The helmet on the left is from 2005’s Revenge of the Sith. The helmet on the right is from the 1977’s A New Hope.
Note: These helmets are sourced from Master Replicas and are not actually screen used helmets, although the attention to detail on both helmets makes them good for comparison.
I have noticed that Darth Vader’s “widow’s peak” on the outer helmet drastically shifted from film to film. In A New Hope, it is very pronounced, lessened for The Empire Strikes Back, and then totally removed for Return of the Jedi.
It appears that if a scene requires Vader to appear sad, contemplative, or distressed, removing the widow’s peak allows for that interpretive expression that can then be filled in with context and music, as apposed to the widow’s peak which makes Vader continuously have a furrowed brow, making him look angry and hard to sympathized with him.
I think they did not bring the widow’s peak back into Revenge of the Sith because we’re still supposed to understand there is a human inside that chassis of darkness.
Before anyone cries about this being anachronistic, it isn’t. The costume shifts from every film. In A New Hope Vader wears his robes over his shoulder piece, then he never does it again in the follow up films. So it is safe to say that Vader’s style options were very limited over those twenty years in the iron lung, but he exercised his fashion agency to the fullest extent.