The logic of HDR for photos with bright and dark areas

Started 11 months ago | Discussions thread
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RossK
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The logic of HDR for photos with bright and dark areas
11 months ago

Hi, I wondered if someone could help with something that puzzles me about photographing with HDR (I’m just learning about DSLR after getting a Canon6D). I see the concept of 3 shots, one underexposed, one over exposed and the camera combining. But to me if you combine the two bright and dark extremes together you are back with the end result of a middle average like the middle exposure photo that had areas too dark and too bright.

In other words, if you feel part of your scene is too bright, to me it seems that by taking a second photo correcting it a bit, but also a third where you make it worse, the merger average takes it back to square one.

Or, is it the case that rather than overlaying the whole underexposed image on to the whole overexposed one as I’ve described, is the camera instead effectively taking the best of the 3 versions of the brighter area of the image and the best version of the darker area and combining them, like effectively sort of “stitching”.

In the field, the example I’ve had is photos of narrow canyons where the bottom of the canyon has no direct sunlight so to the eye the walls are brown and patterned, and higher up they are glowing orange cliffs thanks to reflected light, and then you see higher brighter cliffs towering up in the direct sun hundreds of feet higher.

However taking this photo it’s not what you get. Then the bottom of the canyon comes out pretty dark without its brown colour and walls are almost black, meanwhile the top is pretty bleached out and loses its lovely deep orange which becomes very pale and may even be just white.

If I were to expose correctly for the bottom to make it look nice, the problem is that the higher walls actually disappear in the highlight glare. But if I expose on the top of the cliffs in order to reveal the full height of the canyon and keep their real colour, the bottom becomes just a silhouette or even black.

Now I’ve read about HDR’s 3 versions. But if I get the too dark bottom and then combine a 2nd version that makes it better with a 3rd version where it’s been made even worse, does it not just average out somewhere back in the middle that I wasn’t too happy with at the start. And the same with bits that were a bit too bright before using the HDR.

Or have I misunderstood the capability of HDR and it actually ignores the photo areas where you’ve darkened the too dark bits and the other one where the too bright bits have been further brightened, and it is capable of combining just the improved versions of the two extreme halves of the image.  But I must say when I tried HDR I didn't get that much difference.

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