Hi,
If you use the 12 MP main lens (26mm), the default zoom level is always 1.0. If you press the telephoto icon, the camera changes to the 29mm tele lens and by default displays a 2x zoom. I don't have an S20 at hand, so I can't say how zoom levels between 1.0x and 2.0x are handled. Perhaps it's the 26mm lens up to 1.1x and from there on it's the 29mm lens. The icon on the screen tells you always which lens is being used. You can adjust the settings so that the 29mm tele lens takes a full 64 MP image.
As you've noticed, this 29mm "tele" lens is not truly and optically a telephoto. It's almost as wide as the main lens and the zoom effect is achieved with software tricks and cropping the 64 MP sensor. You can get a pretty sharp 2x image but after that the image quality starts to take a hit. 3x zoom is already noticeably softer because the sensor can't deliver any sharper image. And because the "tele" lens is so wide, you also see typical wide-angle problems such as geometrical distortions that are common for all wide-angle lenses. Subjects tend to get a bit roundish, bloated and stretched, especially near the edges of the image. Even if you zoom in to, for example, 5x, these optical distortions remain because the lens itself is only 29mm, no matter what the software-created zoom level is.
The purpose of this setup? Maybe cost savings. For comparison, the S20 FE has a real optical 3x tele lens (76mm if I remember correctly) and it produces very sharp 3x images. It really blows the S20/S20+ tele lens out of the water at 3x. But on the other hand the FE uses the smallish 12 MP main sensor for 2x tele images, and those are a bit softer than images taken with the S20/S20+ 64 MP sensor. I've used both phones extensively and personally prefer the S20 FE camera setup because of the much better tele lens, even if it's only 8 MP. The main sensor/lens is the same in both phones.
So if you mostly use the ultra-wide lens, main lens or max 2x zoom, then the S20/S20+ is perhaps a better choice. But if you need more high-quality zoom, then the FE or Ultra phones are better. And to be honest, anything over 5x is rubbish on the S20/S20+ or the FE.
I hope this helped!
