Now that we've explored the various imaging methods, we should recap and highlight some of the different techniques you can use in building images suitable for output on halftone and contone devices. We say "recap," because we've mentioned most (if not all) of these in previous chapters, though never in one place.
The first and foremost difference between contone and halftone imaging is the required image resolution. It's quite a bit harder to work out the resolution needed for halftone output than it is for contone, so we'll deal with halftone output first.
Tonal and color correction
We talk a great deal about compressing tonal range ("targeting") for halftone output in Chapter 6, Image Adjustment Fundamentals, so we won't go into it here. Contone output needs less in the way of tonal and gamut compression than halftone output, because contone devices generally have a greater dynamic range and a wider gamut than do halftone devices. However, this can bring its own problems, particularly when you have a scanner with a tendency to oversaturate some colors, as do many inexpensive scanners (and even some expensive ones). Keep a watchful eye on saturated colors. Some dye-sublimation printers feature a magenta that's almost fluorescent!
As we noted back in Chapter 9, Sharpeness, Detail, and Noise Reduction, contone images need less sharpening than halftone images. But that doesn't mean they don't need any at all. Halftones, again because of their coarse screens and significant dot gain, mask details and edges in an image; sharpening can help compensate for both the blurriness of the scan and the blurriness of the halftone. And, halftones being what they are, you have a lot of room to play with sharpening before the picture becomes oversharpened (most people end up undersharpening).
In contone images, however, there's a real risk of oversharpening. Not only should you use a lower Amount setting for unsharp masking, but also a smaller Radius. Where a Radius less than one is often lost in a halftone image, it's usually appropriate in contone images. In this context, inkjet printers tend to behave more like contone devices than halftone devices.
This last item, image mode, isn't really dependent on what output method you're using. However, because we still see people confused about image mode, we thought we'd throw in a recap here, too.
Again: if you're printing to a color contone device that outputs to film (or if the image is only seen on a color screen), you should leave your image in RGB mode. Contone and hybrid devices that print on paper use CMYK inks or toners, but in most cases you'll get better results sending RGB and letting Photoshop or the printer handle the conversion. If you have a good profile for the output device, you can preview the output using Proof Setup, and convert the image from your RGB editing space to the device's space at print time (we discuss this in the next section). If you're printing separations, though, you need to send a CMYK file.
We've tried many times to build Photoshop Classic CMYK setups for CMYK dye-sublimation printers, but it simply doesn't work. Photoshop's separation engine is geared toward halftone output, where the ink density remains constant and the dot size varies. It simply can't handle the variable density on dye-sublimation printers. It would work for inkjets if we could control the inks directly, but since we can't, it doesn't.