The answer will vary depending upon what your needs are. I have spoken with many professional archivists who are scanning rare documents. Almost universally, the answer I have received is they scan documents in color at 400 dpi and save the images as .tiff. Based on their response, I have scanned about 90% of my images at 400 dpi and saved them (as they did) in .tiff format. The images are large (many exceeding 10 megabytes) and that’s ok. This is my family history; I am scanning not only for my personal interests but also for the generations that will follow. Now when I share the images, I may save duplicate images as .jpg (usually less than 1 meg in size), but the original .tiff is intact providing the key source of information.
Ok, that’s what I do. The following is a discussion about scanning resolution, giving more insight and help to provide better understanding about the concept of resolution. The best resolution to scan your images depends on how they will be used—on screen or in print. For most users, it is rare that you will need thousands or even more than 200-400 dpi pixels of resolution. per inch, which means that If you scan 6×4 inches at 150 dots per inch (dpi), then you create “6 inches x 150 dpi” x “4 inches x 150 dpi” which = 900 x 600 pixels. But if you are scanning 3×2 inches, then 300 dpi creates the same 900 x 600 pixels of resolution.
There are essentially five questions to take into account when scanning:
- Size of the original image: Are you scanning a 3X5 photo?
- Size of the printed image: Will the image you are scanning be reduced or enlarged?
- LPI at which your image will be printed: What type of printer and paper will you be using? LPI (lines per inch) are affected by those choices. Your final image resolution is usually about double the LPI at which the image will be printed.
- Resolution multiplier: What is the resolution multiplier? This number is usually 2.
- Editing of the scanned image: Will you be using a higher resolution to compensate for what is lost during editing? It is usually better to scan at a higher resolution than you will be using. When you edit photos, there is a loss of pixels. Scanning at the higher resolution helps to compensate for what is lost when editing and resaving; ( remember though, the larger the resolution, the larger the file size.
The following are examples of how to calculate your final image resolution:
- If the size of your image will not change, multiply LPI by your resolution multiplier. LPI x 2. For example, an image destined for a brochure on uncoated paper using offset printing needs a final resolution of approximately 133 x 2 or 266.
- If the size of your image will change, multiply LPI by your resolution multiplier and then by the size of your final image as a percentage of the original. LPI x 2 x %.
- For example, if our image for the brochure (above) is to be enlarged by 25%, then it will need a final resolution of 133 x 2 x 125% or 333. If it is to be reduced by 25%, then it needs a final resolution of 133 x 2 x 75% or 200. (I’ve rounded up in each case.)
- If you don’t know the percentages, it may be fine to scan at the final resolution arrived at by one of the formulas above. However, since most scanned images need some type of image editing, scanning at a slightly higher resolution is often advantageous. It is easier to “throw out” unneeded resolution after scanning than it is to restore lost resolution after the fact. Just remember to reduce your image to the final resolution before placing it in your document for printing.
Typical LPI Guidelines
Output/Paper: Typical LPI
- Screen printing: 35-65
- Laser printer/photocopier (copier or matte laser paper): 50-90
- Laser printer/photocopier (coated paper): 75-110
- Quick printer (uncoated or matte bond paper): 75-110
- Offset printing (uncoated paper): 85-113
- Offset printing (coated paper): 120-150+
- High quality offset or gravure (such as glossy magazines): 150-300+