So I’ve recently bought a canon 750d because I’m wanting to get into photography and videography, however I’m having major trouble trying to work out the autofocus for video! When I put it into movie mode it it will often go in and out of focus when recording and it’s extremely frustrating! Why does it do this and can any help me! Also the video looks nice and sharp on the lcd monitor but when put onto laptop it looks rubbish and the focus going in and out is really noticeable! Any help much appreciated!
That is the way it works. It has to use contrast detect during video and it is not very good. The t6S was a little better, but not much. The T7i should be much better since it has phase detect on the sensor.
That being said, you should use manual mode or carefully select your focus point.
Video shot with many DSLRs comes with certain limitations, as kvbarkley has noted. Think of your video capability as being a "bonus" function built in to a still camera body.
It may also help to shoot your video in manual exposure mode and manually focus your scene. If DOF is critical keep your aperture closed down a bit and compensate, as needed, by using a slower shutter speed and/or increasing the ISO.