How a Digital Camera Works

Lens to Image Sensor to Media

Photography—whether film or digital—is all about light. The light reflects off the scene in front of the lens, then passes through the lens and the open shutter. This process is similar to the way light passes through the lens of your eye to the cones and rods at the back of the eye and on to the optic nerve. It’s after the light passes through the open shutter that film and digital cameras start to work differently. With film photography the light that passes through the lens exposes the light-sensitive film. The film records the light through photo-molecular reactions that embed a latent (nonvisible) image in the film. The latent image becomes visible when the film is chemically processed. With digital photography the flow of light is the same, but the flow of information is not. When the light passes through the open shutter, it hits the image sensor, which translates that light into electrical voltages. The informationis then processed to eliminate noise, calculate color values, produce an image data file, and write that file to a digital memory card. The camera then prepares to take the next exposure. This all happens very quickly, with a tremendous amount of information being simultaneously processed and written to the memory card.


The Lens

Camera lenses are surprisingly complex and sophisticated in their construction and design, containing a series of elements—typically made of glass— that refract and focus the light coming into the lens. This allows the lens to magnify the scene and focus it at a specific point. After many years of working with digital cameras, we’ve come to the conclusion that digital photography requires higher-quality lenses than film photography. Film has a variable-grain structure, whereas pixels are all the same size; this means that digital cameras operate at a disadvantage when it comes to capturing fine detail. In addition, an image sensor is more sensitive to light hitting it directly as opposed to hitting it at an angle, resulting in a slight loss of sharpness particularly around the edges of wide-angle lenses. The resulting loss of sharpness can be compensated in part by using a lens of the highest quality. We’ve also learned that although you can correct one or two less than sharp images within reason, it is absolutely not enjoyable to correct hundreds and thousands of them. A bright, sharp lens is something you will never regret photographing with.

Focal Length

 Different lenses provide different perspectives, and what makes those lenses different is their focal length. Focal length is technically defined as the distance from the rear nodal point of the lens to the point where the light rays passing through the lens are focused onto the focal plane—either the film or the sensor. This distance is typically measured in millimeters. From a practical point of view, focal length can be thought of as the amount of a lens’s magnification. The longer the focal length, the more the lens will magnify the scene. At longer focal lengths the image circle projected onto the image sensor contains a smaller portion of the scene before the lens . In addition to determining the magnification of a scene, the focal length affects the apparent perspective and compression of the scene. In actuality, the focal length isn’t what changes the perspective. Rather, the change in camera position required to keep a foreground subject the same size as with another focal length is what changes the perspective. For example, if you put some objects at the near and far edges of a table and then photograph the scene with a wide-angle lens, the background objects will appear very small. If you photograph the same scene with a telephoto lens, you’ll need to back up considerably to keep the foreground objects the same size as they were when photographed with a wide-angle lens. This changes the angle of view of the subjects, so that the distance between foreground and background objects appears compressed.

Lens Speed

 Lens speed refers to the maximum aperture (minimum f number) of the lens. A “fast” lens has a large maximum aperture, allowing the lens to gather more light—a very useful feature in low-light situations such as in the early morning or in candlelight . A “slow” lens has a maximum aperture that is smaller and lets in less light. A fast lens might have a maximum aperture of f1.4 or f2.8, whereas a slow lens might have a maximum aperture of f5.6. A fast lens is more desirable and is usually more expensive. A proper exposure depends on the camera ISO setting, the aperture size, and the shutter speed to allow an appropriate amount of light to reach the digital sensor. Opening the aperture allows you to use a faster shutter speed. A fast lens with a larger maximum aperture means that you can use a faster shutter speed than would be possible with a slow lens—a significant factor in low-light situations and fastmoving situations. A fast lens also helps the camera focus. Because the lens transmits more light, the camera will be better able to acquire proper focus, even in relatively low-light situations.

The Viewfinder and the LCD

The viewfinder is aptly named; it enables you to find the view you want of a scene and compose the shot. This is the essence of photography. Although seemingly a simple aspect of a camera, not all viewfinders are alike, and it’s important to know the differences between them and how a viewfinder can affect your photography. With many cameras, the viewfinder is a small window you look through to see how the scene looks. This is referred to as an optical viewfinder. Not all compact digital cameras include an optical viewfinder, but nearly all of them feature an LCD that serves double duty as a viewfinder and as a way to review your photos. The smallest cameras frequently do not have an optical viewfinder and rely entirely on the LCD for this purpose. On most DSLR cameras the LCD is only for reviewing photos and changing camera menu settings, though some newer cameras offer a live view feature that adds viewfinder functionality to the LCD screen.

LCD: Advantages and Disadvantages

We all love instant gratification, and that is the primary advantage of the LCD display on a digital camera. It allows you to preview the photo you are about to take and review the one you just took to check for exposure and composition or to share with everyone around you. In addition, you can also look at any of your images at any time after you’ve taken them. With the non-SLR digital cameras, the LCD display adds another dimension by allowing you to use the display as your viewfinder. Instead of putting your eye to the camera to compose the scene, you can hold the camera in front of you and compose based on the preview display on the LCD. This provides a little more freedom of movement, but also provides the opportunity to take images with unique perspectives. We’ve all seen the crowds of photographers trying to take pictures of the same subject, with those to the back holding their camera over their head, clicking the shutter release, and hoping for the best. The LCD display on a digital camera takes the guesswork out of that situation. You can take pictures up high, down low, around the corner, or elsewhere, and still know what you’ll end up with. One of the big disadvantages of the LCD is that it can be very difficult to compose a shot or review your images outdoors on a bright, sunny day. For composing a photo, you’ll find yourself shading the LCD with your hand, and when reviewing images, you may have to seek the shade of a tree, hold the camera inside your camera bag, or even put a jacket over your head to get a good look at your images. Turning up the brightness of the LCD display may make it easier to see in bright light, but it also changes the appearance of the image; so you can’t really judge proper exposure based on the LCD preview alone. Your best bet is to shade the display as best you can to review your images, and use the histogram feature for exposure evaluation.

The Image Sensor

Film does the job of both recording and storing the image photographed. With digital cameras these jobs are split between the image sensor and digital media .  The image sensor replaces film as the recording medium. The sensor actually consists of millions of individual light-sensitive sensors called photosites. Each photosite represents an individual pixel. These photosites, or pixels, generate an electrical voltage response based on the amount of light that strikes them. The analog voltage response values are then translated into digital values in a process called analog to digital conversion—or in geek speak, A to D conversion. The voltage information that has been translated to discrete digital numbers represents the tonal and color values in the photographic image. There is a certain irony in the fact that at the very instant of its creation a digital image is not really digital at all! Even though digital cameras take pictures in full color, the sensors are unable to see color. They can only read the luminance, or brightness, values of the scene. Colored filters are used to limit the range of light that each pixel can read so that each pixel records only one of the three colors (red, green, or blue) needed to define the final color of a pixel. Color interpolation is used to determine the remaining two color values for each pixel.

Types of Sensors

There are different types of image sensors technology, but the most widely used image sensors in digital cameras are CCD (charged coupled device) and CMOS (complementary metal oxide semiconductor).

CCD
   CCD sensors capture the image and then act as a conveyor belt for the data that defines the image. These sensors use an array of pixels arranged in a specific pattern that gather light and translate it into an electrical voltage. When the voltage information has been collected by each pixel as an image is taken, the data conveyor belt goes into action. Only the row of pixels adjacent to the readout registers can actually be read. After the first row of data is read, the data from all other pixels is shifted over on the conveyor belt so that the next row moves into position to be read, and so on. The CCD sensor doesn’t process the voltage information and convert it to digital data, so additional circuitry in the camera is required to perform those tasks.

CMOS
 CMOS sensors are named after the process used to create the components— the same process used to manufacture a variety of computer memory components. Like CCD sensors, the CMOS sensors contain an array of pixels that translate light into voltages. Unlike on the CCD sensor, the pixels on a CMOS sensor include additional circuitry for each pixel that converts the voltage into actual digital data. Also, the data from the sensor can be transmitted to the camera’s circuitry in parallel, which provides much faster data transfer from the sensor to the camera circuitry . Because of this significant difference in how the data is processed from the sensor, CMOS sensors are also known as APS sensors (active pixel sensors). Because circuitry is used at each pixel site in a CMOS sensor, the area available to capture light is reduced for each pixel. To compensate for this, tiny micro lenses are placed over each pixel on CMOS sensors to focus the light and effectively amplify it so that each pixel is able to read more light. Because sensors using CMOS technology are able to integrate several functions on the actual image sensor, they use less power and generate less heat than their CCD counterparts.