not to believe one’s eyesUsed when something one sees is so surprising that it is hard to believe. The pronoun changes to match the subject, as in « je n’en crois pas mes yeux » (I can’t believe my eyes).
FR: Devant un tel spectacle, il est difficile de ne pas en croire ses yeux.
EN: In the face of such a sight, it’s hard not to believe your eyes.