What is the difference between qadar (divine decree) and naseeb (allotment or share)? And are the consequences of human misconduct, such as poor eyesight due to neglect or prolonged use of glasses, considered a written qadar or a result of human shortcoming?

There is no difference between qadar (divine decree) and naseeb (allotment/destiny). Everything that befalls a servant is by Allah’s qadar and was written 50,000 years before the creation of the heavens and the earth.

This does not contradict the servant’s obligation to take lawful means, strive, and exert effort. As the Prophet, peace and blessings be upon him, said: "Act, for everyone is facilitated for what he was created for." Indeed, ruqyah (incantations) and medicines are also part of Allah’s qadar.

A Muslim should be keen on what benefits him, seek Allah’s help, and not despair. If something disliked befalls him, he should say: "Allah decreed, and what He willed, He did," and not say, "If only I had done such and such," because the word "if only" opens the door to the work of Satan.