There are two different avenues to go down to fully answer this question.
One is that most of the UFOs that are seen and then photographed could have been seen by anyone if they were looking at the right place at the right time. But, the newer digital cameras have such a good eye that they will capture an object that is so small that the majority of people don't notice it at the time. Had someone else pointed out the object to them, and they had binoculars or a telescope, they COULD have seen it.
Secondly, there are reports from sane individuals who state without pause that they have seen UFOs which seem to appear and disappear right in front of them. Some researchers propose that UFOs possess a "cloaking" device, like something right out of Star Trek, which enables them to render themselves visible or invisible at will. This could explain some of the photographs where the UFOs were not seen by human eye, but were visible to the electronic eye.