Berachos 25a - Part I

Gemara: תניא כותיה דרב חסדא לא יקרא אדם ק"ש לא כנגד צואת אדם ולא כנגד צואת כלבים ולא כנגד צואת חזירים ולא כנגד צואת תרנגולים ולא כנגד צואת אשפה שריחה רע ואם היה מקום גבוה עשרה טפחים או נמוך עשרה טפחים יושב בצדו וקורא ק"ש ואם לאו מרחיק מלא עיניו "We have a B'raisa which aids R' Chisda's position: One may not recite the Shema in front of [various types of unclean substances]. If they were in a spot ten tefachim above or below him, he may even sit next to these substances, and recite the Shema. If they aren't [ten tefachim above or below him], he must distance himself from them as far as he can see them."
The gemara implies that the distance he must distance himself from them is based on his vision. Therefore, someone with 20/20 vision would have to move more than someone with 20/60 vision. Also, someone who is blind would not have to move away at all!
In fact, by a blind person the halacha is different. The Shulchan Aruch, in Orach Chaim 79:1 says: ומלפניו, צריך להרחיק מלא עיניו, אפילו בלילה, או שהוא סומא (תשובת הרשב"א סי' קס"ה וב"י בשם הר"י) שאינו רואה אותה, צריך להרחיק עד מקום שאינו יכול לראות ביום "If the objectionable substance is in front of him, he has to distance himself to a distance where he can't see it. Even at night (or if he's blind and can't see it) he has to distance himself to a distance where he wouldn't be able to see it during the day." On this, the Biur Halacha elaborates: "It is obvious that a blind person must evaluate the distance based on an average person's eyesight. At night, a person who can see must evaluate the distance based on how good his vision is during the day."
What I'm wondering is: A person with eyeglasses or contact lenses - what is considered normal vision for him? Is it as far as he usually sees (in other words, as far as he sees while wearing glasses), or, as far as he naturally sees?