What is tethered shooting? In a nutshell, it consists of wiring up the camera to a personal computer, typically a laptop, and use special software that recognize the camera and pulls images as they are shot to display them on the computer screen.
Why would you do that? There are plenty of good reasons. First, reviewing images on a computer display is much more comfortable than doing so on the camera's tiny LCD; if viewing comfort is important to you, tethering shooting might just be the ticket.
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Next, you could use a projector to broadcast images on a big screen, which is great for public event, photo seminars or ceremonies... applications are plenty.
Finally, the tethering software - by downloading the picture just after they are shot - provides you with an instant backup of the images. Depending on the software's abilities, you can also rate the images or delete the bad one on the fly, greatly speeding up the overall workflow.
What do you need to get started? First, the camera must support tethered operations, typically by the way of the MTP/PTP protocol. Most modern DSLR cameras from major manufacturers support this mode. If your camera is less than 2-3 years old, chances are that it supports PTP already, check your camera's manual to find out, it should be in the section describing the USB connectivity.
Next you need an USB 2.0 cable long enough to give you the freedom of movements required by the conditions and your shooting style. The USB 2.0 standard limits the maximum cable length to 5 meters (15 feet) but it is easy to find USB 2.0 extenders/repeaters that allows much longer distances between the camera and the computer.
Last, you need a software application to download and display the pictures. Here the choices depend on the platform your computer is running on. Sometimes the camera ships with tethering software in the box, sometimes you must purchase dedicated software from the camera manufacturers. Additionally, several third party software developers offer tethering solution for most cameras, on both the PC and Mac platforms.
Some of them allow you to remotely control the camera, for example to fire the shutter by clicking a button on the computer, in addition of the download and display functions described above. The most complicated solutions requires you to setup the file download in one utility, then the image viewing in another, while the simpler ones offers integrated functions performing both the communication and the viewing within the same application.
Digital Camera Owners - Discover Tethered Shootingusb cable 20 feet