Genesis 27:1

And it was when Izhak was old and his eyes were darkened from seeing,--because when his father was binding him he had seen the Throne of Glory, and from that time his eyes had begun to darken,--that he called Esau his elder son, on the fourteenth of Nisan, and said to him, My son, behold, this night they on high praise the Lord of the world, and the treasures of the dew are opened in it. And he said, Behold, I am.

ותכהינה עיניו, due to old age, as we find in Samuel I 3,20 when the eyesight of the High Priest Eli failed due to old age.

ותכהינה עיניו, just as what happened to the High Priest Eli who had not protested the wrongs committed by his sons. (Samuel I 3,13) In that instance the expression used is ועיניו קמה ולא יכול לראות, (Samuel I 4,15) Neither Avraham nor Yaakov experienced such problems in their old age at the age at which Yitzchok was struck by this affliction. They were both far older than Eli who died at the age of only 98. In fact, the Torah reports Avraham’s youthfulness in 25,1 when it reports him as taking another wife. Yaakov, with all the troubles he had experienced during his life was still able to see at 147 years of age, though with difficulty, (may be was very shortsighted then) as in Genesis 48,8 he is reported as clearly seeing Joseph’s sons, though they may have resembled each other so that he could not tell apart which one was Efrayim and which one was Menashe, This was in spite of the Torah (48,10) having told us ועיני ישראל כבדו מזוקן, “Yaakov’s eyes were heavy with old age,” (problems keeping the lids open?) At any rate, he could see, though not with clarity.

ויהי כי זקן, the word זקן is a verb in the past tense. The Torah relates that Yitzchok kept on aging until his eyesight was badly impaired. Yaakov experienced a similar problem; (48,10) In fact most people experience the problem of failing eyesight in their old age. In some instances this is a sudden phenomenon, in others it is a phenomenon experienced gradually. In the case of Yitzchok he was struck with this affliction more than 20 years before he died. He misinterpreted the onset of this phenomenon considering it as foreshadowing that his death would occur soon. In the event he was proven wrong. However, this is what prompted him to call in Esau at this time to bestow his blessing upon him before his death. Perhaps this failure of his eyesight at a premature time was one of the afflictions with which the righteous are sometimes afflicted. It may be that the reason why the Torah bothers with telling us this detail is to inform us that G’d subjects some of the righteous to some afflictions in this life in accordance with His wisdom. Some commentators attribute Yitzchok’s premature blindness to the smoke of the incense burned by his daughters-in-law to their respective deities. (quoted by Rashi).

ותכהינן עיניו מראות, “his eyes had become weak from seeing (too much).” One opinion cites Exodus 23,8 where the Torah tells us that accepting a bribe corrupts, i.e. blinds the eyes of even normally honest judges. Yitzchok accepting venison hunted by his son Esau, caused him to overlook the numerous shortcomings of his son Esau without rebuking him. An alternate explanation of the word: מראות, “from seeing.” G–d weakened his eyesight in order to enable Yaakov to secure Esau’s blessing for himself. This is presumably the reason why Rashi wrote that his eyesight became impaired from the smoke rising from the animals offered as sacrifices to a variety of idols. (Tanchuma, 8) The Midrash there explains this by means of a parable. When a person has two implements at his disposal, one made of bone, the other of earthenware. When he chooses to perform certain tasks with the earthenware implement for which it was not designed, it is liable to break into pieces. On the other hand, if he were to perform the same task with the implement made out of horn, it would not break into pieces. Yitzchok who was Avraham’s son, i.e. made of earth as G–d had made Adam, could not tolerate the smoke from these offerings of his daughters-in-law. Rivkah, being a woman, i.e. her origin was the bone of the first woman from which Chavah had been constructed, was not affected by that smoke.

ויקרא את עשו בנו הגדול, “he called his senior son Esau.” From this adjective used by Yitzchok when calling Esau, we learn that when fortune smiles on the wicked it is permitted to flatter them. An alternate explanation for the word הגדול: seeing that Esau was such an important person at that time, replacing Nimrod, he is described as “the great.”