I have a Canon EOS 650D with 18-135 f/3.5-5.6 IS STM. When shooting in LiveView, I find it harder to achieve focus when it's in the dark, and sometimes it can not lock focus despite my touching the screen to mark the AF point. Why does this happen? What makes the difference between LiveView and through optical viewfinder? Is there any tips to improve this ? (I used LiveView because I needed to hold the camera high above my head, so I could not use the optical viewfinder to see the captured area).
This is the difference between CDAF (contrast detection) with integrated PDAF hybrid pixels vs full PDAF(phase detection). When using live view, the camera's imaging sensor is exposed and it tries to focus by using a combination of hunting for focus and checking the image contrast mixed with some hybrid pixels that contain limited phase detect focus capability. Generally speaking, CDAF should work better in low light, but it also takes longer and doesn't work well if contrast is low.
When in optical view finder mode on the other hand, the full PDAF sensor is exposed to the image and can be used to quickly get pretty accurate readings. It still requires vertical or horizontal lines, but doesn't require the same hunting for focus.
It is a little bit weird that you couldn't get focus at all with live view but could under the same conditions with the view finder, but it is expected that it would be a bit slower in LiveView since the PDAF functionality is not as complete.