This particular artwork is a portrait which has been classified as having come from the artist's workshop, rather than definitively from his own hand. It is quite possible that he was heavily involved in the portrait, but perhaps others would finish off elements of the scene. It is also possible that he actually produced the entire painting himself but that insufficient documentation around the piece has led us to be unable to identify him as the sole artist that worked on it. Confusion over the precise source of artworks was common during the Renaissance because of the large numbers of assistants that many famous artists used, as well as the large amount of time that has passed since then, meaning much of the available evidence to aid identification has since been lost or damaged.
Luther is depicted in black clothing which is in line with his Protestant background. He sports a simple flat hat with jumper and coat which are so dark that it is hard to distinguish them from each other. He was attempting to live a simple life without unnecessary distractions as part of his strong religious beliefs. He was keen to set the right example to others too. It is possible that this particular artwork was a copy produced by hand within his workshop, after the artist himself had produced a similar earlier painting. This is supporting by some stylistic elements to the piece, which academics have argued are a little too rigid to have been directly from his hand alone. Cranach used his workshop to increase his output and allow more depictions of Martin Luther to flood across Germany in order to help promote his friend's social campaigns.
This painting was produced from oils on wood and is now owned by the The Metropolitan Museum of Art in New York. They have it dated at 1532 but it is not always on display, so worth checking ahead if you wish to see it in person. It was gifted to the institution by Robert Lehman in 1955, as part of his larger collection. They also own a number of other Cranach artworks too, including some highly complex drawings and woodcuts. An x-ray was completed on this original artwork some time ago but it did not reveal anything too extraordinary, just a few blemishes which would have been corrected later, perhaps from damage sustained to the piece earlier in its lifetime.