10/31/96
pixels?
I would like to evaluate scanner resolution (ie. 2700 dpi) in terms of
pixels.
mcbrande@sierra.net
you scan a 35mm slide (1.25"" by 1") at 2700dpi you'll end up with
approximately a 3400 by 2700 pixel image. If this is then printed with a
300dpi printer then its final size would be (approximately) 11" by 9".
The confusion generally sets in when some printers demand you are
knowledgeable of what it is capable of printing in terms of "lines per inch"
(lpi), which has much to do with its inability of printing one of the 16M
colors as one "dot" (dye-sub printers have this ability), but instead need a
small matrix of dots to dither CMYK into a resemblance of one of 16M colors.
Thus, if a 300dpi printer (ink jets, color laser) needs an 8by8 matrix to
create one of these colors then its true capability is 300/8 or ~37 lines
per inch.
So much for the math ... there is a lot of gray area here such as
"apparent" capabilities for printers to do better than this in spite of the
math ... so begin with an understanding of the terms and experiment ...
hope this helps ...
cheers, shaf
Michael Shaffer - mshaf@oregon.uoregon.edu - mshaf@darkwing.uoregon.edu
Electron Microprobe Facility - Geological Sciences - University of Oregon
http://darkwing.uoregon.edu/~mshaf/shafhome/
When we scan photographic images for reports or the web, we are usually not
pushing the limits of our scanner by any means. If we aim to convert a 4x5
inch photo to a 800x1000 pixel image, then all we need is 200 dpi (pixels
per inch) resolution. Even cheap scanners give that, and such an image is
big for the web. A 1600x2000 pixel image would require 400 dpi which is
still within the capability of most scanners. However, to scan a 35mm
transparency to the same image size would require about 4 times the dpi
capability as the 4x5 Polaroid. Therefore, the new scanner capabilities will
be needed.
There are also the phrases "optical resolution" and "interpolated
resolution". I understand optical resolution to be the actual spacing of the
elements of the scanning mechanism. Interpolated resolution refers to extra
resolution obtained by mathematical (or mechanical?) tricks to see in
between the pixels. It may help, but I would always check the optical
resolution as the better measure of capability.
Now on printing, laser printers will normally need more dots per inch than
you are printing pixels per inch. The reason is that the printer pixels/dots
are only black or white and are dithered together to generate an area of
apparent gray. For example a 3x3 or larger printer area will be needed to
give the impression of shades of gray for a single image pixel. I think that
600 dpi printers are the practical *minimum* for printing gray scale images.
1200 dpi printers are better.
True gray scale printers (e.g., most dye subs) require less dpi to give a
similar quality image than does a laser printer. Of course they will tend to
be more expensive.
Warren E. Straszheim
270 Metals Development, Ames Lab/ISU, Ames IA, 50011
Phone: 515-294-8187 FAX: 515-294-3091
wes@ameslab.gov