This question contains spoilers for the first part of the Mistborn trilogy (and maybe minor spoilers for the second part, not sure). You've been warned.
At the end of chapter 69 of The Hero of Ages we have this bit:
Marsh held up the sheet, flaring his tin to get a better look at its contents in the darkness.
Why would tin help Marsh see in the darkness, and why would he even have trouble seeing in the dark? The way I understood previous descriptions of how an Inquisitor's vision works, they're not dependent on light at all (I mean, they don't even have eyes...). By my understanding all they "see" are the blue Allomantic lines caused by burning iron and steel, but because their command of those metals is a lot more subtle than a normal Allomancer's they can see effectively like normal people (or better).
I also vaguely remember Marsh walking into a dark room before, probably at the Conventical of Seran.
Is this a rare slip by Sanderson, or is there a reason why an Inquisitor would need to enhance its vision in the dark by burning tin? Or did I just misunderstand how their vision works?
(I've finished reading the trilogy, but haven't read any other Cosmere works yet. If the answer to this spoils any major plot points in other works, please add a warning or use spoiler tags.)