Why didn't Death Eaters take advantage of Harry's bad eyesight in a duel?

Question

In Harry Potter, Hermione once says something like this to Harry, "you really do have bad eyesight". So this means Harry can't go around well without his glasses. So in a duel, why didn't the Death Eaters or Voldemort used something like "Accio Potter glasses" and make advantage of his bad eyesight?

Answer 1

Removing Harry’s glasses won’t blind him.

First of all, taking away Harry’s glasses mightn’t really provide the Death Eaters with that much of an advantage. Harry doesn’t need perfect vision to battle, all he needs to do is be able to see the outlines of the Death Eaters well enough to aim his wand at them, he doesn’t need to see them in detail. Harry’s not blind without his glasses - he can still see, just not quite as well as he can with them, as clearly proven in more detail by this answer by Valorum. Without them, he’s still able to recognize Madam Pomfrey and Ron from across the room.

“Everything was slightly blurred. Somebody had removed his glasses. He was lying in the dark hospital wing. At the very end of the ward, he could make out Madam Pomfrey with her back to him, bending over a bed. Harry squinted. Ron’s red hair was visible beneath Madam Pomfrey’s arm.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 21 (Hermione’s Secret)}}

Summoning Harry’s glasses won’t make him unable to see, it’ll just make his vision somewhat blurrier, which isn’t necessarily that useful. More useful would be the Conjunctivitis Curse, which causes its victim’s eyes to swell shut.

“And Krum – you won’t believe this, but he didn’t even think of flying! He was probably the best after you, though. Hit it with some sort of spell right in the eye.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 20 (The First Task)}}

Even if the intent is to render Harry blind, there would be a more effective way of doing that than casting Accio Harry’s glasses.

Death Eaters have more effective spells.

There are far more effective spells for the Death Eaters to choose from than Accio Harry’s glasses. In the pantheon of spells existing, there are several that would be more effective in a battle, particularly for those who intend to inflict harm. With spells like Petrificus Totalus (freezing the target), Reducto (which blasts through solid objects), and Confringo (which blasts its target), Accio Harry’s glasses isn’t anywhere near the top of useful spells to cast.

“As they soared upwards, away from the two remaining Death Eaters, Harry spat blood out of his mouth, pointed his wand at the falling sidecar, and yelled, ‘Confringo!’

He knew a dreadful, gut-wrenching pang for Hedwig as it exploded; the Death Eater nearest it was blasted off his broom and fell from sight; his companion fell back and vanished.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 4 (The Seven Potters)}}

The majority of wizards would have better spells to use than Accio Harry’s glasses. And these aren’t average wizards. The Death Eaters have even more options than the average wizard does, since they wouldn’t be held back by moral standards, so they’d be free to use the deadliest and most harmful curses. They’d be willing to use both Dark and illegal magic, and Barty Crouch Jr. (as Moody) says illegal Dark curses come in many strengths and forms.

“Curses. They come in many strengths and forms. Now, according to the Ministry of Magic, I’m supposed to teach you counter-curses and leave it at that. I’m not supposed to show you what illegal Dark curses look like until you’re in the sixth year.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)}}

Therefore, the Death Eaters would have several different types of spells in their arsenal, both legal spells that are useful to anyone, and illegal Dark spells most wizards wouldn’t consider using.

Answer 2

There may be a combination of factors at play here:

• Did the Death Eaters actually know how bad Harry's eyesight was?

  Consider Hermione's statement in Chapter Three of Deathly Hallows:

  "Harry, your eyesight really is awful," said Hermione, as she put on glasses.

  It sounds like Hermione is surprised at how bad Harry's eyesight is. Hermione has been his best friend for nearly six years. If she was not aware of Harry's eyesight, presumably no one (and certainly not Death Eaters) was aware. Of course, anyone who saw Harry would know that he wears glasses, but a lot of people wear glasses even if their eyesight isn't so terrible.

• Some of Harry's enemies may not have been so bright.

  It is possible that some of the people Harry went up against did not take advantage of this because they were not smart enough. There are several instances where various bad guys are associated with a lack of intelligence. In Chapter Nineteen of Deathly Hallows Ron describes a group of Snatchers as follows:

  "They weren't the brightest. One of them was definitely part troll, the smell off him..."

  In Chapter Twelve of Chamber of Secrets:

  Ron, who had been gazing at Harry, said, "you don't know how bizarre it is to see Goyle thinking."

  And:

  Far too late, Harry and Ron forced themselves to laugh, but Malfoy seemed satisfied; perhaps Crabbe and Goyle were always slow on the uptake.

  And:

  "Azkaban — the wizard prison, Goyle," said Malfoy. "Honestly, if you were any slower you'd be going backward."

  In Chapter Nineteen of Order of the Phoenix we again see how Crabbe and Goyle's intelligence is viewed:

  “Last year’s Beaters, Derrick and Bole, have left now, but it looks as though Montague’s replaced them with the usual gorillas, rather than anyone who can fly particularly well. They’re two blokes called Crabbe and Goyle, I don’t know much about them —” “We do,” said Harry and Ron together.

  “Well, they don’t look bright enough to tell one end of a broom from another,” said Angelina, pocketing her parchment, “but then I was always surprised Derrick and Bole managed to find their way onto the pitch without signposts.”

  “Crabbe and Goyle are in the same mold,” Harry assured her.

  Both Crabbe and Goyle's fathers were Death Eaters, in Voldemort's inner circle. If they were anything like their sons they were probably not very sharp. Indeed, the description of them in Chapter Thirty-Three seems somewhat indicative of this:

  They bowed clumsily, muttering dully.

  In chapter One of Goblet of Fire Voldemort calls Peter Pettigrew a fool for not realizing why it would be a bad idea to initiate their plan before the Quidditch World Cup:

  Because, fool, at this very moment wizards are pouring into the country from all over the world, and every meddler from the Ministry of Magic will be on duty, on the watch for signs of unusual activity, checking and double-checking identities. They will be obsessed with security, lest the Muggles notice anything. So we wait.

  Similarly, in Chapter Thirty-Three of Goblet of Fire Voldemort describes Quirrel as foolish:

  A wizard — young, foolish, and gullible — wandered across my path in the forest.

  Similarly, In Deathly Hallows the Carrows were unable to answer the riddles to access the Ravenclaw common room.

  Based on this, it seems that intelligence was not one of the most prized qualities among Voldemort supporters.

• Wizards don't seem to be very creative.

  Even wizards who aren't particularly unintelligent seem to regularly not think of obvious magical solutions, and generally lack creativity in what they do. I elaborated on this point in this answer, particularly in points 2 and 3. Thus, for the most part, when dueling, wizards stick to regular dueling spells. Many of them simply would never even think about some roundabout way to disadvantage Harry, even if they were aware of the facts and were smart enough to process the information properly.

• Until the last book, no one really took Harry as a threat.

  For most of the series, Harry was just a kid. He was merely a student, and not a terribly good student at that. So for most of the confrontations his opponents didn't really view him as a threat. For instance, in Chapter Thirty-Five of Order of the Phoenix, Bellatrix mocked the idea of him fighting the Death Eaters:

  “You hear him? You hear him? Giving instructions to the other children as though he thinks of fighting us!”

  Additionally, he was usually outnumbered. In Chapter Thirty-Four of Goblet of Fire:

  All he had learned there was the Disarming Spell, "Expelliarmus"... and what use would it be to deprive Voldemort of his wand, even if he could, when he was surrounded by Death Eaters, outnumbered by at least thirty to one?

  And in Chapter Thirty-Five of Order of the Phoenix:

  Harry’s insides plummeted sickeningly. They were trapped and outnumbered two to one.

  In such cases his opponents wouldn't feel the need to disadvantage him by taking his glasses, as they didn't view him as a real threat in the first place.

• His opponents wanted to demonstrate their superiority.

  In some cases, the people Harry was fighting specifically wanted to show that they were more powerful than Harry. They wouldn't want him to be disadvantaged by not having his glasses because that would diminish the demonstration of their superiority. For example, in Chapter Thirty-Four of Goblet of Fire Voldemort gives Harry back his wand and attempts to have a fair duel with him:

  "You see, I think, how foolish it was to suppose that this boy could ever have been stronger than me," said Voldemort. "But I want there to be no mistake in anybody's mind. Harry Potter escaped me by a lucky chance. And I am now going to prove my power by killing him, here and now, in front of you all, when there is no Dumbledore to help him, and no mother to die for him. I will give him his chance. He will be allowed to fight, and you will be left in no doubt which of us is the stronger. Just a little longer, Nagini," he whispered, and the snake glided away through the grass to where the Death Eaters stood watching.

  "Now untie him, Wormtail, and give him back his wand."

  Similarly, Bellatrix tried to toy with Harry in Chapter Thirty-Six of Order of the Phoenix:

  “Come out, come out, little Harry!” she called in her mock-baby voice, which echoed off the polished wooden floors. “What did you come after me for, then? I thought you were here to avenge my dear cousin!”

  “I am!” shouted Harry, and a score of ghostly Harry’s seemed to chorus I am! I am! I am! all around the room.

  “Aaaaaah... did you love him, little baby Potter?”

In conclusion, depending on the situation, there may have been various reasons as to why Harry's opponents did not try to remove his glasses.

Answer 3

Because it wouldn't work. I have vision that is certainly worse than Harry's actually is. (People with perfect vision are always unduly impressed by how glasses distort their vision.) But if you took my glasses away and gave me a gun in exchange and I had to shoot someone 20 feet away, my ability to aim at the fuzzy person over there would not be significantly impaired although I couldn't make out their face. Given a choice between accio glasses and accio magic wand, the latter choice is infinitely preferable. The first hampers. The second wins the fight and isn't more difficult. Or you can just use a killing spell.