| Back to Top |
Scanning Resolution |
| Let us consider flat bed scanning. Take
an example of a scanned 6 x 4 inch source image and a
required printed output of the same 6 x 4 inch size at maximum
printer quality. Scanning at 300 dpi will give an output print
with the required 300 dpi resolution, from a 1440 dpi or greater printer. If
the printer resolution is selected to be 720 dpi then the best
output image resolution will be reduced by a half to around 150
to 190 ppi. Likewise if the 6 x 4 inch source image is to be
enlarged to say 12 x 8 (a 200 % linear enlargement) then the
printed output from a 1440 dpi printer will be 300 divided by 2. That is 150
ppi.
To compensate for a linear enlargement of 200 %,
the source image would need to be scanned at 2 x 300 dpi linear
resolution. i.e. 600 dpi. Conversely if a 12 x 8 inch source
image is to be reduced in size to a 6 x 4 printed output, then
the scanning resolution need only be 300 divided by 2. i.e. 150
dpi. The scanner application software often makes this easy for the user
by enabling the image source size and required output size and
resolution to be predefined. The scanner then scans at the
required resolution. The same principle applies to scanning of negatives or transparencies. For example scanning a 35 mm slide to give a 6 x 4 inch print needs to cater for a linear enlargement of 400 % requiring a scanning resolution of around 1200 dpi. From the table below it can be seen that if high enlargements are required it is worth investing in a scanner with at least 2314 dpi optical (not interpolated) resolution. Note that digital images have an aspect ratio (height/length) of approximately 3/4 whereas traditional film camera aspect ratios are approximately 2/3. The table below is based on digital aspect ratios. |
| Back to Top |
Table of suggested dpi scanning
resolutions |
|||||
| Source Output |
35mm slide (3.5 x 2.4 cms) |
6"x4.5" (15 x 11 cms) | 6.6"x
5" (17 x 13 cms) |
8"x
6" (20 x 15 cms) |
10.6"x 8" (27 x 20 cms) |
| 35mm | 300 | 70 | 62 | 53 | 39 |
| 6"x 4.5" (15 x 11 cms) |
1286 | 300 | 265 | 225 | 167 |
| 6.6"x 5" (17 x 13 cms) |
1457 | 340 | 300 | 255 | 189 |
| 8"x6" (20 x 15 cms) |
1714 | 400 | 353 | 300 | 222 |
| 10.6"x 8" (27 x 20 cms) |
2314 | 540 | 476 | 405 | 300 |
| Back to Top |
Saving scanned images for printing |
| Having scanned the source image at the required resolution the image has then to be saved. It is suggested that the image is saved in an uncompressed format such as TIFF or BMP so that the source file is of the best quality. However, a small amount of compression using JPEG format, say at Photoshop setting of 8 out of 10, still maintains good quality outputs but with greatly reduced file sizes. The JPEG compression is recommended for normal domestic use. |
| Back to Top |
Resizing, re-sampling and saving scanned images for web pages & email |
From the source image, saved as above, the image requires resizing and re-sampling to achieve monitor screen resolutions of around 72 ppi with a suitable image size. To cater for monitors with resolutions of 800 x 600, a suitable screen image size is around 600 pixels x 450 pixels or whatever maintains the proportions required. Advances in LCD monitor resolution should enable screen image sizes of 800 x 600. A suitable resolution for the image would be 72 ppi. This gives a suitable size for viewing on monitors of differing resolutions. The size and resolution of the image can be adjusted to suit using imaging software. The resulting image can then be saved with high JPEG compression, say at Photoshop setting of 2 or 3, to give a final file size of around 25 to 50 kb. This size of file does not take too long to download from the internet. In fact the time taken to download a 25 kb image file would be around 6 to 7 seconds, based on a domestic dial up modem/telephone system operating at a typical speed of 4 kb/sec. |
| Back to Top |
Scanning
Resolution
Table of suggested dpi scanning
resolutions
for 300 ppi output from 1440 dpi printer
Saving scanned images
for printing
Resizing,
re-sampling and
saving scanned images for web pages & email