Thursday, August 13, 2015

About Pureview

It's a little sad that the 2012 Nokia 808 is a better cameraphone for still photography than any of the flagships from Apple, Samsung, HTC, Sony, etc. The 808 is a bit of a specialized beast in that it has a relatively large sensor coupled with an extremely high pixel count. In use it's a lot like using a medium format camera with ISO 25 slide film, i.e., relatively low exposure latitude and very fine detail in certain conditions. The sensor is still small and noisy even at its lowest ISO 50 setting but it isn't uselessly noisy. It can resolve over 18MP (38MP nominal) in good conditions which is higher than most dSLRs. The Galaxy S5 resolves about 6.5MP (16MP nominal).

You aren't using the system at its best if you use the PureView modes. This is something I've argued in the past but I thought I'd post some evidence to back up my claim. Here are tripod mounted crops at ISO 50 at various settings. First off is at 38MP taken with -5 sharpness, ultrafine, 4:3, +0.7 EV. 

38MP original

I added sharpness afterwards because sharpness filters don't actually add in real detail. They do however introduce noise which is information that JPG compression has to allocate bits for. These are bits that are taken from actual image detail. All images are displayed in lossless PNG.

38MP Unsharp Filter

The Pureview image is simply not as sharp, even after applying an unsharp filter. At first I thought it might be focusing error but when downsampling the 38MP file to a quarter the area (9.5MP), we see that the detail levels are comparable. I'd examined other parts and the focal plane is essentially the same in both the Pureview and 38MP image (as close as can be given no manual focusing controls).

8MP Pureview

38MP downsampled to 9.5MP

Applying unsharp helps but the file is much less forgiving than the 38MP file. Increasing sharpness brings out ugly JPG macroblocks as well as aliasing artifacts. Although it is clearly inferior to the 38MP version, the decrease in quality is mild compared to the huge reduction in file size (14MB vs 2.5)

8MP Pureview Unsharp

Automatic mode produced, alongside the default exposure 0.7EV lower than the previous images, a file of around 1MB. Detail clearly suffers.

Is there anything Pureview offers in the signal chain that makes it superior to simple resizing? Not to my eyes. Anyone is welcome to apply their own denoising and sharpening methods, but given the 38MP file's greater tolerance to editing, it is likely the quality delta will only increase.

Given the fact that a 64GB microSD card is only $25 and can hold well over 4000 images at the full quality setting, there's little reason to use the PureView modes for still photography. 

And just to drive the point home: Pureview vs the original - after applying sharpening to both.

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