Filmcameras does not have auto focus. Video cameras have. It is made for amateur users, people using their home cameras on vacation or recording a birthday party. It needs no technical knowledge. Everything is sharp.
Why to use manual focus if auto focus is so easy to use?
The short answer is becouse it gives you more freedom to create your images. It is a bit like when you shoot photos on your DSLR: auto mode is fine, you don’t have to worry about the settings, just hit the shutter button to take the shot. The camera will do the ‘thinking’.
But if you switch the camera to Manual or Program mode, you tell the machine what to do. You create the image you want. You can make the background blurry, with beautiful light flares, while your subject in the front is sharp. You can even take a photo against the son, or other light source, (you can break the rules) by creating stunning visual effects. You can make the subject blurred out and the background sharp. You can make silhouettes against the sunset (would be impossible in auto mode). And its the same with video. Your creativity has more playing field in manual mode.
You can not do all these things in auto mode. The camera does the ‘thinking’. Thinking that it is an error and it would try to correct the image by either changing the exposure, so the light source is as visible as possible (of course, with that loosing the flare effect) or by focusing on something in the background, and sometimes he could not find an object with contrast so it just moves back and forth trying to find the focus…
Professional film cameras have no auto focus option
Of course, the difficult part about that, is that it needs another person to actually focus, while the camera operator is taking care of the movement of the camera and the framing, compositing part. But that’s another story. And we are talking now about high end film cameras, not video cameras (which have auto focus). But let’s get back to DSLR’s.
Use the DSLR in manual mode whenever you can
I am not saying auto mode is the only solution. No. In most of the situations auto mode is the answer. For example if you are in a hurry, no time to think and change settings (events where you can not have the action repeated so you have only on chance to take the shot) using auto mode is a better solution. Here you just need to concentrate on staying in focus.
But if you want to be more creative and have time to play around with lights (and you can re-shoot the scene if needed) switch to manual mode and create beautiful images that your client will appreciate more.