First of all I came across these questions already:
My viewfinder won't autofocus but live view focuses well
Canon 600D Autofocus not working when using view finder but working in live-view
Why is my camera focusing fine in liveview but getting it wrong with the viewfinder?
And they really surprise me because it's basically the opposite of what I experience. I've tried the Yongnuo EF 35mm and the Canon EF-S 10-18mm IS STM, and with both I experience the same: Using the optical viewfinder, I always have blazing fast, accurate AF. However using liveview, it is most of the time impossible to get a focus point at all, forcing me to move around and what not.
The camera is a Canon 70D, supposed to have a good AF system.
In a bright light condition both work perfectly fine so I doubt it's a defect, but in lower light situation I can never get a focus point with LiveView at all - No matter which AF method I select. This surprises me as the answers to the questions I've linked suggest that in low light, the LiveView AF should work even better. Does my camera and its dual-ISO make a difference? If anything I'd assume it'd make it better.
Especially the video AF (which I assume does always use LiveView's CDAF?) for the 70D is praised to be outstanding, compared to its previous models. Again: Can't confirm. I haven't tried video in bright light (I'd assume that'd make it better though), but in low light (it's not that low by the way, it's a single, but bright and close-to-subject lightbulb spotlight) the AF in video is just as bad.
Any explanations/insights/tips/tricks for a DSLR newbie like me?
Additional info: When focusing manually, Magic Lantern's Focus Peak is able to draw in where the focus is too, and it works by detecting contrasts too. Unless I'm missing something doesn't that mean that Canon's CDAF should be able to find a focus point with CDAF too?
As you can see, ML has no issues drawing the focus peak into the right spot instantly. In addition to that: I'm sorry I didn't notice this earlier, but the camera actually does drive up the ISO while focusing when in LiveView. I couldn't get a screenshot of that but the subject was clearly visible (and in focus for a split second) during the AF attempt of the camera in LiveView. It just went past the focus point, and back again. Everytime I'd try to focus. The distance to subject is ~0.35m and my lens can focus up to 0.25m (even tho with the viewfinder I can focus as close as 0.15 sometimes). Just to make sure I've tried a longer (~0.5m) distance - Same results.
I've also just tried to AF on my desk keyboard in absolute low-light (on the LCD, I see nothing but pitch black). The camera was able to focus in LiveView properly and it turned up the ISO to the max. during the focus process.
Here's a screenshot made during the AF attempt - The camera sets the ISO to a guessed ~4000 during the failed attempt:
Conclusion: It's not the lowlight environment. What else could be the cause?
I couldn't get the screenshot timed right, but this is during focus attempt. The camera was able to find focus on first try, even though the light situation is a lot worse.