 The magazine of the Melbourne PC User Group What Is The Best Scanning Resolution? Stephen Davey |
Most people are often unsure of the best scanning resolution for the
photographs they scan on desktop scanners. For many purposes more may not be better.
What to we mean by resolution? In fact we have two terms to deal with - Scanning resolution and
Final resolution. As image/photograph files that have been scanned consist of a grid of pixels (picture
elements), the number of pixels per inch when the image is displayed or printed is referred to as the
final resolution. Just to confuse the issue, what should really be referred to as ppi (pixels per
inch) is normally referred to dpi (dots per inch).
Scanning resolution is the resolution that an image needs to be scanned at to achieve the final resolution.
Obviously if you scan a photograph at the same size as it will be used then the scanning and final
resolutions will be the same. However, if you intend to enlarge the photograph to, say, double the size and
want a final resolution of 100 dpi then the scanning resolution will need to be 200 dpi.
So before scanning any photograph it is important you know what size you want the final photo to be.
For web purposes, size is normally referred to in pixels, while for printing purposes millimetres or
centimetres are normally used.
Most good scanning software lets you enter the required final resolution and the final image size in pixels
or millimetres. This means that you do not need to be worried about the actual scanning resolution - the
scanner software is doing all the calculations for you.
If your software does not let you enter in the final image size then you will have to do some extra
calculations to arrive at the correct scanning resolution.
The rest of this article refers to the final resolution of images.
There are generally three categories of scans made on typical scanners.
Firstly, images that are used on the Web or for e-mail purposes; secondly, images used in documents printed
on your own printers (inkjets/lasers); and finally, images you want printed in high quality brochures or
magazines.
Web Images
Generally Web images are best at about 50 dpi to 90 dpi with scanned images saved as either GIF or JPEG
files. In most cases photographs are best saved as JPEG and logos or images with large areas of flat colour,
saved as GIF. With regard to resolution experiment yourself, viewing your scans in a browser rather that and
image editing
or scanning program.
The higher the dpi the better the images appear . . . but the slower they load.
Home Documents
Images to be printed on an inkjet or laser printed only need to be from 50 dpi to 120 dpi at the most. Try
some tests on your own printer and you will find that it's rare to get any image improvement once you go past
about 100 dpi.
High Quality Offset Printing
The myth is that for really high quality you need to have a 300 dpi image. In fact in almost all cases 300
dpi is far too high. The problem with scanning to a high resolution (dpi) is that the file sizes are much
larger than required, slowing down imagesetting equipment and wasting disc space.
The correct dpi required for printed images depends on the screen ruling used to make the plates. Screen
ruling could be the subject of an entire article in its own right, however all printing companies will
happily tell you what screen ruling they use. PC Update for instance, currently uses a screen ruling of 150
lines per inch.
A good, general industry rule of thumb is to have image dpi at 1.4 times the screen ruling.
So a magazine to be printed at - 133 lpi need 186 dpi images, 150 lpi needs 210 dpi images, and 175 lpi needs
245 dpi images.
The small sample images on the top of the next page show eight TIFF images scanned at a range from 50 to 350
dpi. Grab your magnifying glass and you be the judge of the lowest resolution that gives a good result.
Note that the 300 dpi image is more than twice the size of the 200 dpi image. Can you notice any quality
difference? The images below are copies of the 200 dpi TIFF image saved as JPEG files ranging from 1 (the
lowest quality and smallest size) to 12, the highest quality.
It is interesting to compare the 200 dpi TIFF image and the same file saved as JPEG (quality 12).
Be aware that saving TIFF files as JPEG causes a reduction in file size by compressing the file in a
"destructive" way. The compression is achieved by throwing data away (albeit in a very cleaver and scientific
way). If you resave a JPEG as a TIFF file the image will not regain its lost quality.
The 400% enlargement samples in the split image (bottom right) show in the left pane, part of the TIFF file
scanned at 200 dpi and in the right pane, a JPEG made by compressing this file using the quality setting of
12. On screen the JPEG displays as a higher quality image. Let's see it printed at 150 lpi.
Reprinted from the April 2000 issue of PC Update, the magazine of Melbourne PC User Group, Australia
