There are 4 factors that greatly affect the way Vader's helmet looks in ANH:
1.) The camera pedestal height - Pedestal is a term used in TV (what I do) and film production, it refers to the height of the camera in relation to the ground. Vader is often shot with the camera below his eye level, this is a camera trick used in all forms of photography. When you shoot up at the subject it makes them appear large, dominant, and powerful, these are all characteristics that they what the audience to perceive in Vader. And consequently we see under his dome and can see his brow often.
2.) The lighting - When Vader is lit from above, the shadow of the dome covers his already dark brow, so that its effectively invisible. If he's lit from the side or from below, we can see the brow, because the light is reflecting off the thin line of gunmetal paint along the edge of his brow.
3.) The wacko, crazy, all-over-the-map, placement of the dome throughout ANH - As many of us, and others that take the study of Vader's costume to an almost academic extreme have observed, consistent helmet placement was far from a priority in ANH. In some scenes it appears that it's obstructing his brow, in others it almost looks like it's going to fall off the back of his head LOL!
4.) The tilt of Vader's head - Sometimes his head is tilted so far back, it's like he's looking for a buddy in a crowded bar. In other scenes he looking down at the camera or actors. As I've stated in the other factors, the angle of his head has a major impact on how the dome looks in relation to the viewing angle of the observer.
So taking those four factors into consideration, after weeks of research and tearing my hear out trying to come up with the elusive "accurate" helmet placement, I've decided to be a normal person, and just position it so that it looks good to me. I'm the only one I need to satisfy anyway. To me he looks the most correct when the brow is just barely obstructed, so that's the look I'm going with and I will never criticize anyone else's choice, because no matter where you choose to place it, you can probably find a screen grab to support your choice, and if not, so what, it's your figure, do what you want.
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