Resolution - it’s universally accepted, in photographic circles, that it’s impossible to appreciate an increase in quality when comparing scans at 300 DPI with those at higher figures.
For most purposes, you’re unlikely to see any difference over 250 DPI and that’s for photographic quality. If you’re talking about graphic images - certificates etc. - you’ll get away perfectly well with 150 DPI and if you’re only ever going to look at them on a monitor 75 DPI will probably do.
I’m talking here of 1:1 scans; i.e. assuming you’re reproducing the original at life size. If you want to reproduce something at, say, twice the size, simply double the scanning resolution from 300 to 600 DPI.
JPEGs - the JPEG (or .jpg) format is known as a “lossy” format. It is widely used to reduce the file size of an original - for downloading, uploading to sites like Rootschat, or sending to somebody in an email. The original scanned image is compressed by effectively throwing away information contained in the image thereby producing a file of more modest size.
There are several ways of doing this and for the purposes of this board these are irrelevant but the problem is that, once the information is lost, you can’t get it back. You can apply various levels of compression and, for most purposes, you’ll not notice but in an over compressed image, things tend to break up and look ragged - known as jpg artifacts.
The real problem comes if you take a JPEG image, do some work on it and save it again as a JPEG image. You will effectively be more than doubling the problem so, if it’s pure quality you're looking for avoid JPEGs.
What’s the solution? Simple, use something like TIFF and, if you need a smaller file, make a JPEG from that but keep the original. Most professionals use Photoshop and that has it’s own industry standard file format PSD. Serious photographers save their originals as RAW image files and work on them at home. Cameras that only produce JPEG files take the RAW image and decide for themselves what is best for the image.
