You can't focus an object space telecentric lens to infinity. An object space telecentric lens holds magnification constant regardless of subject distance. If you could focus such a lens to infinity (or thereabouts) then you could just point it at the moon and get closeup images of Neil Armstrong's footprints.

Real object space telecentric lenses have a limited range of focus distances at which the lens can produce focussed images, called the "telecentric depth". As most such lenses are designed for assembly line machine vision applications and can only focus on objects a few tens of centimetres away.

I the datasheet you link to looks like a regular close focus lens. It is possible that it is an image space telecentric lens. This just means the rays exit the lens perpendicular to the sensor and magnification doesn't change if you focus by moving the lens assembly.

To answer your remaining questions (ignoring the spurious issue of telecentricity for now):

• When focussed at infinity objects far enough away for the light rays to reach the lens almost parallel will form a focussed image at a distance of f behind the lens, where f is the focal length. Objects closer than that will form a focussed image behind the sensor (if the sensor wasn't in the way) and will thus form a blurred image on the sensor as each point of light from the object gets spread out.

• Rotating the lens to focus closer simply moves the lens further from the sensor, so the closer object now forms a focussed image on the sensor.