Live View or Live Preview is a function on just about all modern DSLRs and mirrorless cameras which lets you view the final image on the the camera’s LCD screen before you press the shutter rather than looking through the viewfinder.
What’s the point of Live View?
Live View in no gimmick—it’s a genuinely useful feature incorporated into modern DSLRs. It’s not just about seeing the image on a nice big screen versus peeking through the viewfinder—the point is being able to guarantee focus in certain situations. DSLRs use phase-detection methods to focus in milliseconds on the subject and whilst they generally deliver impressive performance, they are not infallible. Even after the effort of micro-adjusting lenses the infinitesimal margin of errors involved from lens-camera compatibility to the manufacturing process itself mean that especially in shots with a very narrow depth of field the camera will sometimes back- or front-focus and leave you feeling a little deflated (or irritated) later. It’s impossible to judge whether the point of focus is spot on through the viewfinder—you can squint all your want but it’s just too small. However, with Live View the sensor image is now previewed up on a larger LCD screen and you have the ability to zoom in on any given spot to confirm the camera’s not amiss.
So why not just use live view all the time? Well, phase-detection autofocus requires some of the light entering the lens to be split and sent towards the dedicated AF sensor. A wall of mirrors achieves this in DSLRs, but in Live View the pellicle mirror must flip up into its upright position to let light through onto the image sensor—this is why when you press the Live View button it sounds like the camera is taking a photograph (in comparison the shutter is very quiet)—and so the AF unit instantly becomes redundant.
To deal with this, in Live View mode the camera switches to contrast-detection to focus. Typically a more accurate but far slower method. It also allows for focusing on any given point (because it is using the image sensor data the number, placement, type, etc. of the phase-detection AF points are irrelevant). And because you can zoom in on any given point through the LCD you suddenly have the option to fine-tune focus manually. This is really the main advantage of Live View. You won’t find it useful for snapping sports but for portraits where depth of field is narrow and the subject remains still it can be invaluable. Also for landscape shots there’s also no need to go and dig through the menu to find the mirror lock up option because the pellicle mirror is already in its upright position.
Sure there are some other advantages depending on the camera like face-detection and the whatnot, but pin-point focus is really what Live View is all about. Just don’t expect your battery to last as long!