I've seen this claim a few times. Based on Altgens' WC testimony.
"This would put me at approximately this area here, which would be about 15 feet
from me at the time he was shot in the head--about 15 feet from the car on the
west side of the car--on the side that Mrs. Kennedy was riding in the car."
He's a lot more than 15 feet away from the President's head at Z312/313. And maybe 30 feet away from Mrs. Kennedy's car door.
Ike Altgens' livelihood depended on him knowing distances, especially in 1940, when he started taking pictures with fairly primitive - by today's standards - cameras. If he miscalculated, the picture would not be in focus.
So, I would tend to trust his judgement regarding distances more than most, if not all, of the people in Dealey Plaza.
Yet, he is not 15ft away.
Perhaps there's something you're missing. : )