Photographic color spaces – sRGB vs Adobe RGB
I recently posted a question regarding photographic color spaces
(sRGB vs Adobe RGB) on an Internet discussion board. The answers I got
were all over the place and frankly, never really answered my original
question. So I decided to do the work and determine the answer myself.
Researching on the Internet proved to be a matter of wading through
hundreds of rehashed opinions, very few of which were supported by
the technical facts that I eventually found.
So, on to the issue at hand … which is the better choice of color space - sRGB or aRGB?
There isn’t an easy answer to this question and to appreciate the answers, one must first fully understand the issues at hand.
This image shows the relative sizes of the sRGB and Adobe RGB colorspaces within the
color universe. The colors shown are not the actual limitations of each
colorspace but rather serve to represent the general directions where each
colorspace exceeds others.
What is a Color Space?
A rather terse definition is: a model for representing color in terms of measurable values, such as the amount of red, green, and blue in an additive color image. A little explanation of that definition is in order.
Consider all the colors that can be seen by the human eye. This group of colors is called the RGB color space. R standing for red, G standing in for Green and the B representing blue. With these 3 colors, most any shade of color can be reproduced; notable exceptions are the metallic colors.
Ok, we’ve established that RGB is basically all of the colors, an infinite number of shades, but there are limitations in the ability of cameras, scanners, computer monitors, printers, etc to reproduce all of these colors. This is where sRGB and aRGB (and other color spaces) come into play.
Your computer monitor is not capable of displaying some color shades, likewise your printer can’t reproduce some shades. The range of colors that each of these devices can deal with is that device’s own color space. The problem with this is that there are thousands of different models of monitors and printers out there, each with their own color spaces. Some sort of standards are required to make the colors of images appear the same over various devices. sRGB and aRGB are two answers to the need for those standards.
What are the sRGB and aRGB Color Spaces?
sRGB is the older of the two and is the Microsoft Windows default. The ‘s’ stands for ‘standard’. sRGB is almost always shorthand for the ANSI designation of: sRGB IEC61966-2.1. sRGB is basically the set of colors that can be seen on the average computer color monitor or color TV.
aRGB was established by the Adobe software company with the ‘a’ standing for ‘Adobe’. aRGB is variously referred to as Adobe RGB, aRGB, Adobe RGB (1998) and SMPTE-240M just to keep everyone as confused as possible. Not all colors in aRGB can be represented on even the top of the line color computer monitors.
Both of these color spaces are subsets of the RGB color space. This means that neither of them contains all the colors visible to the human eye. And this is where the controversy begins.
What are the differences between sRGB and aRGB?
Here we have to get into the way colors are recorded in JPEG images. In JPEG images, there are 24 bits allowed for describing a color. 8 bits are allocated to describe the amount of Red – 8 bits for Green – 8 bits for Blue. With these 24 bits, 16,777,216 different shades of colors can be described. Up to this point, sRGB and aRGB are identical – both describe the same number of colors – the difference is in which colors they describe.
The little diagram at the beginning of this article is a simplified graphic representation of the relative sizes of the RGB color space along with the sRGB and aRGB color spaces. In it, you can see that sRGB is a subset of aRGB – meaning that the entire range of colors represented by sRGB is included in the aRGB color space. But … size isn’t everything!
While aRGB can represent colors with more saturated blues and greens than possible using the sRGB color space, sRGB has the edge over aRGB in being able to accurately represent the colors that are within its range. That is because aRGB chooses to represent more different colors while sRGB can represent finer shades of difference in colors (leading to better accuracy).
Remember that both color spaces are limited to 16 million colors, the difference between them is in which 16 million colors they describe.
Should you use sRGB or aRGB?
Sorry, no simple yes/no response here but I will give a broad sweeping generalization: Only 1% of people who take photos will ever print their photos on a device that will reproduce the aRGB gamut of colors – in other words – sRGB is not just OK, but is preferred for 99% of the photos taken.
sRGB is the only choice for web presentations. sRGB is the standard - aRGB is not an option. Photos that are posted on the web are assumed to be in the sRGB color space and browsers display them that way. If you try to view a photo in the aRGB color space on a web browser its colors will appear washed out. The same is true for email programs (but not email attachements). Conversions can be made from aRGB to sRGB (at a loss in quality) if you need to post an aRGB based photo on the web.
If your camera doesn’t give you a choice of which color space to use, as is the case with almost all point-and-shoot cameras, then you can count on sRGB being the default. If you use Wal-Mart, your local drug store, K-mart, Costco, Wolfe Camera, etc for printing your photos – sRGB is for you – their printers expect and treat all submittals as using the sRGB color space. End of discussion unless these descriptions don’t fit you.
Now for that remaining 1 percent of photographers … I’m going to bet that many of you shouldn’t be using aRGB either! First off, if you have not done each of the following, you are shooting yourself in the foot by using aRGB:
Still not convinced?
Why not use a working space with the widest gamut possible? It is generally best to use a color space which contains all colors which your final output device can render (printer, web, or your own computer display), but no more. Using a color space with an excessively wide gamut can increase the susceptibility of your image to posterization. If you make large editing moves, you’ll find that your ability to do so without introducing posterization or color banding is more constrained by a large-gamut space than it is by a small-gamut space.
In the larger color space of aRGB, the limited number of shades that the file has may be spread out over colors that the output device can not reproduce. Thus, the shades will be spread over a larger range of colors that will increase the distance between the shades and the likelihood of posterization.
What about converting between color spaces?
Converting from sRGB to aRGB (or vice versa) is a mathematical operation, and therefore can be a source for interpolation and round-off errors. This is because some computer has to make decisions for you about how to represent colors in one color space that don’t exist in the other.
In short, converting from one color space to another causes image degradation, and the larger the differences between the color spaces, the greater the image degradation. An original aRGB or sRGB file is always higher quality than one that was created by converting from the other format. There is very seldom any reason to convert from sRGB to aRGB. You will not gain any greater range of colors. In general, avoid conversions if there is any way to do so!
When should aRGB be used?
If you shoot RAW and convert to 16 bit TIFF, skipping JPEG all the way, then, if you have your editing suite properly profiled, go ahead and use aRGB. You most likely know what you are doing and even if you don’t really benefit from using aRGB at least you will not be hurt by using it. Just remember to convert to sRGB for anything that will be displayed on the web or shared via email. Also always verify what color space is expected when sending out files for printing.
There are occasions when aRGB really is the best choice. There is a certain percentage of photographic scenes that contain colors outside the range of the sRGB color space. If you are a professional photographer using a fine art printer that is capable of reproducing these colors and have your computer system properly setup to work with color profiles and you are very experienced with PhotoShop and have a very critical eye for color, then aRGB is likely right for you. Also aRGB is highly recommended when it is required by whoever has contracted for your professional photography services. After all, the art director is always right!
So, on to the issue at hand … which is the better choice of color space - sRGB or aRGB?
There isn’t an easy answer to this question and to appreciate the answers, one must first fully understand the issues at hand.
This image shows the relative sizes of the sRGB and Adobe RGB colorspaces within the
color universe. The colors shown are not the actual limitations of each
colorspace but rather serve to represent the general directions where each
colorspace exceeds others.
What is a Color Space?
A rather terse definition is: a model for representing color in terms of measurable values, such as the amount of red, green, and blue in an additive color image. A little explanation of that definition is in order.
Consider all the colors that can be seen by the human eye. This group of colors is called the RGB color space. R standing for red, G standing in for Green and the B representing blue. With these 3 colors, most any shade of color can be reproduced; notable exceptions are the metallic colors.
Ok, we’ve established that RGB is basically all of the colors, an infinite number of shades, but there are limitations in the ability of cameras, scanners, computer monitors, printers, etc to reproduce all of these colors. This is where sRGB and aRGB (and other color spaces) come into play.
Your computer monitor is not capable of displaying some color shades, likewise your printer can’t reproduce some shades. The range of colors that each of these devices can deal with is that device’s own color space. The problem with this is that there are thousands of different models of monitors and printers out there, each with their own color spaces. Some sort of standards are required to make the colors of images appear the same over various devices. sRGB and aRGB are two answers to the need for those standards.
What are the sRGB and aRGB Color Spaces?
sRGB is the older of the two and is the Microsoft Windows default. The ‘s’ stands for ‘standard’. sRGB is almost always shorthand for the ANSI designation of: sRGB IEC61966-2.1. sRGB is basically the set of colors that can be seen on the average computer color monitor or color TV.
aRGB was established by the Adobe software company with the ‘a’ standing for ‘Adobe’. aRGB is variously referred to as Adobe RGB, aRGB, Adobe RGB (1998) and SMPTE-240M just to keep everyone as confused as possible. Not all colors in aRGB can be represented on even the top of the line color computer monitors.
Both of these color spaces are subsets of the RGB color space. This means that neither of them contains all the colors visible to the human eye. And this is where the controversy begins.
What are the differences between sRGB and aRGB?
Here we have to get into the way colors are recorded in JPEG images. In JPEG images, there are 24 bits allowed for describing a color. 8 bits are allocated to describe the amount of Red – 8 bits for Green – 8 bits for Blue. With these 24 bits, 16,777,216 different shades of colors can be described. Up to this point, sRGB and aRGB are identical – both describe the same number of colors – the difference is in which colors they describe.
The little diagram at the beginning of this article is a simplified graphic representation of the relative sizes of the RGB color space along with the sRGB and aRGB color spaces. In it, you can see that sRGB is a subset of aRGB – meaning that the entire range of colors represented by sRGB is included in the aRGB color space. But … size isn’t everything!
While aRGB can represent colors with more saturated blues and greens than possible using the sRGB color space, sRGB has the edge over aRGB in being able to accurately represent the colors that are within its range. That is because aRGB chooses to represent more different colors while sRGB can represent finer shades of difference in colors (leading to better accuracy).
Remember that both color spaces are limited to 16 million colors, the difference between them is in which 16 million colors they describe.
Should you use sRGB or aRGB?
Sorry, no simple yes/no response here but I will give a broad sweeping generalization: Only 1% of people who take photos will ever print their photos on a device that will reproduce the aRGB gamut of colors – in other words – sRGB is not just OK, but is preferred for 99% of the photos taken.
sRGB is the only choice for web presentations. sRGB is the standard - aRGB is not an option. Photos that are posted on the web are assumed to be in the sRGB color space and browsers display them that way. If you try to view a photo in the aRGB color space on a web browser its colors will appear washed out. The same is true for email programs (but not email attachements). Conversions can be made from aRGB to sRGB (at a loss in quality) if you need to post an aRGB based photo on the web.
If your camera doesn’t give you a choice of which color space to use, as is the case with almost all point-and-shoot cameras, then you can count on sRGB being the default. If you use Wal-Mart, your local drug store, K-mart, Costco, Wolfe Camera, etc for printing your photos – sRGB is for you – their printers expect and treat all submittals as using the sRGB color space. End of discussion unless these descriptions don’t fit you.
Now for that remaining 1 percent of photographers … I’m going to bet that many of you shouldn’t be using aRGB either! First off, if you have not done each of the following, you are shooting yourself in the foot by using aRGB:
-
Selected the aRGB color space (or RAW) as the output of your camera
Calibrated your high-end monitor
Have a profile for your printer (for each paper type you use)
Using color space aware editing programs through all steps
Still not convinced?
Why not use a working space with the widest gamut possible? It is generally best to use a color space which contains all colors which your final output device can render (printer, web, or your own computer display), but no more. Using a color space with an excessively wide gamut can increase the susceptibility of your image to posterization. If you make large editing moves, you’ll find that your ability to do so without introducing posterization or color banding is more constrained by a large-gamut space than it is by a small-gamut space.
In the larger color space of aRGB, the limited number of shades that the file has may be spread out over colors that the output device can not reproduce. Thus, the shades will be spread over a larger range of colors that will increase the distance between the shades and the likelihood of posterization.
What about converting between color spaces?
Converting from sRGB to aRGB (or vice versa) is a mathematical operation, and therefore can be a source for interpolation and round-off errors. This is because some computer has to make decisions for you about how to represent colors in one color space that don’t exist in the other.
In short, converting from one color space to another causes image degradation, and the larger the differences between the color spaces, the greater the image degradation. An original aRGB or sRGB file is always higher quality than one that was created by converting from the other format. There is very seldom any reason to convert from sRGB to aRGB. You will not gain any greater range of colors. In general, avoid conversions if there is any way to do so!
When should aRGB be used?
If you shoot RAW and convert to 16 bit TIFF, skipping JPEG all the way, then, if you have your editing suite properly profiled, go ahead and use aRGB. You most likely know what you are doing and even if you don’t really benefit from using aRGB at least you will not be hurt by using it. Just remember to convert to sRGB for anything that will be displayed on the web or shared via email. Also always verify what color space is expected when sending out files for printing.
There are occasions when aRGB really is the best choice. There is a certain percentage of photographic scenes that contain colors outside the range of the sRGB color space. If you are a professional photographer using a fine art printer that is capable of reproducing these colors and have your computer system properly setup to work with color profiles and you are very experienced with PhotoShop and have a very critical eye for color, then aRGB is likely right for you. Also aRGB is highly recommended when it is required by whoever has contracted for your professional photography services. After all, the art director is always right!