The Talmud teaches that the prohibition to recite shma in the presence of an ervah applies even when the ervah is behind glass, which is considered no less of a violation of “so that He will not see a shameful thing among you” (Devarim 23:15). Some poskim cite this passage as a proof that seeing something through glass is halakhically considered seeing. This issue is raised in several contexts, such as seeing the moon for the purpose of kiddush levanah (see Sha’arei Teshuvah, O.C. 426, citing Resp. D’var Shmuel, 242) and the requirement of the judges officiating at a chalitzah to witness the procedure (see Resp. Sh’vut Ya’akov, I, 126).

However, a distinction is raised between situations where something that would have been visible anyway is now viewed through glass, and situations in which the glass (such as a telescope, magnifying glass, or eyeglasses) makes it possible to see that which would otherwise be impossible.

R. Natan Gestetner (Resp. L’Horot Natan, XI, 80) discusses this question in relation to the issue of the different dates for megilah reading, for cities, walled cities, and villages. In that context, the definition of a city includes areas that are visible from the city. Would such a definition extend to areas visible with the aid of a telescope?

He cites in this context a dispute among poskim (see Resp. Dovev Meisharim I, 1, and Resp. Sh’erit Yisrael, O.C. 11 and 12) concerning two letters of a Sefer Torah that appear to be touching, but a magnifying glass indicates that there is actually space in between them. He asserts that even if one were to take the lenient view in that debate, the megilah situation would be different. In the Sefer Torah case, the halakhah is established by the reality, and the reality is revealed by the magnifying glass. In the case of the megilah, the establishing factor is the seeing itself. If that seeing cannot be done through natural means, it is not called seeing at all.