DPR's DR test plot - JPEG - why 'S' shaped?

Started 9 months ago | Questions thread
Dave_J_E Regular Member • Posts: 139
Re: DPR's DR test plot - JPEG - why 'S' shaped?

xpatUSA wrote:

Ellis Vener wrote:

Hi, the last time that article appears to have been updated was 2006 and it only applies to JPEGs with the camera’s “standard” processing “style” processing.


Although I did not see it mentioned, I think it is safe to assume that the sRGB color space was used. The sRGB color space has a unique tone curve which rises steeply from zero, approximating a gamma of 2.2 in the mid tones, and flattening out slightly in the highlights.

Agreed and I am familiar with the sRGB tone curve.

As to why the “S” shaped curve? It’s a close approximation to how a healthy human vision evolved: Not so well in the dark and attenuated in the bright light.

Sorry, I don't fully understand that, although I am aware of the approximate log response of the human eye. My lack of understanding stems from gamma curves not being S-shaped.

If you want a more straight line tonal response, shoot raw and output the processed results as 16-bit per channel TIFFs using the Pro Photo RGB color space.

Yes already examined with RawDigger raw channel outputs both with 2.2 gamma (can't set 1.8 in RD) and without - but thanks for the tip.

Hi Ted

I believe there are two tone curves at play here, The first is an S shaped TC applied during raw conversion to improve global contrast (linear looks a bit flat), The second is the sRGB tone curve (gamma) which would have been applied when the jpeg was created. You won't see the effect of this when viewing the image but it will show up in the measurements of the jpeg values.

The two diagrams below show these curves with linear and log2 x axis using a gamma of 2.2 and a typical Adobe standard TC (I don't have details of the Canon stuff). The purple one compares to the curve you cited.


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