Images you use for professional projects typically start from high-quality source material, such as digital-camera photos containing several megapixels, or a high-resolution scan of film or a print. As you adjust and refine the image in Photoshop, you might pile on the layers, masks, and channels, increasing the file size. When the time comes to hand off the document, and it needs to be in a different format, you convert the document to the required final format and you're done. For example, if you were preparing images in the CMYK color mode for press, you might simply archive the CMYK files used on the project.
However, preparing a document for a specific medium can remove much of the flexibility in the original document, often flattening layers or reducing the range of colors. For example, if you save an RGB document in CMYK color mode for press, you lose ranges of colors that can't be reproduced in CMYK. That's fine if the document will only be used on press in the future, but it restricts the color quality of the document if you later want to make an RGB version of the document for the Web or television. If you do expect to use a document for multiple media, it's better to maintain a master Photoshop document that isn't specifically tied to any medium. (Of course, if you always work in one medium, or if you are doing a one-time project where the content is unlikely to be reused, you may not be concerned with the ability to reuse Photoshop documents for different media or future projects, and therefore maintaining a master document might not be a top priority.)
If you do want to save a Photoshop document that's ready for all kinds of future uses, I'd recommend saving a master document that includes:
The largest dimensions you expect the document to need in the future. Typically, this can simply be the resolution of the source document. In other words, if you use an 8-megapixel camera to produce a 200 x 300-pixel portrait for a Web page, and you think the same portrait will probably be needed for next year's printed annual report, you definitely want to hang onto the original 8-megapixel document.
Layers and masks. Preserving layers lets you alter the composition for a future use. Preserving masks lets you maintain the transparency of a layer in case you want to use a layer over a different background in the future or restore areas of a layer that are currently hidden by the mask. It's OK to merge layers that you don't think you'll need to rework in the future, but avoid flattening the entire document unless it's required.
The largest color space you expect the document to need in the future. For most uses, the choice comes down to sRGB or Adobe RGB, while demanding photographers may archive using ProPhoto RGB. For the rundown on color spaces, see the nearby sidebar "What's the Right Color Space?"
The highest bit depth you expect the document to need in the future. For images that are properly corrected for tone and color, 8 bits is quite enough. If you expect an image's tones or color to be reworked significantly in the future, you can choose to archive a 16-bit or higher version of the imagebut only if the image started out at that bit depth or higher. For example, if an image started out as an 8-bit JPEG, you'll gain nothing from converting it to 16 bits.
Type, effects, and vector shapes. It's a good idea to preserve these layers because you can edit them at any time without reducing their sharpness or quality. For example, you can edit the text in a type layer at any time in the future. If you flattened or rasterized the type into the document as pixels (by flattening the document or choosing Layer > Rasterize > Type), there's no practical way to change the text.
You'll use this master document as a fully editable source file. When you need to reuse the Photoshop document in another medium down the road, simply save a copy of the document at the required specifications for that medium, resizing, flattening, and converting its color mode and color space as needed.
As you read the rest of this chapter, keep the ideas of a media-independent master and a media-specific final document in mind, because that concept drives many of the choices I cover.
What's the Right Color Space?
Color spacethe range of color available to an imageis widely misunderstood. The situation isn't helped by the number of color space choices for RGB alone (sRGB, Adobe RGB, and ProPhoto RGB to name just a few). Without getting into a book-length discussion of color management, I try to set things straight here.
One myth is that you have to choose one color space and stick with it forever, and that leads to endless arguments about whether a space like Adobe RGB or ProPhoto RGB is better than sRGB. The fact is that you can use different color spaces for capture, editing, and output. It's certainly easier to pick the color space closest to the range of colors of your final output and use that all the time, but that's most practical if you're going to stick with one kind of output all the time. For example, if you only prepare images for press, it's both sensible and simple to edit and hand off final files in the CMYK color space of your press. However, if you maintain master files that you convert to both print and Web media, you'll probably want to use a color space large enough to contain most of the colors you use in all the media for which you prepare images, such as Adobe RGB. (Avoid the temptation of choosing very large color spaces like ProPhoto RGB unless you have a very good handle on color management and 16-bit editing.)
It isn't necessary to convert all documents to the Photoshop working space (specified in Edit > Color Settings) before you edit. When documents come from different sources, it sometimes makes sense to leave them in their original color spaces while you edit as long as those are device-independent (not media-specific) color spaces like Adobe RGB or sRGB. Photoshop can maintain the individual color space of any documents you open, so that's not an obstacle. (Contrary to popular belief, the Photoshop working space setting in the Color Settings dialog box only comes into play when you open an image that isn't already tagged with a color space profile and automatic conversion is on in Color Settings.) You can work in any reasonable color space, and when it's time to prepare the media-specific versions, you can convert to the appropriate color space. For example, you can edit files that arrive in Adobe RGB and sRGB in their original color spaces, and when it's time to create the final images you convert all of them to the final CMYK color space. Or, when it's time to create the final images for the Web you can convert them all to sRGB.
How can Photoshop keep all this straight? If you maintain a properly configured color-management system, with accurate profiles installed for your monitor and printer and embedded in your images, Photoshop and other color-managed programs can use the profiles to reconcile color differences among documents so that they're displayed consistently and so that conversions are predictable. I know color management is a scary term to many, but if you can reach the point where you understand it, the effort is well worth it. For a complete and effective explanation of color management, I recommend the book Real World Color Management by Bruce Fraser, Chris Murphy, and Fred Bunting (Peachpit Press).