much to get everything in the shot, you'll want a small focal length (eg, 17mm) these are often called wide-angle lenses. An easy way to remember this is to imagine that all the light that enters the lens is in the shape of a cone and the focal length controls the height of this cone. Smaller focal length means you have a short cone with a shallow slope so the light cone casts a wide view (ie wide angle lens). Larger focal length means a steep cone so things far away seem bigger (ie telephoto lens). When you zoom in or out you are changing the focal length of the lense. As you can imagine, a larger cone allows more light in which will change your exposure. Conversely, a telephoto lens allows less light to enter, however that is not the only issue. With telephoto lenses, the image magnification, also causes angular camera motions to be magnified. That is, as you zoom in, any movement in the hands holding the camera will be magnified in the photo.
Usually the scene you are trying to capture will dictate what lens or focal length you will use. However, sometimes if you cannot get enough light or you cannot hold your camera steady enough, it might be better to use a lower focal length and resize the image in post processing later.
Besides, focal length, the next most important aspect of a lense is how much light it can let in. Usually the light that enters at the edges of a lense gets distorted so camera lenses limit that light with an O-shaped disk placed between the lenses. The hole in this disk is called the aperture and can be enlarged or reduced by the camera or by rotating a ring on the lens itself. A bigger hole lets more light in but with a more narrow depth of field (which we'll get into later). Aperture is measured in f-values (ex. f/22 or f22) . The lower the f-value, the bigger the aperture so f/22 has a much smaller hole than f/1.4.
When buying a lens or a camera with a lens you will probably see it listed like this: 18-55mm F/3.5-5.6.
The first part means that the focal length can vary between 18mm and 55mm, but the second part doesn't mean the aperture can be from f3.5 to f5.6. It means that the largest aperture can vary between f/3.5-f/5.6. Most lenses can easily produce small apertures f/30+. The hard part is keeping the apertures large throughout the range of focal lengths. That's why the largest aperture that you can use will shrink when you zoom in.
Depth of field is the range of distances from the camera where things will be in focus. Lets say you wanted to take some landscape photos. You would probably like the wildlife in the foreground and the mountains in the background to both be in focus. In this case you want to have a wide depth of field. On the other hand, suppose you wanted to emphasize the subject of the photo, you could use a large aperture to create a shallow depth of field so that only the subject of the photo is in focus. The photo to the left is my baby Sophia sleeping next to her mom. Notice how Sophia is in focus while my wife isn't. This was shot with an aperture of f/2.8.
2. Shutter Speed
Shutter speed is the amount of time that the aperture will remain open so that light from the scene can reach the camera's sensor. This is measured in seconds or fractions of a sec like 1/30. Since most photos are taken with shutter speeds like 1/20, 1/30, 1/100, etc, your camera may show only the denominator of the fractional part like so 1/20 is shown as 20, 1/30 as 30 and 1/100 as 100. Long shutter speeds like over 1 second, will usually be displayed with a double quotation mark, like 2".
The combination of shutter speed and aperture determines how much light gets to the sensor, i.e. the exposure. For example, if you let light through a large aperture for a short time, you can still get the same exposure by letting light through a small aperture for a longer time. The biggest drawback to a long exposure is that the camera and the scene has to be perfectly still during the entire time that the shutter is open, otherwise your photo will be blurry. This type of blur is known as motion blur. In the photo on the right, you can see how the the people who were moving look kinda ghostly while everything else is pretty sharp. The woman in the center remained almost perfectly still during the shot so she had almost no motion blur. This was a 2 second shot (shutter speed) at f/8.
3. ISO Sensitivity
Also known as the ISO speed, the ISO sensitivity of the camera's sensor is the third and final component of determining a photo's exposure. Camera sensors usually have the ability for you to increase or decrease its sensitivity. You can think of this like the volume knob on your car radio. If you get a weak signal, you can up the volume, but the more you turn the knob, the more noise you will get. ISO sensitivity works the same way. The higher the sensitivity, the less light you will need, but at the cost of more noise in your photo. Ideally, you want to use the smallest ISO possible for the aperture and shutter speed that gives you good results. Often times the lowest shutter speed that minimizes your motion blur is the driving factor for the ISO you use. Its usually easier to correct ISO noise in post processing than motion blur.