What has the camera captured in "A Photograph"?

The camera has evidently captured the speaker's "suited and booted grandpa": his grandfather, dressed in his best. The sight of his grandfather makes him think of a heavy wooden door. He would be as difficult to push against as such a heavy door would be, figuratively at least. Apparently the speaker's grandfather was a tough nut to crack, so to speak; it was difficult to get to know him, to get him to open up (pun intended! You know: to open up emotionally or like a door opens up). Anyway, once the man does begin to share his life with another person, it is like being pulled "into an open room where history / reclines vacantly on an armchair." The speaker's grandfather has evidently been through a lot in his life, and when he eventually feels comfortable enough to share his experiences, one can learn so much about history and the world.