Answer:
The poem emphasizes the desperation that the author felt for having been blind for half of his life and for this reason, he feels that his life is meaningless.
Step-by-step explanation:
This sonnet deals with the loss of vision that the poet suffered, a pitiful fact for those who have poetry as art, a circumstance that made him practically dependent on a third party, who collected his words.
In this poem he uses the word "light" as a metaphor for vision, so when the author speaks "When I consider how my light is spent" he is speaking of his worn out vision and this left the rest of his days a world dark and wide.