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One special image evaluation issue that you need to pay attention to and this is kind of a sneaky one is whether your original image is a true continuous tone image and I like to use the word contone, like a photograph or if it's an image that's built out of dot patterns. Because they may look similar to the human eye, in fact, they are designed to look similar to the human eye. But in fact when we get to the scanner, they are not the same at all. Let me show you the difference between a true contone or continuous tone photographic image, and one that is built-out of patterns of dots.
On the right, we have an image that was scanned from a true continuous tone photograph. The building blocks are actually microscopic. In fact, they are atomic in size and dimension. They are either a film-based emulsion type building block or they are a silver halide crystal if it's an old grayscale photograph, and the crystals are literally on the size of the atoms. It's just a collections of small atoms, much, much smaller than the resolution in which you're scanning your images. So your scanner will never ever actually see those building blocks. Let's take a look, let's just zoom in on this image here on Christina's forehead, no matter how close we get, you see you don't see any building blocks of the image at all.
It's a true continuous tone, particularly when you can look at something very smooth like someone's skin tone, you don't really begin to see any building blocks until you actually see the scanned building blocks, the pixels. So the process of scanning an image or capturing it with a digital camera actually reduces the resolution of a continuous tone photograph. It takes something that is very, very super high resolution and breaks it up into pixel-sized building blocks, in this case, one three-hundredth of an inch on a side. That's what a continuous tone image looks like. Notice, there's no original pattern in that image that we have to worry about at all.
No matter what scale you look at it, it's true continuous tone. The only thing you change is, is just a tonal value of the image itself. On the left on the other hand, we have a scan of a book, and there is a type of course and then there's a " photographic image" that has been reproduced. And let's zoom in on that like we did the previous image and notice as we start to zoom in, it looks like a continuous tone image, except for as we begin to zoom in more and more and more you start to see that dot pattern. You see because printed images are not printed as continuous tone unless they are actually a photographic print.
When an image is printed on a printing press or a laser printer or an Inkjet printer, any of the standard printing devices that you and I use today, the image is actually reproduced as a pattern of dots. There are several terms that are used to describe this, the most common one is the most ancient one. It's called the halftone dot. In the process of reproducing a photographic image as a printed image to allow us to reproduce it cheaply at very little cost and multiple, multiple many, many copies is using this halftoning process. Conventional halftoning looks like this where you see you have an equal spacing of dots and we vary the size of the dots in order to create the perception of a grayscale to the human eye when we zoom out.
That's the way a conventional commercial printing works, with a lot of printers like laser printers and particularly Inkjets, instead of varying the size and having equal spacing, they vary small halftone dot and they vary the spacing. But in either case, you end up with a dot pattern. This is a very obvious one because it's a fairly coarse grained halftone dot pattern. But the result is you don't end up with a smooth continuous image like we do over here with the Santa Claus image, you end up with actually a pattern of dots. As you move down the production path things get worse.
This halftone dot image when you scan it, not only reproduces the halftone dots, but if you notice at various enlargements here, you see the pattern that's coming across on screen here, let me go down a little bit more. That pattern changes and you get this very obvious black and white pattern. That's all being created by the interaction of the halftone dot pattern in the scanned image that's been reproduced by the scanner. And the pattern or array of pixels that you have actually displayed on your screen, the same thing happens when you take this halftone dot scanned image and try to reprint it with another pattern of dots. Ooh! Things get real ugly in a hurry.
So it's very important that you recognize if your image is built-out of a dot pattern versus a true continuous tone photograph before you start your scan process, which is one of the reasons why I recommend that if you are going to be doing much scanning at all and particularly of printed images, you should always have a little magnifying glass with you and you can look and see if you have a true continuous tone or if you've got halftone dot pattern. You would be able to see that pattern with the 10 power magnifying glass. Obviously, during the scanning process, our goal is going to be to remove this halftone dot pattern as much as possible, so we don't end up with all this interaction on screen or even worse, when we go to reprint this image again.
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