how much can changing light enter the eye shap what you see?
I'm interpreting your question to be: "How much can changing the light entering the eye change the shape of/how objects are seen? Would manipulating that light just stop you from seeing certain things?"

Manipulating the light entering the eye is the exact premise of a virtual retinal display (VRD):


  From Wikipedia –  A VRD is a display technology that draws a raster display (like a television) directly onto the retina of the eye. The user sees what appears to be a conventional display floating in space in front of them. 


Google Glass and other technologies (virtual reality headsets, heads-up displays) operate through relatively similar projection mechanisms. Light defines everything the eye sees (outside of hallucinations, and even those can be induced by flickering light). Also, the way your brain processes visual information (photons received by your eyes) can be "tricked," which is discussed in this BBC article. Finally, your brain's ability to process visual information can be altered by external forces (e.g. drugs, motion, or even strong magnetic fields).