I don't know why there would be prior art. You can have numerous methods to achieve the same result, so just because cameras have had HDR doesn't mean there's prior art. Nobody can patent HDR itself, but you could patent your "specific method" of implementing it.
This is just like when it was discovered Apple filed for a facial recognition patent. Everyone was whining "how can Apple patent facial recognition". Apple wasn't trying to patent facial recognition (which would be impossible), they patented a specific method for doing it. If you search the USPTO you'll find numerous facial recognition patents. Some for still images, some for video, some need bright light, some work in low light, some are highly secure, some require a single frontal picture, some require multiple pictures from the front & sides and on and on. Apple's version was a low security system that required minimal processing power suitable for portable devices and their patent specifically stated that.
Yet people still whine on thinking Apple was trying to patent something that already existed (facial recognition).