What are the downsides to using iPhone for HDR?

Question

HDR was added to the iPhone in IOS 4.1:

Take great photos that capture a wider range of light intensity using the new high dynamic range (HDR) setting on iPhone 4, which automatically combines multiple exposures into a single HDR image.

So it seems that this is using the multiple image exposure blending approach to HDR. Other than a reduction in contrast, are there any other side effects of this process to keep in mind when choosing whether to use HDR on the iPhone?

Answer 1

Quality / file size

If the camera saves the image in a format with a greater bit depth, the files will be larger. If it on the other hand saves it in the same format as regular images, the image quality will suffer in some conditions, as it has to squeeze a larger dynamic range into the same number of bits.

Answer 2

The biggest issue I see with multiple exposure photos is that anything moving in the picture (or a shaky hand) will add aliasing/"ghost features" to the resulting photo.

Depending on the algorithm (I have not tried the HDR feature), this may be more or less prominent. For example, a bird flying through the photo could, with a clever algorithm, be cancelled out from the sky for 2/3 exposures. The bird wouldn't be HDR, but you would not get three transparent birds in one shot.

Example of a HDR photo with ghosting and how the photo would look without ghosting:

(From this article)

Answer 3

I think that iPhone HDRI implementation could be an Exposure Fusion process, rather a true HDR + Tone Mapping process that would lead to a much more artifact result and would require much more CPU power to be processed.