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Another comparison of Sony A7rII raw formats

ErikKaffehr

Well-known member
Hi,

I assume DNG uses Huffman coding that is lossless, need to check on that. Huffman coding is just more efficient data storage, every bit of data is kept uncompressing Huffman coded data.

Yes, it is lossless, virtually lossless is in a sense an oxymoron. On the other hand we may keep in mind that most things are approximate. Our representation of an image is approximate, the height of Mount Everest is approximate, although measured with a remarkable precision.

The number Pi cannot be represented exactly, but we can use any number of decimals to describe it, a common representation is 3.14159 a longer one can be found here: http://www.geom.uiuc.edu/~huberty/math5337/groupe/digits.html. Neither representation is exact, at least one of them may be good enough.

Raw images contain noisy data. If you would make two identical exposures the value of the very same pixel would vary quite a lot, just because of photon statistics. Say a pixel would have a value of 6000 in one exposure, next value could be 6075 and third exposure would perhaps 5925. The value would be distributed around 6000 with a standard of deviation of 77. 95% of the samples would be with two SD from 6000, that is 95% of the samples of the pixel would be between 5846 and 6144, just based on the statistical properties of light.

Best regards
Erik




Hi,

Using high ISO is essentially under exposure. The Sony A7rII is said to make use of an Aptina patent to improve SNR at 640 ISO and above, essentially by reducing full well capacity (*).

I did develop the images in DCRaw and into linear gamma space, and that didn't change the conclusion.

What I noticed is that with exposure pushed 5 EV the dark areas turn brownish in the compressed image while in the uncompressed they stay neutral. This also applies to the DCRaw conversion. I would also say a significant deviation, perhaps not visible in real world images, but very much observable in experiments.

I cannot explain the colour shift but it seems to be related to compression.

The reason I don't see artefacts on edges is probable that edge contrast is not high enough. The delta compression is lossless under a wide variety of conditions, but it will have artefacts if contrast is high enough, like on the star trails often shown.

I will check out 3200 as I still have the setup standing.

Best regards
Erik

(*) According to the said Aptina patent the photodiode is often connected to a capacitor to increase full well capacity (FWC). The voltage from the pixel is proportional to captured photons / FWC. In the Aptina patent the condenser is connected to the pixel trough a transistor that can disconnect the capacitor at a certain ISO, thus raising the output voltage from the cell.
 

Stefan Steib

Active member
For all the People who need to save and work on a file that is 5 stops under, the new 14 bit uncompressed is obviously a godsend.
For all the other ones , we (I am definitely included) mostly got a (kind) psychological treatment by Sony, taking customers wishes serious, even if they knew,
it would not cut the gordian knot of infinite image quality. So we are all still on the search of the holy grail, the next even better camera.

Good ! :thumbup:

This remembers me so much about the Open Raw initiative. That was........long ago.

Greetings from Germany
Stefan
 

dmward

Member
Someone has to explain why the 5EV exposure push is part of evaluating the benefit of a lossless file.

Naturally, if there is very little data in the photo sites that are near the black end of the tone curve, compressing that data is risky.

That's why, I thought, that Sony skewed its compression to the highlight side where there are many more bits defining photo site luminosity.

I've read a bit about the hips in the data curve as ISO increases, that again seems to be to protect data in the black photo sites.

It seems to me that pushing the curve up 5EV will, by definition, skew the data, even for uncompressed files.

Personally, I am much more interested in determining how much better the gradation is in highlights with the uncompressed file format. My quick evaluation suggests that there is and that justifies using it for everything in my view.
 

Wayne Fox

Workshop Member
For all the People who need to save and work on a file that is 5 stops under, the new 14 bit uncompressed is obviously a godsend.
Seems like it’s quite common for some parts of an image to be 5 stops underexposed. Many exposures are capturing scenes with large dynamic ranges, which means that setting a “normal” exposure will leave parts of the scene 5 stops “underexposed”. Sort of the wrong term, but certainly if we based the exposure on that particular part of the scene we would open up the exposure 5 stops to get a normal exposure.
 

Wayne Fox

Workshop Member
Personally, I am much more interested in determining how much better the gradation is in highlights with the uncompressed file format.
I would agree, although as I have mentioned in a previous post or thread, I struggle with “color crossover” between highlight and shadows when working with my a7r files. I’m hoping the new lossless raw will help that as well.
 
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