Do You Really Need All That Resolution?
It seems even the scanner makers are now caught up in the more-is-better routine.
A few years ago 300dpi 16-shade grey was an incredible scanner, to-day it seems
if you dont have a 600dpi 24-bit scanner youll likely to get drummed out of the
local DTP users club. But do you really need 600dpi?
It is quite possible that, depending on your application, you could produce some
very pleasing and fully professional results with less investment. I suppose I should
explain my first question now. I wont assume anything, so dont let me insult your
intelligence, just overlook any riduculously elementary parts.
- A flatbed scanner is merely a series of CCDs (charge coupled device = light sensitive
integrated circuit) mounted in a stationary row that light reflected from a piece
of flat art is allowed to pass over. These CCDs register presence or absence of
light (ON/OFF) thus producing a pixel electronically. Since they are mounted in
a single row that is the way the electronic file is created, row by row. Essentially
the CCDs are reflected one row of the flat art at a time until the image is completely
- That being the case, resolution or the number of pixels written based on what is
reflected is controlled two ways. The number of pixels horizontally is controlled
by how closely the CCDs are placed next to each other along the single row. The
number of pixels vertically is controlled by how slowly the light bar and mirror
inch along the length of the flat art thus reflecting onto the CCDs. Therefore,
the more CCDs and the smaller the steps of the advancing light bar the greater the
resolution. Currently there are five major scanner engine manufacturers (many re-packgers)
and they all buildtheir systems essentially the same way. The highest resolution
flat bed scanning system currently is physically limited to 600 spots (pixels) per
linear inch. You may say wait a minute I've seen 1200, 2400, and even 9600 dpi ratings,
but note I stated the physical limit is 600 spi. This is true,there currently is
a real physical limit as to how many CCD ICs can be placed side by side in one inch
and that limitation is 600 right now.
- This 600dpi physical limitation has been breached by what is known as interpolation.
Interpolation is a software/firmware process whereby the scanner essentially samples
two pixels and averages (often times using more complex formulas) the two pixels
together to form an extra pixel (or more) in the middle. Better scanners now do
this in hardware, but some still rely on their scanning software to do it (often
uninvolving the user). But nevertheless, this higher resolution is only psuedo-data.
That is, it is data being create by averaging and not by actually sampling it from
the original art.
- Another interesting development is the scanner manufacturers that are indicating
their resolutions in non-uniform terms. For instance, Microtek currently indicates
that their scanners are 600x1200 dpi. While this seems like a higher resolution
scanner than one that is merely 600x600 dpi, think about it for a moment. This measurement
reflects how much data that the scanner can acquire in a square inch, or X x Y.
What would happen if we acquired 600 spots in the X and 1200 spots in the Y? Either
we wouldnt have a square, we would have noticable gaps in one dimension, or the
most likely scenario, there would be overlapping spots in one dimension. Scanners
that have non-uniform resolutions dont actually give you the ability to acquire
image data at this non-uniform resolution, they instead interpolate one dimension.
At 600x600 they interpolate the 1200dpi dimension down to 600dpi (usually done by
merely running the stepper motor that moves the light bar at twice its minimum rate),
or at 1200x1200 they interpolate the X dimension.
- Grey-scale scanners are scanners with CCDs that can differentiate between levels
of light falling on them, rather than just being on or off, the greyscale scanner
can determine if the pixel should be any number of shades ofgrey. Most scanner manufacturers
have stopped R&D once they acheived 256 shades of grey because the current version
of PostScript can only recreate 256 levels of grey, however there are many high-end
systems that can produce much more levels (4096, 32768, etc.)
- Color scanners are nothing more than grey scale scanners that have filtration (most
commonly Red, Green, and Blue) and make multiple passes to generate 256 levels of
each RGB component. The software then recombines the three passes to create full
color. Higher quality scanners perform all three scans in one pass at the same time
to perserve registration (although this is not usually a problem in good three pass
- Imagesetters are unable to produce continuous tones, that is that are unable to
make a pixel different shades of grey like the scanner sees them, instead they use
a very complicated screening pattern to simulateshades of grey to the naked eye.
At higher magnifications you can easily see that a photo is not really a photo,
but is a series of variable size dots.This is called half-toning. Half-toning, therefore,
isnt using all of the pixels to create the dot patterned image, in most cases its
using only half or less of a high resolution scans original data.
- So the question becomes why scan so high if the data won't be used. There is a formula
for this of course. It is, scan at 1.5 times the lines per inch (LPI) of the final
output device. Therefore if you are outputting to a 2400dpi imagesetter at 150 LPI
then the normal maximum resolution you need to scan at is only 225dpi. So if the
un-informed user scanned his photo at 600dpi thinking he needed that high res capability
because he was going out to a high res imagesetter, he would be sending over 9 times
too much data to the imagesetter. This would result in a very long RIP time and
possible crash of the RIP.
- Line art scans (black and white) could, of course, be a good candidate for the high
resolution scanner, but more likely if you plan to use a piece ofline art you can
scan a large original and reduce it. Or better yet,autotrace it into your favorite
illustration program and forget having towork with large, slow bitmaps.
- High resolution color becomes a different story somewhat. It is possible tofind
a continuous tone color output device where it would be nice to output a true continous
tone modification that would rival the original (i.e.-National Enquirer PhotoShop
modification of a 35mm slide at high res and thenreoutput to 4x5 negative on a film
recorder to produce a retouched print).However, continuous tone output requires
very high resolution to produce satisfactory results. This requirement pushes the
upper envelope of flat bed scanning (800, 1200dpi) and becomes a job for the slide
scanner and drum scanner. Unfortunately at these ultra high resolutions the personal
computer becomes a liability. I have worked on a photo retouch of a 35mm slide that
we intend to re-output to 4x5 on a Solitaire film recorder. Unfortunately the full
resolution file (4800 dpi) was over 250 Megabyte! This is definitely not a job for