Manipulating the size of an image can be very confusing to both newbies and experienced users. How to resize an image to exact dimensions, while maintaining picture quality, can be tricky and sometimes plain old lucky.
The overall quality of a picture is determined by the number of colours, resolution, width and length of the picture (for more information on colours, see Here's How Graphics, August 2000, page 124).
RESOLUTION Resolution of an image is simply the number of dots used to make up a unit measure of an image. The most commonly used term is dpi, which stands for dots per inch (there are other units, but this Imperial system still persists and 39.3 dots per centimetre seems odd compared to 100dpi). The more dots you have, the finer detail can be displayed. In the images shown on this page, the first picture is at 20dpi, meaning that 20 dots take up an inch of space, so there is little room for fine detail. The image looks very 'blocky'. At 300dpi, there can be 300 different coloured dots per inch, allowing much more detail.
So why not simply use higher resolutions? There are several reasons. First, more dots per inch means a lot more data. A 1x1in image at 1000x1000dpi has over 10 times the amount of data as the 300x300dpi equivalent. In most cases, you won't notice much of an improvement in the quality of the printed image once you start getting above 300dpi. Factors such as paper stock, ink quality and accuracy of the printer/scanner head can outweigh the increase in resolution. This is easily demonstrated by using glossy photo paper on your printer. Most importantly, the image resolution is not the same as the printer resolution (even though both are measured in dpi!). A rule of thumb is that the printer resolution should be about double the image resolution. So, if you have a 1200dpi printer, then your images only need to be about 600dpi. A 300dpi printer needs an image resolution of only about 150dpi. There are some complex reasons behind this rule of thumb, which won't be discussed here.
DIMENSION CONSIDERATIONS Once you understand resolution, you will need to consider the dimensions of your image. As an example, we are using the standard photoprint size of 6x4in. At 1000dpi, this size has 6000 dots across the top and 4000 down the side (24 million dots in total). Now here is the interesting point: an image that is 500dpi and double the dimensions, i.e., 12x8in, also has 24 million dots.
In essence, it is the total number of dots that will define the size of an image. You can use this information to work out what combination is best for your needs. Using the above example, the dimensions of the 1000dpi image could easily be doubled, while halving the same resolution. Hence, the 500dpi image could be printed at twice its size without any noticeable drop in quality. Likewise, a large image at poor resolution could be scaled down and have its resolution increased to improve its printer appearance.
A monitor only displays images at 72dpi. If an image is intended for on-screen display, such as on the Web, then the file can be even smaller.
Caution: if you reduce the resolution while the dimensions remain the same (or also reduced), then you will be discarding data. That is, the quality of the image will be degraded. For this reason, it is better to save the lower resolution version as a separate file. Only change the resolution once you have finished editing or cropping the image.
HOW TO RESIZE You will need to decide on the right mix between resolution and the dimensions of the picture. To change the size in Paint Shop Pro, select Image-Resize. . Likewise, in IrfanView select Image-Resize/Resample. to adjust the resolution and size of the image. It is best to have the image resampled when the size is changed, which will help maintain the integrity of the picture and prevent unwanted effects creeping in.
As with most aspects of editing graphics, you should experiment to determine what's right for your system. Try printing various resolutions and see which has the right balance between size and quality.