What if the lens adapter (tube) is too short?

Question

Adapting SLR lens to mirrorless camera normally needs a lens adapter or simply a tube or a spacer to get the correct flange distance. If the tube is too long it normally loses the infinity focus but can do some kind of macro and it is easy to find this kind of tube in the market.

What if the tube (adapter) is too short? Does it shrink the projected image size to sensor? For example, to reduce the crop factor when adapting full frame lens to APS-C sensor camera using shorter tube. Does this kind of adapter exist? I don't talk about speedbooster here, purely a tube.

Answer 1

A tube that is too long makes the camera shortsighted. A tube that is too short makes the camera farsighted, meaning that it wastes a good amount of its close focusing range, instead being able to focus behind infinity itself (which is rarely useful). If the tube is considerably too short, the camera will be unable to focus on any real distance including infinity itself. However, if you then add closeup lenses in front, you might bring its focusing ability back into the finite range.