Filing your mega-pixels
This is one subject where I agree, size is a major factor.
Why do my digital photos look fine on the screen, but are dotty and grainy when I print them out? Resolution is the key. You can adjust the resolution in your camera with a two mega-pixel rating, which will give very printable photos up to about 5 x 7 inch.
To keep this as simple as possible, if your camera has a CCD with 480 x 320 pixels (most cameras have more pixels: more like 2890 x 1856 and larger) you will have an image on your monitor of 6.7 inches x 4.4 inches at 72 dpi. If you want to print this in a newspaper, or as a print, you need to increase the 72 dpi to around 150 dpi and this halves the 6 x 4 inches. Print photographic quality at 250 - 300 dpi and your 6 x 4 print would be good quality, but smaller than a postage stamp. In other words, you can juggle the physical size versus the resolution or dpi, but you can't have both. You must set your camera to suit size required before you take the photo!
The file size of a digital photo is in kilobytes, or megabytes and depends on the total pixels in the image.
Generally, images with more pixels give more detail, but they also produce bigger file sizes. So like most good things in life, it's compromise, compromise, compromise.
The images prepared for fast downloading on the web and email are fine on the screen, but far too small for successful quality printing. JPEG images can be compressed to minimise the file size. This involves throwing pixels that are similar to their neighbouring pixels (one blue sky pixel is similar to the one beside it, so a heavily compressed JPEG may well throw away 60 - 90% of sky pixels.) The problem is, that once discarded, you cannot recover these pixels and although the effect is not very noticeable on the screen, it is very noticeable in print.
Do not heavily compress an image destined for printing.
For the sake of copyright infringement, this article was given to me by a friend who had cut it out of a newspaper. I have made every effort to locate the newspaper from which it came, but to no avail. If the said newspaper, or the author of the article requires it to be removed from this site, I will do so without hesitation.
Posted by Ken Ferguson