The way the autofocus system on a camera works is quite complex, it turns out. But essentially the autofocus system on the camera works to find contrast in the photo object that enters the camera. There are 2 ways to detect this contrast. Maybe this doesn't have a direct impact on using the camera, but there's no harm if you know how it works.
1. Phase detection
This method of working usually uses a special sensor to detect the contrast of light entering the lens. The method is to bend light from the lens using a mirror towards 2 autofocus sensors (different from the camera sensor). This sensor can detect differences in the phase of incoming light. When the light has not yet coincided, the lens will continue to move until the light phases have coincided, which means the image is in focus. The focus sensor will tell the camera to stop moving the lens and lock focus.
2. Contrast detection
This system does not use additional sensors and relies on the main sensor to detect whether the light pattern from the lens is sharp.
Source : unsplash
Usually this way of working is on DSLR cameras because it uses a mirror. But DLSR cameras can use both methods. When using the viewfinder, a DSLR will use phase detection, but when in live mode and recording video, it usually uses contrast detection.
Both have their respective advantages and disadvantages. Phase detection is much faster in locking focus, so it is widely used for moving objects. But in terms of accuracy, contrast detection is more accurate even though it is slower. DSLR cameras are usually still superior to mirrorless cameras or compact cameras even though they both use contrast detection.