tunnel vision
noun
Definitions
A restricted field of vision as the result of looking through some type of object or a medical condition.
Example: It took some time for him to get used to the tunnel vision he experienced while looking through a periscope.
The tendency to focus one's attention on one specific idea or viewpoint, to the exclusion of everything else.
Example: After Mark fell victim to tunnel vision, nothing you could say to him would change his mind.
Synonyms:one-track mind