Tethered shooting is the technique of connecting your camera with a phone or a computer so that you can operate the camera from and download the pictures to these devices directly. And the advantages?
- You can keep the camera perfectly steady and still take pictures. This will result in sharper images even in lower shutter speeds.
- You can operate the camera from a distance (useful for wildlife photography)
- Preview on larger and better screens (compared to the built-in LCD of the camera)
- Automate timelapse photography with the apps available on your computer or phone even if your camera lacks the ability
Even entry-level DSLR cameras support USB tethering. Recent models usually feature Bluetooth or WiFi tethering. Some older models can WiFi tether with the help of special adapters. However, even built-in wireless tethering is slower than USB. Being relatively new, wireless tethering might be more buggy too.
There are both manufacturer-provided apps and third-party apps for tethered shooting. Apps from camera makers include Nikon SnapBridge and LUMIX Tether. Free and open source Entangle is an example for third-party apps. It's available in the official repos of major GNU/Linux distros. Tethering is supported by other kinds of apps like Adobe Lightroom also. Although the maker-provided app might work best with a particular camera, third-party apps have the advantage of supporting cameras from multiple manufacturers.
To what extend can one control the camera via tethering? Some guaranteed features include live view, shutter release (i.e., taking a picture), and the ability to download the photo. Focusing and zooming? Depends. However, the preview could be zoomed so that you can perfect the focus manually.