It is natural to accept the artist as the ultimate authority on their own art. We often see people quoting the artist when disagreements about the work arise, and fondly imagining that this should put an end to it. On the flip side, we have a Derrida-esque approach which dismisses the artist out of hand as having nothing of interest to say. These are both positions which have been simplified to the point of idiocy.
Of course the artist is an authority on their own work. On some aspects of it they are the primary authority, and on others they have no particular authority beyond that of anyone else who's been immersed in the work for a while. In all cases, the artist must be taken as unreliable, however. This means simply that we can accept what they have to say, but whenever possible we should cross-check it.
The one narrow region where the artist truly is the authority is this: what does the artist have to say about their own work. Since whatever comes out of the artists's mouth or pen is by definition the right answer here, that pretty much nails that down. Whether this is particularly interesting or not, well, let's see.
One might imagine that the artist is also the final authority on their motivations, their processes and methods, their ideas about meaning and so on. In this case the artist is obviously the primary source -- who else, really -- they are equally certainly an unreliable source. Everyone, talking about themselves and their work, will tend to gloss over the embarrassments and errors, they will tend to exaggerate the flattering moments, and so on. An artist striving to be completely honest might skew the opposite way, in an effort to compensate. Another artist might simply lie about things. A third might genuinely mis-remember elements from the past. In all cases, whenever possible, we must cross-check if we're interested in the truth. Biographers have long known this. They read journals, they interview people surrounding the subject, the search for influences, and then try to synthesize that with what the subject has actually said about themselves and their work, to try to create a best guess at truth.
Arguably more interesting and important are the questions about how viewers react to the work, how the work fits into a larger context, what the work means to us as the viewing public, and so forth. On these points the artist is generally peculiarly ill-suited to comment. Having a unique and intimate relationship with a piece of work, having made it, makes it remarkably hard to imagine how others will see it. Anyone who has submitted a photograph to the inspection of others has experienced the surprise at what other people make of it. A good artist will often be able to get over that, but only to a degree. As for questions of how the art fits into a larger context, the artist will have biases and misconceptions to contend with, and may or may not have any special understanding of the context beyond their own work.
As we proceed from questions directly concerning the artist toward a wider field, the artist's special position as the artisan continues to harm the artist's ability to comment meaningfully, but ceases to provide any special access to knowledge. The artist becomes less and less reliable, until in very large questions their ideas have no more weight, and probably less weight, than anyone else equally skilled and knowledgeable about the area in question.
So, next time someone quotes Ansel Adams at you, now you are prepared.