PDA

View Full Version : Changing from sRGB to Adobe RGB



Michael
11-02-2009, 3:55pm
HI ,

Been trying to get my head around the colour spaces, 8 bit, 16 bit, 32 bit stuff and want to know peoples thoughts on changing from one to the other, especially if it's say from an 8 bit sRGB file to say a 16 bit Adobe RGB or even ProPhoto RGB, can it be done? I can see that downgrading from a 16 bit Adobe RGB would be possible but am wondering if it works the other way around. Wouldn't there be too much interpolation by the computer to add extra shades of colour where it wasn't?

Should I just continue with an sRGB colour setting, I have numerous slides that I have had converted to digital and most are 8 bit sRGB files, some 16 bit sRGB. I know for Web stuff sRGB is the choice.

What about my G10? the specs say it has an sRGB colour space, but I thought that if I saved files in RAW format that they did not have a colour space embedded, am I wrong with this? I thought that a RAW file could be anything that you wanted, maybe I am wrong here.
Be nice to find an easy to understand article or articles on this subject, can anyone point me to something. I have googled this subject and as many results there are there are an equal number of interpretations on this subject.


Mick

ricktas
11-02-2009, 4:13pm
You can change colourspaces, but you cannot 'create' colour where it didnt exist. So going from sRGB to AdobeRGB (adobe being the bigger colourspace) isnt really going to achieve anything.

On the other hand, most DSLR are capable of shooting in AdobeRGB, and then can be reduced to the sRGB space. RAW files do have a colourspace, it is that colourspace that determines the colour data that is in the RAW file. The colourspace can be set in camera (as per previous statement).

Michael
11-02-2009, 4:46pm
Thanks Rick,

That's exactly what I thought regarding sRGB to Adobe RGB, what about 8 bit to 16 bit, I guess it's the same, trying to create info where there is none?
As for the RAW thing I obviously misunderstood what I read about the colour space not being embedded until you tell it when saving to .PSD, .TIFF or whatever.
Unfortunately the G10 does not allow to change the colour space so I guess it's sRGB all the way.
Lots to learn.

Thanks for your help

Mick

ricktas
11-02-2009, 5:55pm
Yeah you can go from 8 bit to 16 bit, but you are creating 'false' data, it is built using the software to fill in the gaps.

sRGB is not a bad colourspace in anyway, it produces nice vibrant colours, and the internet is built on it. You could say it is the Standard! There are bigger colourspaces, but also there are few other things to consider. Most monitors are sRGB, so unless you get a wide gamut monitor (lots of $$$) that is built for AdobeRGB, you can't see some of the AdobeRGB colours anyway. Most printers work in the sRGB space as well, as do most laboratories.

AdobeRGB is probably going to come into its own in the next few years as wide gamut monitors drop in price, and thus become more prolific etc. But for now sRGB is a lot more than adequate.

jev
11-02-2009, 7:09pm
As Rick said, just converting from sRGB to AdobeRGB won't bring do any good (even worth: it will degrade color accuracy) but as soon as you start editing you actually may benefit from the larger color space.

From 8 to 16 bit you prolly won't add information, but you won't lose any either. OTOH, the software that converts may actually detect gradients and be able to actually increase accuracy. Again, if you start editing, depending on the things you do you may benefit from the extra color resolution.

Michael
11-02-2009, 8:07pm
So I'll just continue working in sRGB for my scanned slides, at either 8 or 16 bit as they have been saved.
But I am still unsure about RAW files and their colour space, take a look at the attached video link, http://www.lightroomkillertips.com/2008/video-lightroom-photoshop-and-color-spaces/ isn't this guy saying that we can assign whatever colour space we like to it because it is a RAW file, we can even save it as ProPhoto RGB if we like. Even though we may not be able to print in ProPhoto yet the file will be capable in the future.
Rick, when you say most DSLR's can shoot in AdobeRGB, (which I'm sure is the case)doesn't that just mean that they are saving JPEG's etc. in Adobe RGB and not the RAW file?

Please correct me if I am misunderstanding this as I want to make sure that I am saving all my RAW files in PSD or whatever in the highest colour space available.

ricktas
11-02-2009, 8:39pm
if your camera can shoot in AdobeRGB, then use that colourspace (its bigger).

A colourspace is basically the range of colours a photo can contain. So say you have a basic digital camera (hypothetically) that can show only 256 colours, it would have maybe 20 shades of blue, 20 shades of red etc. Then take sRGB, that might be able to show 1400 shades of red. Adobe RGB is bigger again, and might be able to show 1800 shades of red. This is where bigger colourspaces come in.

Your camera sounds like it is set to sRGB and cannot be changed, so for you, the best colourspace to work in is sRGB. RAW files do contain colourspace information. So not only do the JPGs have a colourspace, but so do the RAW file. By including the colourspace in a file (RAW or JPG), it is telling photoshop, lightroom etc, how much information is in the file.

Lightroom works by default in the ProPhoto colourspace, which is a massive colourspace. You may at times notice the difference in the way your photos look in lightroom to CS4, that is the reason.

jev
11-02-2009, 9:51pm
A colourspace is basically the range of colours a photo can contain. So say you have a basic digital camera (hypothetically) that can show only 256 colours, it would have maybe 20 shades of blue, 20 shades of red etc. Then take sRGB, that might be able to show 1400 shades of red. Adobe RGB is bigger again, and might be able to show 1800 shades of red. This is where bigger colourspaces come in.

Actually, that is not the case. As long as we're talking 8-bit colors, AdobeRGB will be able to show just as many shades of red (or green or blue) as sRGB - they will look different though. Color accuracy ("the number of shades") is defined only by color-depth. 16-bit color depth can describe 65536 individual shades whereas 8-bit color depth describes 256 shades. The colorspace just describes how intense the colors are when on their maximum.

That is what is left out in the aforementioned video too. Although it seems like a good idea to always take the biggest colorspace and work down from there, you actually throw away accuracy. The step Prophoto -> AdobeRGB and than back might result in banding and similar issues unless the color depth is increased too.

Michael
12-02-2009, 10:51am
Jev, from my understanding what your saying is correct that if you tried going from ProPhoto to Adobe or just plain old sRGB and then back again you would most likely end up with banding, but going from Adobe RGB to sRGB would be fine, just dont go the other way.
Rick, still not sure I agree what your saying about RAW files, the colour space (sRGB or Adobe RGB) in camera is only applied to the JPEG format if that's what your saving your files in camera as, the RAW file on the other hand does not have a colour space applied to it until you assign one when your exporting from either Lightrooms ACR to CS4 or your editor of choice, or from CS4's native ACR (essentially the same program).
When you say the RAW file contains colour space info I guess it does but only when you assign it one when exporting to your editor or just saving as a .TIFF or .JPEG files.
I guess all I want to do is make sure all new shots that I take in RAW are saved once exported into my editor (or just saved) in the highest colour space possible to future proof them for want of a better word, and you can do this with a RAW file, well that's how I read this anyhow.
As you said Lightrooms default (and only) colour space is ProPhoto.

jev
12-02-2009, 10:59pm
Okay, a little list of side effects that may happen when you convert from one color space to another. For the sake of argument, I use AdobeRGB and sRGB, but something similar is happening when converting to/fro ProPhoto or Adobe's Wide gamut.

Conversion from large gamut to smaller (from AdobeRGB to sRGB):
- colors may be clipped (because they fall outside the smaller gamut)
- accuracy is retained

Conversion from small gamut to larger (from sRGB to AdobeRGB):
- colors are retained
- accuracy is decreased

Now, if you don't do any edits in the smaller gamut, the decreased accuracy can be considered nil - you won't notice a difference in the results. But if you repeatedly go from one space to the other and back, doing edits in between, you might at some point see banding. Banding just is the result of (repeated) rounding errors and rounding errors get smaller if you increase accuracy.

Thus, if you use LR in combination with PS, why would you do the conversion to AdobeRGB at all? Why not keep working in ProPhoto in PS as well? At 16 bit resolution?

Colour spaces in RAW files don't exist. That is, the sensor still responds in a characteristic way to light. Therefore, a colors space *does* exist at this level, it just is not added in the RAW file. Instead, it is assumed to be known by the RAW processor. Compare it to speed; if a cop pulls you over when you're speeding and he says: "you're doing 130 here where the limit is 100" you know he's talking km/h and not furlongs per fortnight. The RAW convertor will convert from the intrinsic colour space to a working colorspace and than work in that.

Michael
13-02-2009, 10:00am
Hey Jev,

Sounds fair to me, when you say a RAW file has a colour space are you saying that it has already been desigated one such as sRGB or Adobe RGB in camera prior to you assigning it one when saving or going to your editor? Simply speaking (that's me) what I think you are saying is that the RAW file has the ability to fall into any colour space we designate it?, so that if we save or export this RAW file with ProPhoto RGB colour space embedded, it actually has the full gamut of colours that ProPhoto RGB can produce, is this how you understand it?
If this is actually correct I have to ask myself if it is actually better to save these RAW files with ProPhoto RGB embedded or not (seeing as printing in this colour space is very limited if available at all), as you correctly pointed out if we do and then print in sRGB or even Adobe RGB wont we be clipping some of these colours and if that is correct wont we also get some banding?

jev
13-02-2009, 8:51pm
The RAW image has it's own color space, not sRGB or AdobeRGB. That color space, however, usually is not part of the RAW file (I think there are a couple of camera's that actually do embed ICC information in their RAW files, but I didn't check).

The intrinsic color space of the RAW image is defined by the capabilities of the sensor. If you look here (http://www.luminous-landscape.com/tutorials/prophoto-rgb.shtml) and scroll down to figure 3, you can see that the Canon 20D's profile overlaps AdobeRGB all the way. That means you might loose some color information when converting from RAW to AdobeRGB. If, however, you choose ProPhoto (move your mouse cursor over the image), you'll see that allmost all colors that the 20D can generate fall within the ProPhoto color space and than has some room left. Other camera's may have a slightly different color gamuth. If you have a VRML plugin installed on your computer, you can play around with color spaces, overlay them and see what fits what here (http://www.drycreekphoto.com/tools/printer_gamuts/gamutmodel.html).

Michael
14-02-2009, 9:35am
Thanks very much for that Jev, I wasn't too far off the mark, maybe didn't explain it quite right. Whilst the colour space of the sensor isn't quite as big as ProPhoto we are better off saving our files in this colour space otherwise we are losing colour info as the Adobe RGB colour space is smaller than what our sensors are capable of. Maybe not quite that simplistic but shouln't be far off the mark. I'm sure there is more to it but atleast I'm on the right track.