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Scanning-Make Images Smaller | Tech Info Home Page | Tutorials | Scanning |


There are several methods used in making an image smaller in size.

First the image can be scanned in at a lower resolution.

This resolution can be selected prior to scanning by selecting the preferences option in the scanning program. Unfortunately most programs have this is a different location so its hard to tell you where to find it in all cases. The preferred resolution for pictures you want printed in color or displayed on a web site should be in the 75dpi to 150dpi range.

It seems kind of silly to have a scanner that is advertised at using resolutions of as high as 4800dpi and not use it, but remember any resolution above 300 dpi generates files larger than 35mb, that's per page and takes much longer to scan. Also a picture of that size will choke most PhotoEditing programs and make your video card run very slow.

Many times as in the case of the Hewlett Packard DeskScan software a pre-defined setting is provided for you to select such as "Screen" or "Printer". These settings actually change the dpi resolution for you.

Typically color printers advertised at 300dpi will generate images with real resolutions of about 70dpi on the paper. A scanned resolution of 150dpi is plenty for most pictures. Screen resolutions are mostly at the 75dpi range.

Second, an image can be scanned in at high resolution and converted to a lower resolution

In almost all cases, images can be scanned in at the 16.7 (24-bit) colors mode and then converted to a 256-color mode with no noticeable loss of quality . The 16.7 million (24-bit) and 30-bit color settings create very large files on the hard drive because of the large numbers of colors that are stored. A standard 8.5x11 page takes up close to 35 megabytes in the 16.7 million color setting at 300 dots per inch.

Note: A picture that is scanned in at 16.7 million colors and subsequently converted to 256 colors will look much better then a picture that was scanned in originally at 256 colors (even know both pictures will eventually have the same number of colors), 256.

Example:

In "Corel Photo Paint 7.0" the conversion is made by first opening the image and then selecting
Image, Convert To, Paletted 8-bit,
clicking on the "optimized" option, and typing in a value such as 32, 64, 128 or 256 in the colors box
and pressing OK

Third, a picture can be save using one of the compressed file formats, such as JPG, PCX, TIFF or GIF.

The recommended file type for images stored on a web site is GIF (which must be a 256 color format) if you want good looking quality but can sacrifice a bit more space. Typically the GIF compression is not as good as JPG but there is little lose of quality.

The JPG is also good for web sites because it makes very small file sizes but it does this by losing some quality in the picture. The JPG format is a 16.7 million color format unlike the GIF which is 256 colors.

To wrap things up, the smallest picture will be one where the resolution is in the 100dpi range, the picture is scanned in at 16.7 millions of colors , then converted to 256 colors and saved as a JPG or GIF file.

Note: It might seem like a waste to scan at 16.7 million colors setting, convert it to 256 colors and then use the JPG file type which will store the image as a 16.7 million color picture all over again. What helps this to work is that an image converted from 16.7 million colors to 256 will have as a palette only the 256 most used colors and any little used shades of color will be discarded. The last conversion from 256 back to the JPG millions of colors format will again only use 256 colors although the format is setup for millions. In other words its able to store millions but only has 256 to choose from. This smaller number of choices allows for a higher compression.

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