For free I got the HDR Camera app, which has some aggressive tone mapping. Some halos are seen. Less subtle than built-in HDR.
Heres an example of HDR Camera:
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© Benitoite [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. Heres built in HDR on the 2nd gen ipad mini:
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© Benitoite [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. Overall I can tell the tone curve is brighter, showing some detail in the dark areas. Built-in looks generally compressed in the highs, but less detail in the darks. I did not adjust any settings in the app, although there are separate adjustments possible for highlight and shadow compression
I wonder how the non-free apps do against these.