
Beside clipping, the clipping planes influence the spread/precision of the Zbuffer resolution, which is not linear, but instead more concentrated near the viewer (where it is most needed) and lower near far plane.
The general rule of thumb is to have the near plane as near as possible to the camera (1.0), and have the far plane as near as you can affoard without having geometry poping, some fog usually also helps to hide the abrupt transition at the Far plane.
Floating point means just that, floating point ! which in other words means the precision is constant (number of bits available for mantissa doesn't change), but you can vary the exponent (its value, not its bits). This implies that larger numbers can't have fractional detail, because if they do, it will be lost when using this representation. the least significant digits are truncated.
Finally zooming as nothing to do with this. Zooming is an operation that works by changing the projection matrix, so that the mapping of the viewport edges increase or decrease the world volume(projected area) displayed on it.
