Pulborough Computer Club

Graphics Resolution

Graphics files can be enormous and sometimes they need to be. More usually though, they can be reduced to a manageable size without significant loss of quality. This page aims to show you how.

The Wrong Way

OK, so you've bought a new scanner that can scan at 1200 dots per inch and you're determined to use it to the full. In a square inch, that'll give you 1.44 million dots (or pixels) right?.

In normal full colour, we'll need 24bits  per pixel to store the colour information. At 8 bits per byte, that gives a total of 3 bytes per pixel
or 4.32 MB per square inch
or 103.68 MB for a 4x6 photograph.

That's ridiculous. If you're intending to clean the picture up in a graphics program such as Paint Shop Pro, it'll take ages. If you're hoping to attach it to an email, you should be able to send it in about 8 hours - and that's on a good day. I'm sure the recipient will be grateful though.

(Some modern scanners can use 36 or even 42 bits per pixel. That increases the file size even more)

The Right Way

Think about why you're scanning the picture in the first place and then set the scanner resolution to suit.

To Print It

Good quality colour images, such as those found in Country Life or Vogue, are printed at about 200 dots per inch. A square inch would thus consume 120 KB and a 6x4 picture would be 2.88 MB. That's a 36th of what we had before and you'll find the file much more comfortable to work with.

To Display It

(ie. To display it on a screen or send it as an email to be displayed on a screen)

Whilst larger screens are becoming more common, there are still many people around with screens that are 640 pixels wide by 480 pixels high. Your email recipient could well be one of them. So there's not much point in scanning that 6x4 photo at more than 100 dots per inch. That'd give you a file size of 30KB per square inch or 720KB for the 6x4 photo.

Blow Ups

You sometimes want to extract the detail from part of a photograph and then blow it up to a larger size. No problem; you've just got a few more sums to do.

Let's suppose that you want to blow up an area of the photo to 3 times it's original size. To end up with 200 dots per inch for printing, you'd thus need to scan at 600 dots per inch. That's OK, because (with most scanner software) you can draw a box around the small section of photo that you actually want and just scan that section.