Back Contents Glossary Index Forward
How Do Digital Cameras Work?The difference between digital and film cameras is that a digital camera uses an electronic chip, or sensor, to convert light into electrical impulses.
A film camera uses a light-sensitive coating, the emulsion, on a film base to capture an image.
Most digital cameras use a charge coupled device or CCD. I say most because less expensive digital cameras use complementary metal oxide semiconductor, or CMOS, technology. These CMOS chips will continue to get better as time goes on but they will probably never replace CCD devices in high end cameras.
The CCD chip is nothing more than thousands of tiny light-sensitive diodes. These diodes convert light into an electric charge.
The diodes are called photo sites. Each photo site is sensitive to light. The brighter the light that enters the camera and activates the diode the greater the electrical charge that is registered at that site.
The cameras electronics takes the data from all the millions of sites together and makes your picture.
ISO Speed and GainFilm speed, or sensitivity to light is measured by ISO rating. A digital camera does not have this. Rather its sensitivity is referred to as gain. Gain is the sensitivity of an electronic device. See below for ISO equivalency.
When gain is increased distortion is increased as well. In your digital camera this takes the form of artifacts that degrade the final image.
ISO EquivalencyTo make things simpler to understand in relation to film photography digital cameras have an ISO equivalency. This relates the sensitivity of the digital camera to film so that you get an idea of the sensitivity of your digital camera in relation to photographic film.
Most digicams lower the gain when the lighting is good and raise the gain when the lighting is poor. In addition many of the better digicams offer several selectable sensitivities. In this way you can select for low light or higher shutter speeds.
This is fine as long as you keep in mind that you are aware of image quality. Raising the gain produces more artifacts that degrade image quality. The trick is to optimize the image quality while maintaining good exposure.
f-StopsWith only a few exceptions, digital cameras do not have an aperture as film cameras do. This removes your ability to control depth of field.
However, some cameras do have the ability to throw the foreground and background out of focus electronically. Using this feature you can cause a foreground or background to be softly rendered as you would by using depth of field.
The ShutterAs there is no aperture also there is no shutter as exists in a film camera. That is the timing of the exposure is performed electronically, not mechanically as in a film camera.
The better digital cameras that do have a crude mechanical shutter use that shutter to protect the CCD from exposure to intense light. This keeps the CCD from being blinded rather than providing the timing for the exposure.
The use of an electronic shutter means that a digital camera can capture short movies. And why not, as a movie is nothing more than a succession of still images with sound. The CCD or CMOS simply operates at a capture rate of 18, 24, or other appropriate rate, frames per second and you have a movie. That is your digital captures 18 or 24 pictures per second. this us humans perceive as a movie.
Focal LengthThe focal length of a digital camera is less than that of a 35mm camera. This is because the CCD chip that forms the digital image is smaller than the frame of a 35mm camera. And the smaller the frame the smaller the normal lens.
The focal length of a normal lens is the diagonal of the photosensitive frame. A smaller photosensitive surface, wether film or electronic, means a shorter normal lens.
Color From Bits and PhotonsSo where does the color picture come from? Light enters the camera, falls on the CCD-CMOS and is rendered as an electronic signal.
Light is referred to as a photon, or a chunk of light.
The light hits the CCD-CMOS and is turned into an electronic signal. The characteristics of that signal depend on the color, brightness and such of the light.
The signal is then further refined by the cameras electronics and is given a bit value, or color value. Then you see that color on the monitor or in the eyepiece.
Each color is a 24 bit color. Eight bits for Red, eight bits for Blue, and eight bits for Green. This works out to 256 separate and distinct states for each color and 256 multiplied by 256 multiplied by 256 is 16.7 million, or so.
This is a RBG value for each of 16.7 million colors. While this sounds like a lot of colors it is what is needed by us humans to appear real. We can perceive 12 to 14 million different colors and tones. So 16.7 million is just about right.
Twenty-four bit color is needed for an adequate dynamic range as well. This is the range of values in an image from the lightest to the darkest. This defines how much detail a digital camera can capture and display in the highlights and the shadows.
How Many Pixels?As digital camera manufactures tout the number of Megapixels, millions of pixels, in their cameras it is only right that you should think this is important. And it is, but only so far. Let's not forget that lens quality also plays a role.
The total number of pixels is determined like this: a resolution of 1,280 by 1,024 is 1,310,720, which is 1,280 multiplied by 1,024. Or the image contains 1,310,720 pixels.
At some point another million pixels does not improve the picture. This was shown by tests at CNET. Of course, more pixels don't hurt, but that is not the only consideration in relation to picture quality. As in film photography the quality of the lens also plays a leading role.
Resolution of Digital ImagesResolution is measured in pixels per inch or, as pixels are more commonly called, dots per inch, or simply, dpi.
A graphic image's resolution is measured by its width and height in pixels.
The higher the resolution of an image the more pixels it contains and the better its quality. Also, a high quality picture with lots of pixels will consume a great deal of memory when saved and be slow to download as all that information must be transmitted to the user.
This is especially important when you are designing content for the World Wide Web.
It is important to note that device resolution, such as that of a printer or monitor, is the maximum number of dots per inch, dpi, that the device can produce. Monitors vary between 60 and 120 dpi; printers vary greatly.
Here are some resolutions for different applications:
For Web publishing be careful not to use a high resolution because this will create large files that will take forever to download. Don't forget that the greater the resolution the greater the information that must be saved to the file.
Taking The PictureDigital photography, like anything else, can benefit from some basis dos and don'ts. So here are a few:
FiltersA Polarizing Filter is an excellent addition to any camera bag and your digital camera is no exception. It can be used as a Neutral Density Filter to reduce exposure and double as an excellent way to shoot through reflections and to make the sky a magnificently saturated, deep, lovely blue.
Neutral Density Filters These filters are just what the name implies. They reduce the amount of light entering the camera but they do not alter the character of the light. That is the light is not colored or changed in any way. There is just less light. The amount of light these filters absorb can be precisely controlled.
Image EditingMany digital cameras, especially the lower end nonprofessional types, tend to render images with a red bias. To correct for this you can use a host of image editing tools such as Jasc's Paint Shop Pro.
Paint Shop Pro, or PSP, is a fine program with many excellent features, especially if you are making a Web page. It was used in the production of graphics for this e-book. and is a really fine tool for Web authoring, especially in the choice of colors as all colors are shown with their hex number equivalent.
Unless you watched TV in the 1960's the answer is no, but the trivia question is just not that tough.
The really tough trivia question is: Who was Crusader Rabbit's sidekick? Now that's a tough one. And one which I never answer. Should you not know you will just have to go do the research to find the answer.
To get back to photography. Use PSP to correct red bias on photos. Like this:
Adjust until your picture color is correct and save the image. Here is a hint: make small adjustments. If you make a mess of the picture do not save it.
Do not save the updated picture to the original file name until you are very happy with the way it looks and maybe not even then. Rather, give the altered picture a similar, but different name. This way you can always get back to the original.
Now when you print the photo the colors will be faithfully rendered.
Some file formats, like JPEG, use lossy algorithms. That means that every time you change a JPEG file and save it you lose some detail. To avoid this loss of detail copy your lossy file format image to TIFF, or another non-lossy, file format. Edit the TIFF image and then save it as a JPEG.
Digital Graphic File FormatsAfter you make all these beautiful photos you will want to save them for future use. There are several file formats to choose from so it is in your best interest to know something about the various formats.
Bit map One of the three classes that graphic files are found in. Bit map, meta file, and vector. A bit mapped graphic is composed of a collection if individual dots, or pixels. The simplest bit mapped files are monochromes composed of only black and white pixels.
A pixel, or picture element is measured in dots per inch and is the resolution of an image. A graphic with 2,048 x 3,072 has a higher resolution that one of 128 x 256.
EPS, EPSF (Encapsulated Post Script format) This meta file format for graphics contains two elements, the bit mapped image and the PostScript code that tells your printer, or other output device, how to print the graphic.
GIF (Graphic Interchange Format) This is a standard format for images created by CompuServe so that users could exchange images on-line. GIF format is an 8 bit per pixel format. That means that a GIF can have 256 or fewer colors in an image which is very useful for line drawings and cartoons.
GIF files and be animated or have transparent backgrounds. This allows the background color of the Web page to be seen rather than the background for the GIF. Think of this as if the graphic is on glass. You see through the glass to the background of the Web site.
Here is an example of a transparent GIF versus a non-transparent GIF. The transparent
gif is on the left;
You can see that the background color (white) "shows through" the transparent GIF, while the non-transparent GIF has a blueish background.
These images have a border added, the border is not part of the image, so you can plainly see that the transparent GIF truly takes on the background color of the document.
With the transparent GIF you cannot determine where the graphic ends and the background of the page begins.
Indexed Color These files are of two types: those with a limited number of colors and pseudo color images. The first type usually have 256 colors and are adequate for the Web. Pseudo color images are gray scale that display variations in gray levels in color rather than shades of gray. They are typically used in scientific and technical work and can be referred to as false color images.
JPEG (Joint Photographic Experts Group) JPEG is designed to store real world images like landscapes and other natural scenes. Using a 24bit storage format JPEG can record 16,777,216 colors. These files are not necessarily larger in file size than GIF images. JPEG uses a lossy algorithm to do its data compression what that means to you is that when compressed data is lost and it can not be recovered. This loss is designed to affect irregularities to slight for the human eye to notice easily.
Be safe and save your original pictures and edit copies of the original in TIFF, or a similar format, and when you have the edited picture the way you want it save it to JPEG format. But, as always, save all originals files as they are. It is not a good idea to change an original. Male copies and do your editing on the copy.
Vector A vector file is defined in terms of shaped. Each file is a mathematical description of the shapes that compose the graphic and are sometimes called object oriented. the same technology that is used in Postscript fonts is used here and has the same advantage, namely that you can make any image as large as you want and it will retain its quality. Vector files are displayed as a bit map.
Go use that digital camera.
Chapter 14 Chapter 16