Join Taz Tally for an in-depth discussion in this video Understanding dot gain, part of Photoshop CS5: Prepress and Printing.
You may have noticed that whenever you print your images to just about any printing device, they very often print darker than you expect them to and sometimes a lot darker. One of the reason for this is something called dot gain. Unless your working in a fully calibrated or a color managed system, you very often have this dot gain related darkening of your images. If you remember earlier in the course, we talked about how printed images are constructed out of halftone dots. The tonality that you seen in an image, that is how dark it is, what the tonal range of the image is, is determined by the size of the half tone dot as well.
There is a spacing depending upon the screening technology. Well, if you create a halftone dot of one size, and then when you place it on your printing substrate and it gets larger, then your image is going to appear to be darker and print darker. Let me show you how this works. Let's take a look at this here. I've got a little eyedropper with some ink in it. I've got two kinds of paper here. On the left a really absorbent paper and then on the right a less absorbent paper. This might represent on the left-hand side maybe newsprint let's say or very, very poor quality uncoated stock, and notice what happens.
I'm going to take this little eyedropper, and notice a little just drop of ink and I'm going to put it right on that paper. See the size of that halftone dot? Boom, and as soon as it hits the paper, it spreads out. That's dot gain. If you can imagine, thousands of these little halftone dots all side by each, right next to each other and they are all growing like this and you can imagine how much darker your image will look. Now what affects dot gain is the kind of paper that you have. Notice with very absorbent paper, you end up with a lot of dot gain whereas with less absorbent paper like this one on the right-hand side, maybe this is a piece of coated stock or very high quality piece of uncoated stock.
Here when we take that same halftone dot and you drop around there, notice that doesn't gain very much at all. All right! So, dot gain means that your images will print darker. The amount of dot gain you have is determined by the quality of the paper that you have. Let's take a look at the consequences of this in terms of an actual printed image. Here we have two images. On the left, we have a dot gain corrected image. On the right, you have an image that is not dot gain corrected. Now just a cursory examination of these two images, you can see how this is darker and lower contrast.
That's because all of the halftone dots are much larger than they normally would be. Here in the dot gain corrected image, everything is bright and you can see more detail. When you take a look at the details of this image, you can see exactly why this is a higher-quality image. Look specifically at this ridgeline here where we have Kings Mountain. This is in the Yukon, up against its natural sky. You can see there is a good deal of contrast between the two, whereas in the non-dot gain corrected image the tonal ranges are actually merging because those halftone dotes are growing. Down here in the middle tones, we have middle tones and highlights.
You can see lots of detail and good brightness on the dot gain corrected image whereas on this one, you can see all of the midtones emerging with the quartertones, and you lose a lot of detail. In the highlight and quartertone area, like Zip's fur, lots of nice detail and good brightness in the dot gain corrected image but in the not dot gain corrected image, you're losing a lot of that detail. Again, you're getting merging of your tonal regions. So you can see both in a cursory and a detailed examination why you want to make sure that you can dot gain correct your images, that you always want to accommodate for that. Now we're going to move into Photoshop, where I'm going to show you how to do dot gain correction for grayscale images.
Later on, I'll show you how to accommodate dot gain correction in color images when we go through a colored gamut conversion.
- Understanding RGB and CMYK bit depth
- AM versus FM screening
- Working with device color gamuts and profiles
- Making image adjustments before printing
- Choosing the correct file format for output
- Assigning spot and process colors
- Comparing editable and raster type
- Sharpening for print
- Printing to grayscale
- Proofing images
- Recording actions to automate printing-related tasks