freakyclean
05-16-2002, 10:28 PM
This isn't really a pre-press topic but I'm going to approach it from a printing point of view so I'm posting it here.
There are many different colour models with RGB and CMYK being the most common. Each colour model has a gamut or map of colours that it can "show". The human eye is often used as standard, with RGB and CYMK having successively smaller gamuts. This means that CMYK cannot show all the colours RGB can. RGB is used for online images and Photoshop working files and CMYK is used for printing. Why would you use CMYK for printing when RGB shows more colours? Well RGB is not just made up red green and blue but it's also emits light, something you can't do with paper. CMYK was created to economically show the widest gamut on paper. There are other printing methods that will show a wider gamut like hexachrome which uses six colours (Yellow, Orange, Magenta, Cyan, Green and black) they are more costly than CMYK. A hexachrome job for example would cost more than 150% of a CMYK job.
Just for your info photographic film has a slightly larger gamut than RGB in the same shape.
Here a quick run down of the different gamuts: RGB is similar to the human eye in registering blues and reds but is far poorer in the greens. CMYK is poor at producing middle blues, reds particularly orange and middle greens than RGB but produces better magentas, purples and yellows. You can see a graphic sample here: http://www.nla.gov.au/photos/digital/sld017.htm
Just to confuse things there are more than one RGB specification and more than one CMYK specification. These specifications, called profiles, map colours differently each of them covering a different part of the colour spectrum. I (and others) recommend using the Adobe 1998 RGB profile and for CMYK I use a U.S. sheet fed coated or uncoated profile depending on the job I'm working on. Your choice of CMYK profile will depend on how you are outputting your job. For example if you are using a colour laser printer then you should be able to get a profile from the manufacturer.
As far as working with an image in Photoshop it's generally best to work in RGB and only convert to CMYK when all the adjustments have been made and it's ready for output. The reason for this is that RGB holds more colour information than CMYK and converting to CMYK and then making adjustment will degrade image quality. There are also certain features that are not available in CMYK as in RGB most notably some filters. When converting a Photoshop RGB image to CMYK you should always flatten the file first as this will affect the colour in the file.
Part 2 to follow
- monitor calibration and other stuff you didn't learn in grade 5 art class
:)
There are many different colour models with RGB and CMYK being the most common. Each colour model has a gamut or map of colours that it can "show". The human eye is often used as standard, with RGB and CYMK having successively smaller gamuts. This means that CMYK cannot show all the colours RGB can. RGB is used for online images and Photoshop working files and CMYK is used for printing. Why would you use CMYK for printing when RGB shows more colours? Well RGB is not just made up red green and blue but it's also emits light, something you can't do with paper. CMYK was created to economically show the widest gamut on paper. There are other printing methods that will show a wider gamut like hexachrome which uses six colours (Yellow, Orange, Magenta, Cyan, Green and black) they are more costly than CMYK. A hexachrome job for example would cost more than 150% of a CMYK job.
Just for your info photographic film has a slightly larger gamut than RGB in the same shape.
Here a quick run down of the different gamuts: RGB is similar to the human eye in registering blues and reds but is far poorer in the greens. CMYK is poor at producing middle blues, reds particularly orange and middle greens than RGB but produces better magentas, purples and yellows. You can see a graphic sample here: http://www.nla.gov.au/photos/digital/sld017.htm
Just to confuse things there are more than one RGB specification and more than one CMYK specification. These specifications, called profiles, map colours differently each of them covering a different part of the colour spectrum. I (and others) recommend using the Adobe 1998 RGB profile and for CMYK I use a U.S. sheet fed coated or uncoated profile depending on the job I'm working on. Your choice of CMYK profile will depend on how you are outputting your job. For example if you are using a colour laser printer then you should be able to get a profile from the manufacturer.
As far as working with an image in Photoshop it's generally best to work in RGB and only convert to CMYK when all the adjustments have been made and it's ready for output. The reason for this is that RGB holds more colour information than CMYK and converting to CMYK and then making adjustment will degrade image quality. There are also certain features that are not available in CMYK as in RGB most notably some filters. When converting a Photoshop RGB image to CMYK you should always flatten the file first as this will affect the colour in the file.
Part 2 to follow
- monitor calibration and other stuff you didn't learn in grade 5 art class
:)