I should submit as evidence: this was shot handheld, turned out pretty sharp. To the theory that the back being loose could effect focus, the camera was angled downward for this shot and I believe up against my chest, so in theory that would keep the back pressed to the body. It would be on a tripod that the back would have slack. Hopefully, as someone else said, if it were enough of a gap to cause focus issues, I would see it in light leaking as well.