How to create HDR from bracketed shots on your iPhone
Since I switched to mobile photography in 2014, I experiment to recreate some HDRs from bracketed shots that I took a "big camera". The best software I found so far for creating HDRs on an iPhone/iPad is Pro HDR X [iTunes App Store Link]; it works surprisingly well if you know its limitations.
Pro HDR X can create HDRs from up to three bracketed shots from your camera roll:
Tap the menu button
Then the button with the two overlaying images. A dialog appears asking you to chose up to three images from the camera roll.
Pro HDR X does not use the default iOS8 photo picker. That results in an annoying behaviour if you use Apple iCloud Photo Library with "optimize storage on device" enabled: You might not see photos in the photo picker that have not been downloaded. If you encounter that behaviour:
Tap "Cancel" in Pro HDR X to close the photo picker.
Go to the camera roll and tap each of the three photos to view them full screen and to force iOS8 to download the full resolution photo from iCloud.
Switch back to Pro HDR X and tap the icon to launch the photo picker again.
Now you should see the three bracketed photos. Select them, and you'll see the screen below. Note that if the preview of the HDR looks blurry, wait for a few seconds. Pro HDR X is resizing and aligning the photos properly and then automatically updates the preview.
The top three sliders on the right control tone mapping, brightness and contrast. The next three sliders are for saturation, temperature and tint. I got the best results when I made the HDR slightly brighter and then adjust the final image using iOS8 adjustments.
There are two more issues that you should be aware of if you plan to use Pro HDR X to create HDRs from imported photos:
You may encounter crashes if you try to open photos larger than 16 MP on the iPhone. On an iPad 2, I could create HDRs from 24 megapixel photos.
Pro HDR X does not de-ghost.
With de-ghosting, HDR software eliminates motion from bracketed shots. If you have e.g. bracketed landscape shots where leaves moved in the wind, you'll see a "ghost" in the final photo created.
So when you use Pro HDR X [iTunes App Store Link] to create HDRs, make sure:
You use a tripod to avoid camera shake.
Shoot a scene with no movement as Pro HDR X does not do de-ghosting.
Note: In a previous version of this post, I stated that Pro HDR X downscales the photos to 11 MP. It seems that this was wrong. I have to investigate this further.