Also, it might be worth considering what (IMHO … :spin: … but who cares?) “HDRI is all about.”
In the bad old days, light was captured by a voodoo chemical process involving silver salts and very-dangerous chemicals. Most importantly, film captured the entire image, all at once. And it did it within what turned out to be an extremely-narrow “dynamic range.”
Our eyes, on the other hand, constantly scan a scene, and the pupils of our eyes constantly adjust the “exposure.” The “final perception” is built-up in the über-voodoo world of our brain’s visual cortex.
Photographers learned to work within the severe limitations of film (if only because they possessed nothing else) in order to create images that our eyes would willingly accept. (Even though the popular term, “photo-realistic,” is actually positively comical in its implications.)
Video – both CRT-television and subsequent flat-screen technologies – have limitations that are very similar to film, although they employ additive rather than subtractive color.
The first “image file” formats grew from video and they were only concerned with display of a final-image that was undoubtedly scanned from a film source. They were also profoundly-concerned with file size. They were also engineered for the requirements of the display device, which was intended to be “as cheap as possible.”
“HDRI” generically refers to the notion of capturing light values (especially, in CGI work) as they actually are, without regard to the display-hardware that will eventually present the finished image to the audience. Whites can be “whiter than white,” blacks “blacker than black.” There is no(!) “absolute lighting reference,” and that, in fact, is precisely the point. We are, in fact, “merely dealing with very large files of numbers, to be processed as ‘mere data’ by a digital computer.”
Trouble is – you can’t “eyeball” this with your computer monitor, because this necessarily implies an arbitrary mapping of the underlying digital data(!) to colors on your monitor. Which is actually premature.
I suggest that you should simply regard “an HDRI image,” or set of images, to be what they truly are: “a digltal data-set.” When you seek to combine these data in order to produce “a visible image on your hardware,” realize that you are actually stuffing the data into a very-tiny and very-limited shoebox. (However, since this is a non-destructive process, it’s okay.)
HDRI is, shall we say, “a logical world.” The world of numbers in a digital computer. Concerns like “levels” and “contrast” are, on the other hand, physical concerns … firmly tied to the physical constraints of one particular chosen “output media,” such as film or (separately!) video, or (very, very separately) the printed page. You very-necessarily must deal with these issues on your way to (say) a movie-file deliverable, but these are (from the viewpoint of the computer) merely mathematical re-mappings of the original data stream(s), which you will now use to create a new digital output … suitable for presentation upon the aforesaid type of device … in a completely non-destructive way.