A few scanning tips

www.scantips.com


So here's the pop quiz to see if you've got printing down pat. The numbers are intentionally hard too. <grin> The next paragraph needs to be perfectly clear if printing is not to be a mystery.

You have a film scanner, and you scan a 35 mm slide which is 36x24 mm, which converts to 1.42 x 0.94 inches (actually it comes out a little smaller, the cardboard slide mount limits it a bit, but I'll ignore that here, it just gets in the way). You scan it at 1000 dpi 100% scale and get an image size of 1420x940. You expected that, right?   Then you print it, and it prints as 1.42 x 0.94 inches. You expected that too, right?   But that's kinda small, and 1000 dpi is way much for your printer, which likes say 150 dpi best. So you scale the image to 150 dpi by resetting the printing resolution number from 1000 to 150 dpi. Then you print it and it prints at 9.5 x 6.3 inches at 150 dpi resolution. And of course, you expected that too?   It's because 1420/150 = 9.5 inches and 940/150 = 6.3 inches.

Easy, huh?

You could have done this when you scanned the image too. The scale factor was 1000/150= 6.3. Set the resolution to 150 dpi, whatever your printer likes best. Set the scaling factor to 630%, whatever number causes the Output size to show 9.5 x 6.3 inches, or whatever size you want. This is then how the scan will print.

Scanning and printing color prints is exactly the same as film, only the numbers are different. In case this might be causing any concern, I'll say it all again, for color prints:   You scan a 6x4 inch color print at 200 dpi 100% scale and get an image size of 1200x800. You expected that, right? Then you print it, and it prints as 6 x 4 inches. You expected that too, right? But that's kinda small, so you scale the image to 150 dpi by resetting the printing resolution number from 200 to 150 dpi. Then you print it and it prints at 8 x 5.3 inches at 150 dpi resolution. And of course, you expected that too. It's because 1200/150 = 8 inches and 800/150 = 5.3 inches. The size increase ratio 8/6 inches is equal to 200/150 dpi (inversely). For better printers, we cannot go much lower than 150 dpi, or we start seeing less detail. For scanning color prints, we cannot beneficially go much over 200 dpi. So scanning prints is much more limiting than scanning film.

Some scaling problems

Image programs vary. No one program does everything best, they each have different strengths and weaknesses, and you will likely use more than one. Each one handles printing differently. Sometimes the tools are rather strange, and hard to interpret. Simple programs typically do not offer scaling,. but most image programs do provide a menu to set printing resolution. But it is not always clear.

File formats vary too. TIF and JPG image files store this scaled resolution number for printing. The image file knows the size it should print. You can mail your scaled image file to someone, they can print it with menu FILE - PRINT, and it will print at the expected scaled size.

However GIF files do NOT store the scaling resolution number in the file. Nor do Kodak Photo CDs either. But programs must have a number to show for print scaling, and therefore must provide a default number for GIF files. It is often shown as 96 or 120 dpi, dependent only on if you have set Small or Large font size in Windows. Adobe shows 72 dpi, an Apple standard, instead of Windows 96 or 120 dpi. It doesn't matter, this number has no meaning for the GIF file, it is not dependent on the image in any way. It is simply a guess provided when there is no other number given. You are expected to change it. People assume this number is the video resolution, but it is NOT, video has no dpi number (see the 72 dpi arguments in Appendix A). And this is unimportant too, because the resolution dpi of an existing image is used only for printing anyway, it has no other use. The source of this number is simply a kludged constant used by Windows for the purpose of making text fonts be a bit larger on screen than on paper, for better readability (more in Appendix A). It's a number, it's available, and our GIF file needs a number. It's hard to add reason here, but suffice it to say that it is a guess when no resolution dpi number is available, like for GIF image files.

However, this default dpi number will be used for printing GIF file images, probably producing large and unwanted size results. If printing GIF files, you will need to rescale to furnish an accurate resolution number for printing. This should be trivial to do now.


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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