Why are zoomed HDR photos so small?

[debs_droidstuff]

Just got back from a trip and was very curious about the photo sizes my husband took with his Nexus 5X. The resolution setting for the back camera was set to 4:3 12.2 megapixels and was never changed. That was the only camera he used. The only setting he changed was whether or not to turn on HDR.

Out of 277 pictures taken 177 of them ranged from 3-6 MB in total size each.
36 of them were less than 1 MB in size!

After a lot of exploring the Exif data and some experiments I figured out

If you turn on HDR and you zoom in as far as possible, the photo will wind up being less than 1MB in size with dimensions about about 756 x 1008 (height x width) in pixels.

I tested this at home. Here's what I got for dimensions taking pictures in the same location and same lighting:

• No HDR, no zoom: 3024 x 4032 (3.2 MB file size)
• No HDR, fully zoomed: 3024 x 4032 (2 MB file size) * yes the file size was different
• HDR, no zoom: 2992 x 4000 (3.1 MB file size)
• HDR, fully zoomed: 756 x 1008 (362K file size)

Since the phone uses a digital zoom, it doesn't surprise me that the files sizes shrink when zooming but I'm completely puzzled about the difference between HDR vs Non HDR. And a phone that claims to provide 12 megapixel photos only getting 1/12th that size when using HDR is very disturbing.

Note: I cannot figure out the difference in the file size for the non-HDR zoomed vs not zoomed files either. I explored the EXIF data (using ExifTool) and there is no data stored within the file that would explain the different.

 

[LeoRex]

Just a guess... When you are zoomed in, you're not at 12mp. I suspect that fully zoomed in, your using 756x1008.

So the HDR+ size is actually closer to the true size of the photo. Not sure why the non HDR is that high.

 

[PhiPsi32]

That's how digital zoom works.

 

[debs_droidstuff]

Yeah that was my original thought but it just didn't compute given that the file size and resolution of the non-HDR photos was so high. HDR has some sophisticated software algorithms that merge together multiple exposures to extend the dynamic range. If I had to guess, I would think that the HDR photos would actually have more data to work with than the non-HDR ones but that doesn't seem to be the case. It would be nice if some Nexus expert could provide some kind of explanation. Meanwhile I think it's a good idea to avoid zooming when using HDR unless you are happy with tiny files. They are so small they don't even fill a typical 19" PC monitor.

 

[flintlock]

Digital zooming is merely cropping. It isn't really zooming. If you don't zoom at all you can crop later and take your time choosing the crop. Real photographers rarely produce final prints that match the original capture area. They crop later. Of course you don't want to waste too much negative--or pixels.

High dynamic range as implemented in cell phones is a joke.

 

[LeoRex]

Exactly... If you want to zoom in, do it after the fact with a editing tool, because that's all that's happening. Between the two files, the HDR+ file is the true size of the file.

Here is what I'm guessing... When you zoom in and take a regular picture, it takes the shot and then starts multiplying pixels to maintain size consistency with the other pics. Not sure why, but stranger things have happened. HDR+, on the other hand, is a specially processed image, not just slapped together exposure bracketed shots like most phones. So when it does it's thing, it just leaves it at the original resolution.

Again, the non-HDR shot is the one with the wrong sized file.