DPI, Pixel and Megapixels
With the advent of the digital cameras and scanners, photographs collections have become a large part in your family tree collection. Understanding the terminology is the first step in conquering the technology. Digital pictures are created using small elements of colour or black and white bits. Look at any newspaper closely and you will see this in practice, the pictures are a mosaic of colour dots. The term 300DPI simply means that there is an array of picture elements or pixels that span 300 wide and 300 high for each square inch of a digital image. DPI stands for "dots per inch" and is used to describe printed or scanned images. A camera or scanned image is described in pixels, the smallest piece of a picture much the same as the dot in a DPI. The major difference is that a pixel is not associated with a printed photograph's size. The term pixel is used to describe the overall resolution of an image regardless of its size.
The only physically fixed dimensions that a bitmap image has are the number of pixels wide and high in the image (e.g. 900 x 700 pixels) as an example. You can choose to reproduce that bitmap at any physical size, simply according to how large each pixel is made at the time of printing. For example, if each pixel is printed 1/100th of an inch wide, the printed 700 x 900 image would be 7" x 9". Resolution is a measure of how accurately defined an image is, in terms of how many pixels appear in each unit of width or height. The effective resolution in dpi of the final printed image is therefore just the actual width of the printed image divided by the number of pixels shown across that width (i.e. 100 dpi in the example above). Software may allow you to attach a dpi value to a particular file, or may attach one by default (typically 72 or 96 dpi for a screen oriented application, or 300 dpi for a professional print oriented application). But this number is entirely arbitrary and serves only one purpose - it is to allow the software to represent dimensions in "real world" units such as inches, rather than pixels. It does not alter or control the definition of the image itself, merely what dimensions will be shown on the display and, usually, what default printing width will be used.
How to use DPI, Pixel and Megapixels
- Use DPI when buying a scanner or printer
- Minimum Scanner DPI should be 2400
- Minimum printer DPI should be 300
- Use Megapixels when buying a digital camera
- Minimum megapixel for a digital camera should be 3.2
- Set the resolution to 300DPI minimum
- Set Colour-B/W to Colour
- Preview image
- Outline the photo to be scanned
- Set the higher of the length or width to 10"
- Scan the photo
- Save original scans without editing
- Always store originals to CD/DVD or other media
- Crop originals and resave them for use
- For cameras do not use Digital Zoom (reduces image quality)
- Use a photo enhancement program to crop images before printing