Thanks, everyone. I think what I'll do is scan each image twice, at 300dpi and 600dpi, and keep them as .psd files in photoshop.
This will give me plenty of options in the future.
Ray, you say that there should be no discernible difference going from 600 to 300 "provided the latter is printed to only half the size of the former". What if it is printed to the same size?
Charlie
Hi Charlie,
Image resolution is one of those things which everyone finds difficulty in getting their head round at some stage - I certainly did when I converted from darkroom to digital photography over 12 years ago.
As I've said, the "magic" figure to work at is 300dpi. In reality, there is nothing magical about this figure, it's simply that there is general agreement that printing an image which has a higher resolution than this will not give you a better print. As you increase image resolution, say from 150dpi towards 250dpi, your prints will appear progressively clearer. Print ones between 250dpi and 300dpi and you'll be hard pushed to tell the difference. Go higher than 300dpi and I'd defy anyone to tell the difference.
Whilst you're thinking about resolution you also need to think about image size. Double the size of a 300dpi image and, in theory, you'll end up with an image at 150dpi. I say "in theory" because you'll find that, in Photoshop, you will be able to keep the image resolution at 300dpi whilst doubling the size. In reality, Photoshop is moving all your pixels apart and cleverly making up new ones to fit in between. In 99% of cases this will simply result in a less clear image but you can get software which does this quire successfully - e.g. Genuine Fractals or whatever its called nowadays - but I digress.
What you need to aim for is your final image to be 300dpi at the dimensions you want to print it. There's no harm in having it bigger than this - you can easily reduce the size - but try not to store it as a jpeg until you have to - i.e. until you're sure that nothing else needs to be done to it. As I've said jpeg is known as a "lossy" file format and, in compressing your image to a smaller file size, the process throws away a fair bit of information.
To get back to your original question (!), let me answer it like this; if you have a 600dpi image the size of an A4 sheet, it will print happily at A3. If you were to reduce the image to 300dpi, it would print happily at A4 but would look poor printed at A3 as, in effect, it would then be a 150dpi image.
Essay over!
Ray