sRGB vs. RGB Locked

Started Apr 22, 2013 | Discussions thread
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John King
John King Forum Pro • Posts: 14,941
Re: Why use sRGB for JPEGs...

Gidday Mike

Mike_PEAT wrote:

As  you already stated, websites use sRGB and so do most consumer printers (drugstores, Costco, Walmart, etc.).

Here are some other facts:

- A JPEG image can have a total of 16,777,216 colours consisting of 256 shades of red, 256 shades of green, and 256 shades of blue, and that is true whether you use sRGB or AdobeRGB.

The difference being that with aRGB one can map into a 16 bit colour space.
However, one should never re-map an 8 bit image into a 16 bit colour space ...

- While AdobeRGB has a larger colour space or "gamut", each of the 256 shades of each colour are spread further apart in the spectrum; the difference between the darkest shade and lightest shade is larger, but there are still only 256 steps between the two extremes of each of the red, green, and blue colours, so the difference between each of those 256 steps is larger/coarser.  To visualize this hold up your hand...your hand represents an image of a sky with many shades of blue.  With your fingers tightly together there is a smooth transition between colours (fingers)...this is an sRGB JPEG image.  Now spread your fingers far apart and note the gaps between them where there are jumps between the shades of blue...this is an AdobeRGB JPEG image.

- With the steps between shades being larger/coarser in AdobeRGB, this can cause blotchiness in especially in skintones and skies, and this could cause someone to think that a lower colour depth or more JPEG compression was used in the image, giving a lower mark as a result.

This is only true if one is re-mapping an 8 bit image into a 16 bit colour space, specially a very wide gamut one such as ProPhotoRGB. Far less of a problem with aRGB.
I would never recommend re-mapping any 8 bit image into a 16 bit colour space; nor re-mapping a narrow gamut colour space into a wider gamut colour space. The results, as you rightly point out, are usually (always?) horrible ...

The converse is true if one is shooting RAW and mapping straight into a 16 bit colour space.

The problem is not in the absolute number of colours that can be represented. Most images do not have even a tiny proportion of 16M+ discrete colours in them.
However, the distance between colours can easily exceed even a 16 bit aRGB colour space.
Highly saturated reds, greens, yellows, aquas (cyans), purples, etc all fall into this category.

That is why I use a 16 bit ProPhotoRGB colour space from the moment I ingest any RAW file. The results are worth it - doing absolutely nothing extra, that is! HDD space is cheap. An image that is destroyed by its out of gamut colours not being captured is ruined for all time.

- Very few people know this, but most common monitors and laptop screens on the market are "TN" based (inexpensive) which only use 6 bits per colour, 64 shades of red, 64 shades of green, and 64 shades of blue for a total of 262,144 colours (they dither to mimic the 16.7 million colours), and of course are optimized for sRGB; this is 90-95% of the consumer/office displays on the market.  Only the more expensive monitors using IPS or PVA technologies are 8bits per colour showing the entire sRGB gamut, and fewer still are "wide gamut" that cover the entire AdobeRGB gamut.  (The reason why most LCD monitors are only 6bits per colour is to speed up response times of displays, the time it takes a pixel to go from light to dark, and reduce the ghosting that occurs with motion in video games and movies due to having process the extra colours...unfortunately with this reduction of viewable colours make these monitors unsuitable for serious photographers.)

Only too true. We both use ASUS ProArt PA246Q wide gamut (aRGB) monitors. They are IPS panels, 10 bit with a 12 bit colour lookup table (LUT). Connected using HDMI, which supports the high bit EDID in the monitors.

- Monitors are measured in NTSC colour (which similar to the Adobe1998 colour space).  Most TN LCD monitors (majority of the monitors on the market) can show only about 40-75% of the NTSC colour gamut (72% of NTSC is roughly equivalent of 100% of the sRGB colour gamut).  An LCD monitor using S-IPS technology (usually costing double or triple the cost of a TN monitor) can show about 80-90%.  Only the most expensive monitors labelled as "wide gamut" approach 100%.  Besides the monitor technology used is the type of backlight that lights up the display...most use fluorescent light which has a limited spectral range...the wide gamut monitors use LED backlights which don't have this limitation.

A very common problem. It also explains why many people see "noise" in images where there is none; and are critical of colour and exposure where there is no problem. I chucked out my wife's old monitor because it could no longer be calibrated acceptably. She now also has an ASUS as above.

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Regards, john from Melbourne, Australia.
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