Question: One of the lenses in my glasses falls out on occasion and needs popping back into the frame. Can I do this on Shabbos?
Answer: The Gemara (Shabbos 138b) writes that if an oven leg broke on Shabbos, then it is assur miderabanan to move the oven and the leg as one may come to fix it which would be assur mideorasia. The Mishna Berura (308:37) explains that one would either transgress the issur of boneh (building) or makeh bepatish (the finishing act).
The Rema (OC 308:16), however, writes that if a chair broke before Shabbos and one sat on it before Shabbos, then they may continue using that chair on Shabbos.
Thus, R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 15:77) writes that one can’t put a lens back into its frame on Shabbos.
R’ Eliezer Waldenberg (Tzitz Eliezer 9:28) and R’ Binyamin Zilber (Az Nidberu 8:33:4), however, write that there would be no issue in popping the lens back into the frame, as this isn’t considered firmly fixing and one doesn’t need to be concerned that they will fix it.
R’ Betzalel Stern (Betzel Hachachma 6:123) adopts a middle position. He writes that this scenario is similar to that of the broken chair. Thus, if the glasses broke on Shabbos, one wouldn’t be allowed to fix them and they would be muktze. If the lens had come out before, then one would be allowed to pop it back in on Shabbos.
In conclusion, if the lens had come out before, one could pop it back in to its frame on Shabbos. If they broke on Shabbos, it would be better to find a spare pair of glasses. If absolutely necessary, one could pop it back in.