[thrtythreeforty]

The naïve optical instrument will be diffraction limited. The resolving power of a lens, basically how "sharp" the resulting image will be, goes down as you decrease the size of the aperture relative to the focal length (that is, as the f-stop number goes up).

A telescope that could zoom into an exoplanet would have an f value of a kajillion or so.

[stouset]

You’d need a 90km aperture to get a one pixel image of an Earth-sized exoplanet at 100ly.

[thrtythreeforty]

Honestly that would be an incredible image by itself.