How does specificity in an image contribute to its sharpness and vividness?
I agree with the previous response, but I want to share a few other ideas with you about this topic. The writing of S.I. Hayakawa, who wrote a book called Language in Thought and Action, is responsible for some of my ideas here, as is Louise Rosenblatt, who gave a great deal of time and thought to how we read.
Hayakawa wrote about levels of abstraction in his book, and used "Bessie the cow" to demonstrate how we can think about Bessie on various levels. We can think of a specific cow that we might actually know, we can think of cows standing in the field, we can think about cows as beef on the table, or we can even think about cows as the basis for buying beef futures on the market.
So, when someone refers to a cow in a book, which cow comes to mind? That will depend on your own particular experiences and knowledge about cows. If the writer doesn't care very much which cow comes to your mind, he or she can just use a general word, and you may or may not get the picture in the writer's mind. Smilarly, if someone refers to a dog in a story, which dog do you see? It could be a dog you know, or some generic dog that you think of when you do not have a good description.
That brings me to Louise Rosenblatt's idea, which is that the reader makes meaning of the text with what he or she brings to the text. The image that the reader "sees" is a product, not of what the writer meant, but a product of the reader's experiences and knowledge.
To the degree that we can all agree on the attributes of cows or dogs, the writer can be assured that the reader will not think about horses or cats, but beyond that, the reader is making meaning on his or her own.
However, if the writer describes a large, black dog with one blue eye and one brown eye, which is wagging its long shaggy tail, the writer is assured that the reader will see a very specific dog. The reader can still fill in some meaning, but the writer has presented the picture that he or she finds necessary for the story.
We write because we want to communicate ideas or experiences. If we do not provide a sufficient degree of specificity, our unique ideas and experiences cannot be successfully communicated to the reader.
If an author did not use specific details in his or her imagery, the images would not be sharp or vivid and they would not be as compelling as they are when details are given. Perhaps we can think about this by looking at some lines from Shakespeare.
Look at this passage from Romeo and Juliet, Act I, Scene 5.
What if Shakespeare had had Romeo say "Wow, she's beautiful. She looks like a treasure!" That would give us the same overall meaning, but without any of the sharpness or vividness. Instead, we think of her as brighter than the lights in the room. We look at her like a bright jewel against a dark skin. Both of these images tell us much more about how Romeo sees her than we would know if Shakespeare just said that Juliet was beautiful.