D800 sRAW and sRAW2

Started Mar 1, 2012 | Discussions thread
DigVis
Forum MemberPosts: 97
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Re: I don't need sRAW and sRAW2.... its silly
In reply to seanhoytphoto, Aug 3, 2012

seanhoytphoto wrote:

My two cents....

An sRAW is essential for many working professionals where a 36MP raw all day long is prohibitive. Not only does it chew up memory it also consumes an insane amount of time on the post processing end. A stupid simple solution would be to pixel bin 4:1. This would give me a 9MP image which is sufficient for 90% of my wedding/portrait work while simultaneously increasing DNR/SNR and reducing memory requirements. It's a simple matter of averaging the readouts via software coming from the ADCs. This is something they could add to the firmware today.

It is not as straight forward as you make it out to be.

The raw data contains 36M 14 bit samples = 63 MB. A 9 MP 14 bit RGB image would occupy 47 MB, a reduction in file size of only 25 %. Generating that 9 MP image would also require one to apply a decent demosaicing algorithm to reduce aliasing(moiré) (that is one of the advantages of the raw format lost). In order to get the most out of the subsampled image, one should also apply a slight sharpening prior to the subsampling, however that would make it even less raw.

In order to further reduce the amount of data, one can transform the RGB into a color space where the luminance is separated from the chromaticity and subsample the chromatic channels even further. Doing that perfectly would require the white balance to be known (yet another raw aspect lost), however one can get reasonable results even if the WB assumption is a bit off. By dropping every other chromatic sample, the resulting image is down to 32 MB, and by dropping three out of four samples, we are down to 24 MB. A lossless compression may reduce this by a further 10–15 %. We have now lost a significant amount of spatial information (the two chromatic channels are just over 2 MP each) and have something that is very far from raw. This basically corresponds to Canon's s/mRAW formats.

Now compare this to the D800 compressed 12 bit RAW format giving 29 MB 36 MP images, where almost no relevant information is lost, where the full spatial resolution is retained and where the result is still a proper raw file.

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