isolate the variable

the act of focusing on an appealing portion of a person's body with a general disregard for the undesirable regions

Steve: Could you isolate the variable on that?
John: You know it. She's huge, but those are some mighty fine sweater cows.

Bill: You see that?!?!
James: You serious? She's hideous.
Bill: C'mon you gotta isolate the variable.
James: You right. She needs to turn back around though.