Brace yourself now, this subject gets into some pretty technical stuff and I apologize for this but you really need to understand at least the basics of the technical workings of a scanner especially if you are planning to buy one. Now understanding what happens when a scan occurs will help you in buying a scanner and also when ordering high quality scans from a service bureau. And the first term you should know is color depth or bit depth. You will see it expressed as twenty-four bit, thirty-six, forty-two, and forty-eight bit depth. Now I can go into a short course on what all this means, but you would probably demand your money back for this tutorial if I proceeded to put you to sleep in that fashion. All you need to know is that the higher the bit depth, the better the scans. It is not the only spec you need to know, but it is the first. Bit depth refers to the scanner's ability to detect minute variations in color shade across the spectrum. You can find a good forty eight bit flat bed scanner for about a hundred and fifty to two hundred dollars. Now you are obviously not going to use a scanner like that in professional pre-press applications, but the average homeowner doing everyday scans, a forty-eight bit scanner in a hundred fifty to two hundred dollar range will serve you very, very well. The resolution you should look for is no less than twenty-four hundred DPI or PPI. Now the next spec you need to know is Dmax or dynamic range. Dmax refers to the range of light intensity from the darkest darks the scanner will detect to the lightest lights, combined with its ability to capture and transmit those signals with a minimum of video noise. Most flat-bed scanners have a DMAX rating of around two to two and a half, which is plenty of dynamic range for scanned prints. For scanning negatives and slides, you should use a dedicated slide scanner with a DMAX of not less than three and a half. By comparison, most photographic prints DMAX out at about two and a half. Slide film can go as high as three point eight to four, which is why you should use a scanner with a DMAX of at least three point five when scanning slides. Now there's at least one scanner on the market that quotes a DMAX of four point eight, which is laughable because there is not a thirty-five millimeter film in the world that exceeds a 4.0 Dmax, so what's the point. Now do not use a flat-bed scanner to scan your slides or negatives. They just do not have the dynamic range to render a good scan plus the light sources are lower quality. Now they are fine for prints and graphics scanning but they are very inefficient as slide and negative scanners. Signal to noise ratio is never quoted in scanner specs, not even in the manual when you buy it, but it is also a very important spec. You can have the best lenses in the scanner, the best light source and the best AD converter in the world in your scanner, but if it is full of noise, your scans will look awful, especially in the darker areas of your image. Now let me show you what video noise in an image looks like; it's not pretty folks. This is video noise; see the bands going across here, the horizontal banding, you see these white blotches, these lighter blotches on the darker areas of this image, and this is beautiful, I am going to have to have this rescanned because this is a beautiful image of the Yosemite valley in the wintertime. It has got all kinds of noise, look at all that; I mean that is just atrocious, that is what you do not want. Now let me show you an image with almost no video noise. I thank goodness for it, too, because this is a really nice photograph, if I do say so myself. Let me do some corrections here, before we go any further. . . I'm looking for my history to get this image back the way it should look and I will lighten it later. But let us take a look at the darkest area of this photograph right here and I will really get and close, and show me the artifacts. There aren't any. And let's increase the gamma on this photograph, and look, there is a little bit, there is a little bit of artifactoring at the very edge of this dark area but that is acceptable, that really is acceptable. Down here where it counts, you don't see any artifacts. So we know that a good scanner has a high signal to noise ratio, much in the same way stereo specs work. Now if you are familiar with the signal to noise ratios in stereo equipment, then the same concept applies in image signals generated from scanners. The more noise in the circuitry of the scanner, the worse your dark areas of your image are going to look. The only real way to ascertain the relative signal to noise ratios in scanners is to do a test scan and then look at the dark areas for noise artifacts. Now sometimes you cannot do a test scan but ask for one before you buy any scanner. One rule of thumb, the higher the cost, the better the scans. You will see slide and negative scanners priced in the hundred and fifty to two hundred and fifty dollars quoting all kinds of groovy specs. What they do not tell you is that the scanners are full of video noise and the light source is often very, very inferior indeed. You get what you pay for in a scanner.
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