It seems that everyone has a scanner these days, and while serious quality scanning is a trade and artform in itself far beyond the scope of this e-zine (and my knowledge for that matter) I thought some info on simple scanning would be useful. In this article I'll concentrate on resolution.

One of the most common questions I here about scanning is "What resolution should I scan my photos to make a print?", or slight variations on this theme.

There are four variables you have to consider when scanning,

• A - Original size
• B - Input resolution
• C - Required output size
• D - Output device resolution

In general A and D are fixed, you decide C and you calculate B. Let's go over these terms.

Original size
Fairly self explanatory I guess, we'll talk in inches as that's how most of this is done.

Input Resolution
This is the setting you will set on you scanner and the subject of this article.

Required output size
Up to you, once again we'll use inches.

Output device resolution
For printers most people work with a figure of 300dpi. I know most printers state 1440 dpi or some such but, while technically true, it's a bit misleading when talking about producing photo quality prints.

300dpi is generally excepted as being the resolution at which the human eye cannot see the individual pixels. Some people prefer to use 288dpi as this is a submultiple of the printer's native 1440 and they feel that the result may be better because the mathematics is straightforward and therefore likely to produce a better print, I've never seen any difference.

The formula for calculating your scanning resolution is very simple, we just need to know the amount of enlargement from the original to the resultant print, and multiply that by the output device resolution.

Below are some case studies.

6x4 print from 35mm slide or neg

width of original = 1"
width of print = 4"
enlargement = 4x

4 x 300 = 1200

scan original at 1200dpi

A4 print from 35mm slide or neg

width of original = 1"
width of print = 8"
enlargement = 8x

8 x 300 = 2400

scan original at 2400dpi

any print from another print of the same size

width of original = say 4"
width of copy = same
enlargement = 1x

1 x 300 = 300

scan original at 300dpi

web page image from neg or slide

width of original = 1"
width of web image = say 4"
enlargement = 4x

4 x 72 = 288

scan original at 288dpi (usually round to 300)

Here we see the Image Size dialogue (in Photoshop) immediately after scanning the slide as per the above example.

Note that the image size is roughly 1" (2.25cm) where we wanted 4".

Change the resolution to 72 and check "Resample image" if not already checked.

Note that the width is now 9.38cm, not really 4" but that the original was not quite 1" so that's about right. Anyway these physical measurements don't mean that much on computer screens, they have a nominal resolution of 72 dpi but that's just a rule of thumb.

For example a high quality screen may have a dot pitch of .25mm, this equates to about 100 dots per inch, not 72, so the image will appear smaller than the 4" you had in mind.

So in the above example the image is 266 pixels wide, 266 divided by 72 is 3.7 so this photo should be 3.7" wide on your screen.

But all our pixels are different sizes.

The truth is that everyone viewing your photo on a web page will have different screens and see the image slightly differently so don't get too hung up on the actual size of the width and height. It's the number of bytes the viewer has to download that is important and I'll cover that another day.

So hopefully that will help you next time you need to scan a photo. It's quite simple and can be a lot of fun.

It's also quite common to scan everything at the highest possible resolution, archive the result to CD, then scale it (in a similar way to the last case study) each time you have a different output requirement.

In this way you only have to scan once. This is a useful technique for images that will be around for a while and that may be used in several different ways.