Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Whenever something is between you and the object that you are photographing, that something gets included in the picture along with the object you are photographing- whether you like it or not.

Martin's left arm was between Mary and his right arm. Therefore, she could not photograph his right arm without also photographing his left arm. 

For instance, in this picture below, the motorcycle cop is being shot from his right side. Both his arms are visualized, but it would be impossible to capture his left arm without capturing his right from where the photographer is.