Daf Yomi - Sukkah 35 - Magnifying Glass

Rashi and the Ran write that the reason that small spots at the top of the esrog invalidate the esrog whereas spots on other parts do not invalidate the esrog is because the top of the esrog is the area which is clearly visible to the eye. The Magen Avrohom in Orach Chaim 648:16 quotes the Mabit who writes that a discoloration at the top of the esrog will only invalidate the esrog if it is visible to all. If the discoloration is so small that one can see it only by gazing intently and others cannot even see it, the esrog will still be deemed as hadar. The Shearim Mitzuyanim B’Halacha writes that this ruling is the source for those who maintain that a black spot or discoloration which can only be viewed with a magnifying glass will not invalidate an esrog. It is noteworthy that there are those who use the magnifying glass to validate a spot that others expressed concerned about.