lens experiment illustration (image courtesy of Pearson Scott Foresman: Flickr.com)
How Does a Camera Capture an Image?
Photography is an art form that touches everyone in some form or another. It's a way to record history, share experiences or just to save a memory. People use cameras of all kinds for all reasons. You can use an underwater camera while snorkeling to photograph brightly colored fish. You might grab a disposable camera on the way to the hospital to record the birth of your first grandchild. You can even pull out your cell phone and snap a shot of a friend while walking in the park. While there are as many different types and styles of cameras as there are reasons to use them, there is only one major difference: manual (film) cameras versus digital cameras. While both take an image and turn it into a photograph, they do it in completely different ways.
The Lens
The first major component of a camera is the lens. Both manual and digital cameras have lenses that work the same way. A lens is a slightly curved piece of glass or, in some cases, plastic, that redirects and refocuses light to create a real image. The two types of lenses are concave, where the surface caves or bends inward, and convex, where the surface bends outward; your standard camera lens is convex. A good way to understand how a lens works is to light a candle in a dark room, hold a large magnifying glass between the candle and the wall, and try to focus the image of the candle on the wall by moving the magnifying glass closer to and further away from the candle. By moving the magnifying the back and forth, you are actually mimicking the way you would focus a camera lens. Different lenses create different effects. If you use a shallow, flat lens, you will get a "zoomed in" effect where you will a smaller portion of the image, but it will appear larger. Using a deep, round lens, will yield a wider field of view where the object(s) will appear smaller.
Recording the Image: Manual Cameras
In a manual camera, the real image is recorded by film. There are two parts to film: the base and the emulsion. The base is a thin, flexible, transparent piece of plastic that holds the emulsion layers. The emulsion is made up thin layers of gelatin (yes, the same thing Jello is made out of) that contain light sensitive chemicals. The emulsion determines whether the film will produce black and white or color photos. Color films have layers that are red, blue, or green sensitive that produce dyes when developed. Black and white film records the intensity of the light, without any color. Film records all information in negative, which is to say that the lightest or brightest parts of the picture will be the darkest on the film, while the darkest areas of the picture will be the lightest. This also works with colors: the red sensitive layers will create the cyan parts of the final photo, the blue will create magenta, and the green will create yellow. This makes the negatives, or developed film, look very strange. But not to worry, once the negatives are processed and the images are turned into photos, the original colors and light return.
Recording the Image: Digital Cameras
Instead of capturing an image on film, a digital camera records it electronically. Light is focused onto image sensors that can be read by a computer. According to Kodak, image sensors are made up of two components: pixels, or "picture elements," that collect light in the form of electrons and output circuitry that converts the signal from the electrons into a voltage. The brighter the light, the more electrons are collected, and thus creates a higher voltage. The voltage is what is recorded by the "memory" element of a digital camera such as a memory card or other storage device. The more pixels there are, the higher the resolution of the image (because it can record more light information). You get the color information from color filters that cover the pixels. As for the image sensors themselves, there are two main types: CCD and CMOS. CCD sensors create higher quality, higher resolution photos; they also have higher light sensitivity than CMOS sensors. CMOS sensors, on the other hand, use less power. CCD sensors are more common in professional grade cameras, whereas CMOS sensors are found in cameras such as those in cell phones.
Processing the Images
There is one last step to "capturing" an image, so to speak, and that is converting the image to a photograph. Film is developed using a chemical process that includes a developer, a fixer and a rinse. The colors are inverted (red to cyan, blue to magenta and green to yellow) as are the light and dark areas. The developed negatives can be used to make prints in two different ways. With the invention of computer scanners, you now have the opportunity to scan the negatives and have a computer interpret the images and print off a copy. This often produces lower-quality images; to get higher quality, you must expose the negatives to light sensitive color print paper. This is what is done at professional studios and at photo processing labs. If you are using a digital camera, all you need to do is insert the memory/storage device into a computer. From there, you can view, edit and print your photos. At a photo processing lab, they will use your digital images and print them on the same light sensitive photo paper that they use to print images from negatives. This paper is much higher quality and will last much longer than anything printed from a standard computer printer.
Tags: CMOS sensors, camera lens, digital camera, higher quality, image sensors, light sensitive