Rava says that we do not suspect the witnesses of having an exceptionally good eyesight. The implications is that it is something possible, yet we do not take it into account. My questions is: why not? Doesn't "VeHitzilu" require that every possibility be taken into account? at least to acquit the witnesses. The Ritva (s.v.Mahu) raises a related question (a contradiction in Rava) and confirms that indeed we never suspect a good eyesight and do not accept such a claim. Tos in Yevamos 116a (s.v. Hocho) says that if the witnesses raise this as a defense they are believed (to the point of executing the defendant?). But this reinforces my question, (as pointed out by the Ritva, see also Tos in Makos 7a s.v. Dilma) if such an argument is acceptable when raised by the witnesses then the judges should raise it themselves because of "VeHitzilu"? Furthermore, if such as argument is not acceptable (as claimed by Ritva and other Rishonim quoted by him) could the judges themselves test the witnesses' eyesigh? What about an (or 2) expert witness(es), could they testify about the witnesses eyesight?
Tosfos Shantz actually says that in this case we consider the Edim as Zomemim only with regard to the defendant. However, as far as the Edim themselves are concerned, we will not prosecute them since it is possible that they have unusual eyesight.
However, other Rishonim write that we invoke the principle of "Holchim b'Nefashos Achar ha'Rov" as long as the Edim themselves did not actually claim that they saw the event from the distance.