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 question 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?
You have a good question. In fact, Tosfos Shantz actually says that in this case we consider the Edim to be Zomemim only with regard to their testimony against the defendent. As far as the Edim themselves are concerned, though, we will not prosecute them since (as you said) it is possible that they have unusual eyesight.
However, there are other Rishonim who write that we invoke the rule of "Holchim bi'Nefashos Achar ha'Rov" to punish the witnesses as Zomemim as long as they themselves did not claim that they saw what happened from the distance with their exceptional eyesight.