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 Begining (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 is the printer resolution you will be using. The Epson Stylus Pro will print at 180 dpi, 360 dpi or 720 dpi. 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, and the image file will be half the size as that other guy's. Which means, of course, that your computer will need only 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 720 * 0.67 * 9.39, which is 4530 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 4530 dpi.

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 * 4530 * 0.85 * 4530, which is 19,187,041. The Kodak DCS-460 creates an image with only 6,230,160 pixels. I cannot create the print I require using this camera. It just isn't good enough, even though it costs over $10,000.

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 next to useless!)

Inexpensive 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 scan only 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, but not good enough. The optional Base*64 resolution is 4096 x 6144, or 25,165,824 pixels. Bingo! Base*64 will do the job, with 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!

Ed Note: Somewhere among scanning devices is the right product for you. Next month, Crimson Star will evaluate film scanners and flatbed scanners.