I am going to speak from the type of subject matter that I photograph most often. I think the illusive term "soul" is not indicative of luminosity. Luminosity is a technical matter. I think that soul is about images that engage. I think that in order to produce photographs that engage we must first become engaged.
By that I mean that we must become actively involved it the act of seeing to the level that we are not just producing images about "things". When we do this then we are seeing the "nature of things" and are capable of making images about the "nature of things".
Far too often I look at something and my mind fills in the "rest of the story". That is I see a tree crested hill with the shimmering grass of the meadow below (for example). And what occurs is that my mind says "oh, but I have seen this before and this is how this looks". When this occurs I cease to see. As someone said "I don't photograph because I have a more critical view". That "critical view" is my mind "filling in the rest of the story".
When I recognize this critical or judging aspect within myself and can put it aside then I see some really interesting things. I begin to see the "nature of things". If I could just for one moment take on the vision of my first hour of life and portray my impressions of that which is around me then I would be a rich photographer. Why? because I will be portraying that which most of my fellow man have ceased to see.
If this "nature of things" isn't "soul" then I don't know what is.