“RAV ARYEH CARMELL cites his mentor, RAV ELIYAHU DESSLER zt’l in a footnote to MICHTAV ME’ELIYAHU (volume 4, p. 355), who teaches that the law does not change even though the reason for the law is difficult for us to understand. We must accept the rulings of the Chachamim, whether they are lenient or stringent. Rav Dessler points out that the reason mentioned in the Gemara for the ruling of the Chachamim is not necessarily the only possible reason. The Chachamim gave the reason that was most obvious in their day, which, in this case, was the universally accepted biological fact that lice spontaneously generate. There are other reasons for their rulings (see also RAMBAN to 12a, DH v’Ha Pligi, in the name of the Yerushalmi), and thus we may not conduct ourselves differently. Rav Dessler suggests, in the case of lice, the following explanation. It is a known principle that Halachah considers only factors that the senses can detect. Accordingly, since the egg of a louse is so small that it cannot be detected by the eye (at a normal distance), the Halachah does not consider it at all, and thus the louse is given the status as a lower degree of life-form, for which there is no prohibition to kill.
RAV MOSHE FEINSTEIN zt’l rules that the Halachah does not change, despite a change in our understanding of the natural world (see IGROS MOSHE, CM 2:73:4). He reasons that even if nature, or our understanding of nature, has changed, the Halachah does not, because the Halachah depends solely on the situation at the time of the original ruling of the Chachamim. (See also CHAVAS DA’AS 98:3, and CHAZON ISH YD 5:3.)”
I added the italicized words. Generally, as in the minimum size of blemishes in an esrog, this is defined when viewing it at about 1-2 feet which is how it is normally seen, although of course many view it, when checking, much closer and even use a magnifying glass.