It's the thing i hate the most about this phone. This happens because the AI is programmed like that, because the zoom lens has a 3.5 aperture it needs alot more light to take pa picture so if the AI detects that the zoom lens doesnt have enough light ( from the range of 4x - 10x zoom ) it automaticly switches to the main camera ( 108 MP ) and its actually digital zoom. The most annoying thing is i had alot of cases i wanted to use the optical 4x zoom but the camera didnt let me because the phone thought there is not enough light but in my cases there was more than enough and i had to struggle to trick the AI somehow to switch to telephoto lens. I dont get it why they just dont add the feature to turn off this automatic switch between cameras because it works very very bad.
EDIT : Also the switching of cameras occurs also when the telephoto lens can't focus close enough so it just decides to switch to the digital zoom of the main camera even though clearly the telephoto has a better focus at some cases. Just wondering why i rushed to buy this phone because if i waited a bit longer i definitely would not have bought this phone. Shame on you samsung i didnt think a day would come that i would say thi but apple has outperformed you in every possible way with the iphone.