Technical Help

We provide these helpful tips as a resource for you. If you don’t find the information you’re looking for, please give us a call 846-1055 or e-mail us at spgm@ncifcrf.gov.

Scanning & File Size Tips

Simple Scanning Mathematics
You can size an image precisely for its intended use, if you know what its final size will be, and the resolution appropriate for that use.

1. The resolution you need is determined by the nature of your final product. Are you producing a Web image? Onscreen show? Slide? Publication print? This will determine your scanning resolution in dots per inch (dpi).
   Note: Pixels per inch (ppi) and dots per inch (dpi) are not precisely the same thing, but for our purposes in this calculation, they can be used interchangeably.
2. The formula: wf x dpi / wo = scanning dpi
   wf = width of the final image
   dpi = dots per inch
   wo = width of original
3. An example: You’re scanning a 5x7 (vertical) photo to go to a printed publication. The final image is going to be 3 inches wide. It will need to be at 300 dpi at its final size, because it’s going to printed.
   Width of final image (in.) x dpi needed = width of final image in pixels.
   3” x 300 dpi = 900 pixels
   Width of final image in pixels / width of original image (in.) = scanning resolution in dpi.
   900 pixels / 5” = 180 dpi
   Therefore, you need to scan your 5x7 photo at 180 dpi.

Approximate File Size Chart
Sometimes it’s handy to be able to tell at a glance whether an image has enough data for its intended use, without having to open it for editing. In order to do this, you need to know how image size and resolution relate to file size. Below are three tables showing appropriate file sizes for a color image used for different purposes and at different sizes. In each case, the image is oriented horizontally (landscape).
Web or Onscreen Show (72 dpi) – includes PowerPoint

*Even a very good quality JPG is much smaller than a TIF of the same image – generally only 1/3 to 1/4 the size of the uncompressed TIF image. Also the JPG format is more appropriate for Web applications than is a TIF. Ideally, by using compression, JPG files can be kept to less than 100 KB, when intended for use on the Web.

Laser Printer (150 dpi)

Final Image Size	Pixels Across Top Edge	File Size (TIF / JPG)*
4x5	750	1.33 MB
5x7	1050	2.60 MB
8x10	1500	5.28 MB

Offset Printing (300 dpi) – includes journals and book chapters

Final Image Size	Pixels Across Top Edge	File Size (TIF / JPG)*
4x5	1500	5.28 MB
5x7	2100	10.35 MB
8x10	3000	21.10 MB

Choosing the Right File Format

Format	Purpose	Drawbacks
TIF	Designed for use with print output devices, and moves successfully across PC/Mac platforms	Files tend to be relatively large, which may slow down performance of onscreen shows
GIF	Good for flat color graphics for web applications, and does not produce compression artifacts	Limited color palette (256 colors)
JPG	Good for photographic images for web or onscreen use; handles smooth gradations of color	“Lossy” compression can introduce undesirable artifacts in image
EPS	Good for vector graphics, and sometimes helps move text and images from one application to another	Requires postscript printer for printing, not recognized by all software, not intended for raster images
PDF	Will print to almost any print output device, free downloadable “reader” enables any user to view a document exactly as it was intended to appear	Proprietary file format cannot be easily edited without full version of source software