What is it that makes witness testimony evidence, or is it evidence at all?

As far as I can tell, moon hoax theories tend not to be based on extensive witness testimony. Most issues seem to relate to engineering, photography, physics, etc. The Kennedy assassination, just to use the example of which I am most familiar, was witnessed by hundreds of people and witness statements can be all over the place on many issues. Conspiracists take advantage this variance and often like to "mine" the witness testimony for discrepancies as direct evidence of a coverup, etc.

As measurements or recordings of an event, I consider witness statements as being more general measurements than the witness makes them out to be. As an analogy, it is like the witness is stating a measurement in inches, but in reality his "ruler" is marked only in feet. It doesn't mean the statement is wrong. Rather, we shouldn't take it so specifically. Thus, two accounts which differ on some details aren't necessarily at odds. At a more general level, they may be consistent.

What makes a witness statement true? Is it agreement with other witness statements? We have a fairly good grasp of how the image points on a photograph came to be, from the action of the light source on the objects, through lens, to the film, and through the development and reproduction process. But what is the corresponding mechanism with witnesses? Is there an equivalent linkage from the event to the statement describing it?