the usages and the meaning of “at all”?

Question

I didn't get the following sentence straight.

• Excuse me, but can you read at all?

As far as I Know, at all used in negative statements to emphasize what you are saying.

This sentence is from a reading article as following:

Tom is an old farmer. He lives a long way from the town.One day he goes into town to buy something. After shopping, he goes into a restaurant and sits down at a table.When he looks around, he sees some old men put on their eyeglasses before reading their newspapers.

After lunch, he goes to a shop to buy glasses, too. The man in the shop makes him try on a lot of glasses, but Tom always says, "No, I can't read with these."

At last the man asks Tom, "Excuse me, but can you read at all?"

"No, I can't!" Tom says, "If I can read, do you think I will come here to buy glasses?"

My question is: Can the question the man asks Tom be rephrased to:

"Excuse me, can't you read at all?"

What is the difference between them?

Answer 1

The sentence the way it is written is a question which does not imply any commentary on the part of the asker. "Can you read at all?" to me merely indicates that who's asking wonders at the ability of reading of the other person. On the contrary, the sentence "Can't you read at all?" seems to express surprise, or disappointment; it adds some degree of commentary or judgement on the part of the speaker.

Answer 2

At all means "any amount". So, he's asking if the person has the ability to read any amount - though you would not say "any amount."

He can read a little bit. He can't read at all, not even his name.

Do you like him at all? The skill/ability/affinity is on a scale (from nothing to a lot). I guess I like him a little.

Can you read at all? and Can't you read at all? mean the same thing with a different nuance or attitude. The second sentence the speaker is expressing surprise that the listener probably cannot read.