When you scan a stamp or cover, there are two settings you will have to make in the scanning program: resolution and color depth. Here are some tips to help you make the best settings for your purposes.
There are three color depth settings that will be useful when scanning philatelic materials. The one you'll probably use most often goes under different names in different manufacturers' scanner programs: True Color, 24 Bit Color, and Millions of Colors are some of the terms I have seen. Images scanned with this color depth can display over 16 million different colors permitting accurate rendition of all the color variations in the item being scanned.
Scanner programs usually offer a 256 Color mode. Images scanned with this color depth setting display a maximum of 256 different colors. 256 Color images can be very realistic, particularly for originals that have a limited range of colors to begin with. The advantage of 256 Color images is that the files are significantly smaller than true color image files. This color depth may be suitable when scanning stamps that are printed in only one or at most a few colors.
Gray Scale or 256 Grays mode discards all color information and creates an image that can display black, white, and 254 shades of gray in between. Gray Scale images can be useful for specialized purposes, such as when you are creating an image for evaluation of pert condition or centering.
When in doubt, I suggest that you always scan in True Color Mode. You can always use your graphics program to convert a True Color image to 256 Color or Gray Scale mode.
Resolution
Selecting the resolution of your scan is perhaps the most important choice you'll make. In selecting a resolution, you need to take into account the intended use of the scanned image. Here's why.
Just like the scan you make has a dots per inch resolution, what the image is displayed on does too. Whether the image is displayed on a screen or is printed on paper, there is a resolution associated with the output device. A computer monitor might have a resolution of 72 or 96 dpi, while a laser printer might be 600 dpi and an ink jet printer can be anywhere from 300 to 720 dpi and even higher. The optimum results are always obtained if the scanned image's resolution is selected based on the resolution of the output device. There's more to consider, however – specifically, the desired output size. Let's consider the situation for images that are to be displayed on-screen.
The default display of images on the screen uses one screen pixel for each pixel in the image. This gives the ideal viewing quality. Of course you can force the image to display at a different size, larger or smaller than its "natural" size. Let's see what happens when an image is displayed on-screen at a size other than its natural size:
- If the image is displayed larger than its natural size, the display software will have to interpolate the extra pixels that are needed and the final result will lose detail and sharpness.
- If the image is displayed smaller than its natural size some of the information in the image will be discarded. You could have gotten the same result with a smaller (lower resolution) image and saved on disk space (and download time too, for the Web).
When scanning images for screen display, you can calculate the ideal resolution as follows. First, determine the relative size at which the image will be displayed. If the image will be displayed at half size, this factor will be 0.5; if it will be displayed at twice its actual size, the factor will be 2.0. Then, multiply this factor by 96 (the most common screen resolution). The result is the scanning resolution you should use. If your scanning software does not offer the precise resolution you calculated, select he nearest value. For example, if the ideal resolution is 192 dpi, use 200 dpi.
There's a simple formula you can use to determine the ideal scanning resolution. First, let's define some terms:
SR = ideal scanning resolution in dots per inch
DR = resolution of final display device in dots per inch (96)
OW = width of the original being scanned in inches
DW = width at which the image will be printed or displayed in inches
Then:
SR = DR ´ DW / OW
With this formula, you can easily determine the ideal resolution at which you should scan. Unfortunately, things usually aren't that simple. There are a number of factors that can, and usually do, prevent you from using that "ideal" resolution when scanning:
- You are not sure of the final use of the image - how it will be reproduced and at what size.
- There are multiple uses intended for the image - for example, you want to make printed copies as well as display it on a Web page.
- The calculated ideal resolution is an intermediate value, such as 117 dpi, that is not supported by your scanner.
In these and other cases, the general rule is to "move up." In other words, you should always move to a higher resolution rather than to a lower one. Thus, if you cannot use the ideal resolution of, say, 117 dpi, you should scan at 150 dpi rather than at 100 dpi. Likewise, if you plan to use the image for printing at 300 dpi as well as for screen display at 72 dpi, use 300 dpi when scanning if possible. Scanning at a higher resolution captures the maximum amount of detail, from the original. You can always throw away some of that information by reducing the image's resolution after scanning (in your graphics program), but you cannot regain information if the image was scanned at too low a resolution. The process of changing an image's resolution is called resampling. If, for example, you scanned an image at 300 dpi for printing, you could resample it at 72 dpi for screen display.
Another advantage of scanning at high resolution is that the resulting images are easier to edit. Certain editing operations, such as rotating an image, give better results with high resolution images. Rotating an image can be useful if the item was not perfectly aligned in the scanner, and is usually easier than rescanning.
