A digital camera (or digicam) is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. They can play images on a screen after recording; store many images on small memory device and delete images to free storage space. Most cameras, can record moving video with sound and still photographs. Some give basic image editing features and some have a GPS receiver to produce geo-tagged photographs. They are also put into mobile phones (or camera phones). The Hubble Space Telescope is a specialized digital camera.
The optical system works the same as in film cameras, typically using a lens with a variable diaphragm to focus light onto an image pickup device. The diaphragm and shutter admit the correct amount of light to the imager, just as with film but the image pickup device is electronic rather than chemical. The resolution of a digital camera is often limited by the image sensor (typically a CCD or CMOS sensor chip) that turns light into discrete signals, now 10 megapixels camera are common.
They can connect directly to PC using different interfaces as
Serial port – Early cameras used it.
USB – It is now the most widely used method.
Firewire – It is not widely used but offered on few cameras.
Wireless – Some use wireless connections, via Bluetooth or IEEE 802.11 WiFi.
Mobile – Some high-end stand-alone digital cameras also use cellular networks to connect for sharing images by Multimedia Messaging Service (MMS) or by sending a picture as an email attachment.
A common alternative is the use of a card reader which may be capable of reading several types of storage media, as well as high speed transfer of data to PC. Use of a card reader also avoids draining the camera battery during the download process, as the device takes power from the USB port. An external card reader allows convenient direct access to the images on a collection of storage media. But if only one storage card is in use, moving it back and forth between the camera and the reader can be inconvenient. Many computers have a card reader built in, at least for SD cards. After the images are on the PC, the picture file, which is usually in JPEG (Joint Photographic Experts Group) format, can then be imported into documents. Most JPEG files have a .jpg file extension. High-end camera might support the uncompressed TIFF format. TIFF (tagged image file format).