From: Ken Watson (watsok@frii.com)
Date: 10/28/02-11:11:24 AM Z
I typed this last Friday but erased it thinking it was too much detail on
the internals of scanners.
IF you have a scanner with a spec of 48bits of color this is supposed to
mean 16 bits of Red, Green and Blue. 16 times 3 is not the same as 2 raised
to the 48th power. So you can see there is already some questionable
specification going on.
The problem a lot of ( almost all ) flat bed scanners have is their
sensitivity to Blue. This is compensated by adding more blue phosphor to the
lamp ( it has a blue cast to it). IF you scan color negatives, the base
color of the film filters out a very high percentage of blue light. So what
happens is that you actually end up capturing fewer electrons than 2 to the
16th so you have a A to D converter working on noise for 3 or so LSB's.
Which means you most likely have only really 12 bits of resolution or "36"
real advertised bits. This sometimes causes artifacts in the black areas of
an image.
AS FOR THE 2400 / 4800 RESOLUTION . Any scanner using a stepper motor can
only , accurately, get about 2400 scan lines to the inch. This scanner
likely has a sensor that is configured for 2400 pixels to the inch. So
anything more is a software guess. Taiwan Inc. ALWAYS uses the lowest cost
sensor. Which means lowest quality because the size of the sensor is
directly related to cost. Smaller sensor, less light captured, bigger error
in color. If the silicon size is the same for a 2400 DPI vs a 4800 DPI
sensor, the 4800 DPI sensor gathers about 1/4th the light, all things being
the same. A 4800 DPI scan will take about twice as long for other reasons.
If you are scanning for a genealogy project you really do not need more than
600DPI. This will make your files size manageable and make the image about
2X what can be viewed on a regular CRT. Most of the above should be of no
consideration for your application.
Ken
With so many messages recently about films for scanning, I'd like to
reverse the subject line to ask a somewhat off-topic question.
I was about to buy a Microtek ScanMaker 5900. I want to use it to scan
in black and white prints and black and white negatives up to 4 x 5" (as
well as scanning in text). My chief use for the scanned prints and
negatives will be to put images in a genealogical database. I want to
be able to print out the pictures as well as send them by e-mail. (I
have been assuming that I will get the best results from scanning in the
negative and using the bundled Adobe PhotoDeluxe 4, which I presume can
convert the negative to a positive. This will certainly be faster than
first making a silver print, but perhaps I am wrong about the quality
obtained. Any thoughts on this matter?)
The specs for the 5900 sounded good: optical resolution 4800 x 2400 dpi
and 48 bit color. However, it has no mention of the bit depth for grey
nor dynamic range. Just now in a list of features on another site I see
"input type - color". The price is $180, about half the next lowest
priced scanner which will take 4 x 5. It has only USB interface {which
is fine for me), and the more expensive ones have FireWire or LAN
connections (which I don't need) and which presumably increase the
price.
Does anyone know anything about this scanner? Does the fact that only
color is mentioned mean that it will not be satisfactory for black and
white? As you can probably tell, I have never used a scanner nor
Photoshop, so any information will be helpful.
Suzanne Izzo (bi3@georgetown.edu)
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