Digital Darkroom Basics, Part 2: Digital
Cameras
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Last month we learned how to select a color printer that would give us the best results possible for less than $2500. Please take a moment to review that column before continuing. Back To The Beginning (Input) The second step in equipping your digital darkroom is to decide how your image will get into your computer. Do you need a digital camera, slide scanner, flatbed scanner, handheld scanner, frame grabber or will you use Kodak PhotoCDs? All of these work, but what do you really need? You need to know and apply this secret formula: Input Resolution = Output Resolution * Quality Factor * Scale The output resolution usually is the "Line Screen Frequency," or half-tone screen size that your printer uses. This term is based upon technology from the world of offset printing, not typical inkjet printing in your office. For example, a high-quality magazine may use a 133-lpi screen. Companies that make inkjet printers have not been very eager to report the actual screen resolution of their printers, since these values are well below the quoted print resolutions.The Epson Stylus Pro will print at 180 dpi, 360 dpi or 720 dpi. Recently, Epson took a bold move and published a silly formula that you can use to determine the "working" Line Screen Frequency of their printers: ( Printing Resolution / 3 ) * ( 1 / 2 ) = Line Screen Frequency If you want to print at 720 dpi, the actual screen size would be 120 lpi. This does not sound nearly as impressive as the printer resolution, which is why it is such a well-kept secret. The quality factor is based upon the screening method used by your printer. If your printer uses half-tone screening, the quality factor ranges from 1.5 to 2.0. If your printer uses error diffusion screening, the quality factor ranges from 0.67 to 0.75. The scale is simply the ratio of the output size to the input size. No doubt, you immediately realized that by using a printer that employs error diffusion screening, your required input resolution will be half that needed by some poor soul whose printer uses half-tone screening. Your images will scan twice as fast, but the image file will be half the size as that other guy's. Which means, of course, that your computer will only need half as much memory and half as much hard disk space. And yes, it will print almost twice as fast too. The Correct Answer Is... Time for an example. I want a full size print on 8.5 x 11 paper. From the Epson printer guide, I locate the minimum margin figures and determine that the maximum print area is 8.0 x 10.33 inches. Being a darkroom person, I already know about aspect ratios, and calculate the aspect ratio of this image to be 1.29. I crop a 35mm slide to get the image that I want, then adjust the dimensions slightly to give an aspect ratio of 1.29. Let's say the cropped image is 1.1 x 0.85 inches. The scale factor is 10.33/1.1, which is 9.39. Therefore, the required input resolution for this print is 120 * 0.67 * 9.39, which is 755 dpi. This means that if I require an 8.0 x 10.33 color print, printed at 720 dpi, using error diffusion, made from an original image 1.1 x 0.85 inches, then I must have a device capable of scanning at 755 dpi. Expensive Toys Digital cameras create the digital image directly and are evaluated differently. They report the number of pixels captured in an image. I can calculate the pixels required in my example image: 1.1 * 755 * 0.85 * 755, which is 532,973. There are now many digital cameras on the market that can capture this level of quality, but prices are still high if you need "real" lenses. Cheap Thrills Frame grabbers digitize a video image into a given size. I use a Tamron Fotovix to convert a 35mm image into a video signal. That signal is fed to a Snappy frame grabber that creates images 640 x 480 pixels. Each image contains a total of 307,200 pixels, not even in the same galaxy as my requirements. (The Snappy is great for what it does though. I also have a ComputerEyes Color Video Digitizer from Digital Vision, and it is garbage!) Cheap digital cameras also quote their capacities this way. The Epson Photo PC has a maximum capacity of 640 x 480 pixels, which is equally good for, um, something else. Handheld scanners are limited to about 400 dpi and only scan reflective materials. A Possible Solution Kodak's PhotoCD service will digitize your 35mm image at several resolutions, then place them on a CD-ROM. The highest normal resolution is 2048 x 3072, or 6,291,456 pixels. Respectable indeed, and more than enough. The optional Base*64 resolution is 4096 x 6144, or 25,165,824 pixels. Talk about pixels to spare! Too bad it's not readily available. Too bad Base*64 images are more expensive. Too bad you have to get a whole batch of them done at once. Too bad we've run out of space for this month! [Next month we will learn how to evaluate film scanners and flatbed scanners.]
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