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Exporting JPEGs for the Web from Lightroom
There seems to be quite a bit of confusion regarding the exportation of JPEG files from Lightroom. The "Export" window itself is rather straightforward, but you still have to know which setting is more appropriate for your needs. In particular, three settings may need further explanation.
Screen Sharpening
In a complete image processing workflow, there are commonly three different kinds of sharpening applied at different stages and for very different purposes.
It is important to understand that most of the sharpening is necessary even if one does not want to apply it "for effect", because of various limitations along the workflow. It should be noted that a basic sharpening workflow is not aimed at correcting soft images (caused by focusing error or motion blur), but at maintaining optimal detail in files that already contain as much as they can.
- Capture Sharpening (or Input Sharpening) is applied first and is necessary to restore loss of detail inherent in digital capture. That is particularly obvious when anti-aliasing filters are placed in front of image sensors to reduce moiré, as is very often the case (except on most digital backs, the Leica M9, and others). That is why the default behavior in Lightroom/Camera Raw and other raw processors is to systematically apply at least a basic amount of capture sharpening, which must then be refined manually (depending on various factors such as frequency). Note that this is true only when shooting raw files—shooting JPEGs means that the camera has already applied sharpening, so additional work on the file must be done carefully.
- Creative Sharpening is usually applied locally on specific regions of an image that require it most (such as the eyes and mouth when working on a portrait). This is the only kind of sharpening that is applied "for effect", where the photographer decides whether he wants his image to look natural or more crunchy.
- Output Sharpening is applied at the very end and is totally dependent upon the destination of the image. Luckily for us (and thanks to the work of Bruce Fraser and the guys at PixelGenius), Lightroom makes it incredibly easy to properly perform this task, which used to require a lot of trial and error. The thing is that if the image is going to an inkjet printer on glossy paper, the sharpening applied is not the same as the one applied for matte paper, and is not the same depending on the resolution of the print, and is very different from the one applied for viewing an image on screen. On one hand, inkjet printers inherently introduce a certain loss of detail because of the nature of the technology itself, so some additional sharpening must be applied—sharpening that would definitely look ugly if the image were to be viewed on screen. On the other hand, images need to be resized to be of an appropriate size for viewing on screen, and resizing an image calls for interpolation algorithms, which also means a loss of per pixel detail. This is why Screen Sharpening is required, not "for effect", but to maintain optimal detail when the image is to be viewed on screen.
Applying a "Standard" amount of screen sharpening to JPEG files exported for viewing on a web page is a very good idea—not to make the images look crunchy, but to retain an optimal level of perceptible detail.
Note that whether you intend to export images for the web or to print them, output sharpening is not something Photoshop will do automatically or even offer. When you want to do it yourself in Photoshop, the burden is on you to apply the optimal sharpening for a given destination—good luck! Do yourself a favor and just don't do it that way... Either print/export from Lightroom, or use PixelGenius' PhotoKit Sharpener and skip output sharpening altogether in Lightroom.
Resolution
When exporting images for viewing on screen, the resolution of the file has absolutely no meaning, so whatever value you put in that field has no importance and, contrary to what you might have been told, will have no effect on the exported file—files won't have more or less detail, and file size will not be affected at all. The important factor when exporting files for the web is the size (in pixels), not the "resolution".
Traditionally, images consumed on a computer screen have been set at 72 ppi (so you might as well put that), but that really depends on the resolution of each monitor—something you have no control over when you publish images on the web.
Color Space
If a file is going on a web page, because we cannot know if the browser used to view the image will support color management, we should aim for the common denominator (it won't support it) and choose the sRGB color space (the default presumption when color management is not supported).
The gamut of the sRGB color space is smaller than the gamut of the AdobeRGB (1998) color space, which itself is smaller than the gamut of the ProPhoto RGB color space—that much is true. But "color spaces" must not be confused with "color models".
The fact that most commercial printers offer very limited gamuts has nothing to do with the fact that they work in CMYK (a color model, not a color space). One simply has to have a look at Bill Atkinson's book to realize that commercial printers are technically able to achieve excellent gamuts and color fidelity when they put in the required effort.
In other words, the CMYK color model itself does not define the gamut, so it would simply make no sense to state that "CMYK has a smaller gamut than color space X".