system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that if you cut a small piece out of a hologram you can still see the entire image in it, instead of just a part of it like you'd get from cutting up a photograph?
gpt
With a photograph, a lens is used to focus the scene on the film.  Each part of the film only gets the light from one part of the scene. 

With a hologram, there is no lens.  Each part of the film gets the light from the entire scene.  

Whether you're cutting a photograph, or a hologram, you only get the information that fell on that part of the film.  With a photograph, that means information from only that portion of the image scene went through the lens.  With a hologram, that means information from the entire scene.

However, a hologram is like looking through a window.  You can see the whole scene, and if something in the foreground is in the way, you can move a little to the side so that the foreground object is no longer in the way.  This is called parallax.  When you cut up a hologram, it is like blocking out the window so you can only look through a small part of it.  You lose the ability to  move side to side (or up and down) to look around objects.  You lose parallax.