Resolution is a technical term that means similar but not identical things if you're talking about scanners, printers or monitors. It is often used to indicate how much detail is visible in an image.
Pictures are scanned as rows of very tiny square dots called pixels. In a picture all the pixels are the same size. The number of pixels per inch (ppi) is the scanner resolution. You can increase the resolution by making the pixels smaller, so that there are more pixels in an inch. Generally, increasing the resolution produces more detail in the image, and the picture looks better when you print or display it. But there is a plateau above which there is little or no visible difference even with increased resolution. There is no need to scan prints higher than 200 ppi or negatives and slides beyond 1600 ppi for use with most printers. Prints have little detail beyond 200 ppi and none beyond 300 ppi. Negatives and slides have little detail beyond 1600 ppi and none beyond 2400 ppi.
Pictures are printed as rows of very tiny dots, but these are called dots, and printer resolution is measured in dots per inch (dpi).
A scanner's pixels and a printer's dots are not the same, even if they have the same resolution. When a pixel is scanned, that pixel can be any one of over 16 million different colours. Colour printers use four colours: cyan, magenta, yellow and black (plus light cyan and light magenta in the case of Epson Photo printers). For a printer to show one of these 16 million colour shades the scanner recognised, it has to print several of its coloured dots in the right proportions to make us think we are seeing the right shade. Since the printer has to use several dots to mimic one of the scanner's pixels, the printer has to have a higher effective resolution than the scanner.
Computer monitors also use pixels which means that each pixel can be any one of the 16 million colours, and each pixel scanned in a photo is represented by one pixel on the screen.
Make sure that your monitor has been set up for True Color or at least High Color. (In Windows 95, 98, ME click Start > Settings > Control Panel > Display > Settings)
Your monitor can be set up for several resolutions and hence display different numbers of pixels across the screen.
Typical values are:
For example a 35mm slide (1.5 x 1 inches) scanned at 500 ppi will give an image 750 x 500 pixels, just right if your monitor is set for 800 x 600 pixels.
Of course image manipulation software such as Photoshop or Paint Shop Pro can magnify or shrink the picture to fit the screen.
There are practical limits to how much detail we can see, and how much resolution a printer or monitor can reproduce. Also, as the resolution of the scanned picture increases, so does the disc and memory space required to store it, the longer it will take for Photoshop to modify it and the longer it will take to print. Doubling the resolution will create a file of four times the size which will take four times as long to process.
When you want to scan an image, most scanners present you with a settings window. Usually you have the choice between setting the Input resolution and size, or the Output resolution and size. Using the Output settings is the best way.
If you original is a black & white picture, select Grayscale as the mode. This will reduce your file size by a factor of 3.
If you want to print your picture choose Output settings. Set the size to the final print size (in inches or centimetres) that you want and set the resolution to 150 dpi. (No more, no less; trust me!)
Now crop the picture. The crop box will keep its proportions as you adjust it and the scanner will automatically sort out what scan resolution to use to make your final picture the right size.
Your file size will be about 6Mb for an A4 picture or 12Mb for A3.
If you only want to use the picture on the screen, eg on a web page or to email it to your friends, set the Output size in pixels.
Set the size to the number of pixels you want for the image. 575 x 365 is a good size as it will fit everyone's screen. Ignore the resolution setting. It doesn't mean anything on the screen.
Crop the image. Whatever area you select will be scanned at the appropriate resolution to give your chosen number of pixels.
The common file formats used for saving your images are:
If you alter a file that has been saved as a .jpg, then save it again as a .jpg, the loss of quality will accumulate. It is best therefore to save the original scan as a .bmp or .tif or whatever your image manipulation software uses (eg .psd for Photoshop), and work on that file. If you want to make a .jpg to email, then save a copy as a .jpg, leaving the original file intact.
When manipulating your image in Photoshop or Paintshop Pro etc, you are working on the pixels. You can change the magnification on the screen using the magnifying glass. However don't be surprised if your picture looks ragged if you choose a magnification other than 25%, 50%, 100%, 200% etc. (when you click "Fit on screen" for example. Always check at 100% before printing.)
At this stage the print size is irrelevant.
It is only when you acually start the printing process that the printer software takes over and changes the pixels into dots for the printer, using the size and dpi information that is electronically stored in your picture file. (This is why there is usually a pause before the printer actually starts printing)
If you need to change the image size, only do it by a small amount. (eg after minor cropping). It's best to rescan the picture if you change your mind about print size or the crop in a major way. This is why it is best to do all your cropping at the scanning stage.
DO NOT CHANGE THE PROPORTIONS OR THE NUMBER OF PIXELS. This will cause the image to be resampled. Your computer will invent pixels by guesswork and your image quality will suffer.