Epson and Other Scanner Matters

My first contact with flatbed scanners was in 1996, when the computer project group at school got a shiny new HP Scanjet 4c, with a screamin' Pentium 133 with 32 megs of RAM to go along with it. This HP could already push a megabyte per second over the SCSI bus. Now the first two scanners in our family weren't quite as high-tech – first a Mustek ScanExpress 12000P, later a Plustek OpticPro UT24. Cheap, slow, plasticky things (the UT24 isn't remotely dustproof, real awfully cheap build). This only changed many years later when I was looking for an A3 sized scanner (much easier handling of schematics) and ended up with a used Epson Expression 1640XL – a huge, heavy, nearly bulletproof pro-grade monster. And so the story begins…

Note that this page is still in beta and may be a little rough around the edges.

Contents

Introduction

The turn of the century was a pretty wild time as far as flatbed scanners were concerned. They really broke into the consumer mainstream around this time, with people wanting to scan both documents and family photos (often negatives as well). CCD sensor resolution downright exploded, with semi-pro models going from 1200 dpi in 2000 to 4800 dpi in 2003, ultimately topping out at 6400 dpi in the mid-2000s when people realized that optics had become the limiting factor and weren't likely to improve much further. Dynamic range handling and ADC resolution also improved steadily. Consumer models tended to use the parallel port, later USB, and often were quite "dumb" and slow (far from the limits even of an ECP/EPP port or USB 1.1, which can transfer about 1 MB/s each). More upscale ones would use a SCSI interface and had more internal processing power and speed to offer; in the early 2000s, IEEE 1394 (Firewire) and USB 2.0 found use instead.

Epson (Seiko Epson Corp.) had been making scanners for quite some time at this point. From this page it would appear that their oldest models went EOL/EOS (?) in 1992, so those must have been introduced in the mid-late '80s. (The oldest model I could find at the well-curated Epson America support website was the ES-300C a.k.a. GT-6000, a 300 dpi color model introduced in mid-1990.) Basically they've been around for just about as long as HP with their Scanjets, with similar roots in the business sector (mere mortals would have been unable to afford anything but a little hand scanner at the time, a woefully flawed concept btw). You can find a lot of custom ASICs and gate arrays in their scanners, pointing to high levels of vertical integration. (That does not appear to apply to CCD sensors and A/D components though, which in the service docs I've seen were sourced from Sony and either Burr-Brown or AD, respectively.)

Let's start in 1999. This is the year that saw USB peripherals take off. By that time, Epson had some really nice scanners to offer – the A3-sized, 800dpi Expression 836XL / GT-12000 behemoth (SCSI + parallel, w/ focus adjust), its A4-sized Expression 800 / GT-9600 companion (SCSI only), remaining stock of the older 600 dpi Expression 636 / GT-9500 that was still being offered (SCSI + parallel), and the then-new Perfection 636 / GT-7000S (SCSI) as the most consumer-oriented option. Transparency units and partly ADFs were available for these. Below that it gets a bit sketchy but I think there was remaining stock of the older GT-5500 400 dpi model (SCSI or parallel), and the GT-2200 (SCSI or parallel). They had dabbled in the consumer market with the Perfection 600, a rebadged UMAX, in early '98, but had quickly run out of stock.

Epson Flatbed Scanner Timeline

I have taken the liberty to dig through the online resources available and figure out the product lineup from about 1998 onwards into the recent past. Older models are not likely to be too interesting since they usually won't even connect to something resembling a modern PC (who'd still want to put up with non-PnP technology like a SCSI scanner, or who still has a matching SCSI host adapter, for that matter?), and even older ones still did color separation by light source, i.e. switch lamps. Note that there may be some overlap that is hard to convey in such a table, I have usually included a note though. Availability refers to Germany.

Along with each model, some basic specs are given: Always-installed extra features like transparency unit (TPU) or automatic document feeder (ADF), connectivity, sensor resolution, bits per color channel (take x3 for full color spec), Dmax (maximum optical density), speed at 24 bit color, and available options. Note that speed is given for the sensor resolution stated unless specified otherwise – it is not a value that is directly comparable between different resolutions (the same time per line at a higher resolution means that the device is actually processing more quickly). The value is also given for "high-speed mode", which usually is not what you are going to use in practice unless running a prescan.

Year Pro A3
(graphics)
Pro A3
(docs)
Pro A4 Cons 1st Cons 2nd Cons 3rd
1998A Ex. 836XL
GT-12000
ES-8000
  • SCSI + PP
  • 800 dpi + AF
  • 12 bit in/out
  • Dmax 3.3
  • 15 ms / line A3
  • Trk TPU & ADF opt
Ex. 636
GT-9500
  • SCSI + PP
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 8 ms / line
  • Trk TPU opt
GT-5500
  • SCSI
  • 400 dpi
  • 10 bit in, 8 bit out
  • Dmax ?
  • 5.8 ms / line
? P. 600
(UMAX Astra 1200S)
  • SCSI
  • 600 dpi
1998B ?
1999A Ex. 800
GT-9600
  • SCSI
  • 800 dpi
  • 12 bit in/out
  • Dmax 3.3
  • 7.5 ms / line
  • Trk TPU & ADF opt
P. 636
GT-7000S
  • SCSI
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 8.1 ms / line
  • TPU opt
GT-5500
  • SCSI
  • 400 dpi
  • 10 bit in, 8 bit out
  • Dmax ?
  • 5.8 ms / line
GT-2200?
?
1999B P. 636/U
GT-7000S/U
(add USB)
  • SCSI / USB
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 8.1 ms / line
  • TPU opt
P. 610
GT-6600U
  • USB
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 16 ms / line
2000A GT-10000
ES-6000
  • SCSI
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax ?
  • 4.0 ms / line A3
  • ADF opt
Ex. 1600
ES-2000
  • SCSI + USB
    (1394 opt)
  • 1600 dpi + DF
  • 12 bit in/out
  • Dmax 3.3
  • 9.2 ms / line
  • TPU & ADF opt
P. 1200S/U
GT-7600S/U
  • SCSI / USB
  • 1200 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 6.5 ms / line
  • Photo w/ TPU, ADF opt
2000B
2001A Ex. 1640XL
ES-8500
  • SCSI + USB
    (1394, Eth-10/100 opt)
  • 1600 dpi + AF
  • 14 bit in/out
  • Dmax 3.6
  • 13.2 ms / line A3
  • Trk TPU & ADF opt
GT-10000+
ES-6000H
  • SCSI
    (1394 opt)
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax ?
  • 4.0 ms / line A3
  • ADF opt
~ 2003B

+ 2001B:
GT-30000
  • Duplex ADF
  • SCSI UW/N (1394 opt)
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax ?
  • 0.79 ms / line A3 (!)
  • 37.5 kg incl. ADF
~ present
Ex. 1680
ES-2200
  • SCSI + USB
    (1394, Eth-10/100 opt)
  • 1600 dpi + DF
  • 16 bit in/out
  • Dmax 3.6
  • 9.2 ms / line
  • Trk TPU & ADF opt
P. 1640SU
GT-8700(F)
  • SCSI + USB
  • 1600 dpi
  • 14 bit in, 8 bit out
  • Dmax 3.3
  • 8.7 ms / line
  • Photo w/ TPU, Office w/ ADF
~ 2003B
P. 1240U
GT-7700U
  • USB
  • 1200 dpi
  • 14 bit in/out
  • Dmax 3.0
  • 7 ms / line
  • TPU opt
P. 640U
GT-6700U
  • USB
  • 600 dpi
  • 12 bit in, 8 bit out
  • Dmax 3.0
  • 16 ms / line
2001B
2002A P. 2450
GT-9700F
  • TPU
  • USB2 + 1394
  • 2400 dpi
  • 16 bit in/out
  • Dmax 3.3
  • 11.0 ms / line
P. 1650
GT-8200U(F)
  • USB
  • 1600 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 10 ms / line
  • Photo w/ TPU
P. 1250
GT-7200U
  • USB
  • 1200 dpi
  • 16 bit in/out
  • Dmax 3.0
  • 36 ms / line
2002B
2003A P. 3200
GT-9800F
  • TPU
  • USB2 + 1394
  • 3200 dpi
  • 16 bit in/out
  • Dmax 3.4
  • 14.3 ms / line
P. 2400
GT-9300UF
  • TPU
  • USB2
  • 2400 dpi
  • 16 bit in/out
  • Dmax 3.3
  • 11 ms / line
~ 2004B
P. 1660
GT-8300UF
  • TPU
  • USB2
  • 1600 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 8.0 ms / line
2003B
2004A GT-15000
ES-7000H
  • SCSI
    (1394, Eth-10/100 opt)
  • 600 dpi
  • 16 bit in, 8 bit out
  • Dmax ?
  • 3.8 ms / line A3
  • ADF opt
P. 4870
GT-X700
  • TPU, fICE, pICE
  • USB2 + 1394
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.8
  • 16.9 (27?) ms / line
P. 3170
GT-9400UF
  • TPU, fICE
  • USB2
  • 3200 dpi
  • Dmax 3.4
  • 16 bit in/out
  • 8.7 ms / line
  • ADF opt
~ 2005B
P. 1670
GT-8400UF
  • TPU
  • USB2
  • 1600 dpi
  • 16 bit in/out
  • Dmax 3.1
  • 7.68 ms / line
2004B Ex. 10000XL
ES-10000G
  • USB2 + 1394
    (Eth-10/100 opt)
  • 2400 dpi + AF
  • 16 bit in/out
  • Dmax 3.8
  • 16 ms / line A3
  • 8.0 ms / line @1200 dpi refl.
  • Trk TPU & ADF opt
P. 4180
GT-F600
  • TPU, fICE
  • USB2
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.4
  • 17 ms / line
  • ADF opt
P. 2480
P. 2580
GT-F500
GT-F550
  • TPU, + film loader on 2580
  • USB2
  • 2400 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 11 ms / line
2005A P. 4990
GT-X800
  • Trk TPU, fICE, pICE
  • USB2 + 1394
  • 4800 dpi
  • 16 bit in/out
  • Dmax 4.0
  • 12.3 ms / line
~ 2007A
2005B P. 4490
GT-X750
  • Photo w/ TPU, Office w/ ADF
  • fICE
  • USB2
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.4
  • 16.96 ms / line
~ 2009B
P. 3490
P. 3590
GT-F520
GT-F570
  • TPU, + film loader on 3590
  • USB2
  • 3200 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 14.5 ms / line
~ 2007A
2006A P. V700
P. V750 Pro
GT-X900
GT-X970
  • Trk TPU, fICE, pICE
  • USB2
  • 6400 dpi / 4800 dpi + DL
  • 16 bit in/out
  • Dmax 4.0
  • 12.3 ms / line (V700 @4800 dpi)
  • 10.8 ms / line (V750 @4800 dpi)
2006B P. V350
GT-F700
  • TPU, film loader
  • USB2
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 21.5 ms / line
~ 2009B
2007A GT-2500
(doc scanner)
  • Duplex ADF
  • USB2
    (Eth-10/100 opt, std in Plus)
  • 1200 dpi
  • 16 bit in/out
  • Dmax ?
  • 4.9 ms / line
  • 2.5 ms @600 dpi
  • 1.2 ms @300 dpi
  • 27 ppm mono, 11 ppm color
+ 2008B:
GT-1500
  • ADF
  • USB2
  • 1200 dpi
  • 600 dpi ADF
  • 16 bit in/out
  • Dmax ?
  • 1.5 ms / line @300 dpi
  • 12 ppm @200 dpi, ADF
2007B P. V500
GT-X770
  • Photo w/ TPU, Office w/ ADF
  • fICE
  • USB2
  • 6400 dpi
  • 16 bit in/out
  • Dmax 3.4
  • 16.98 ms / line @4800 dpi
~ 2012B
2008A
2008B GT-20000
  • USB2, SCSI
    (Eth-10/100 opt, incl. in N Pro)
  • 600 dpi
  • 16 bit in/out
  • Dmax ?
  • 3.8 ms / line A3
  • ADF opt
2009A P. V300
GT-F720
  • TPU
  • USB2
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.2
  • ??? ms / line
2009B
2010A P. V600
GT-X820
  • TPU, fICE, pICE
  • USB2
  • 6400 dpi
  • 16 bit in/out
  • Dmax 3.4
  • 21.0 ms / line
2010B
2011A P. V330
GT-F730
  • TPU
  • USB2
  • 4800 dpi
  • 16 bit in/out
  • Dmax 3.2
  • 21.8 ms / line
  • 36 s, A4 @600 dpi, 24 bit
2011B
2012A
2012B

Legend:

Cons 1st, 2nd, 3rd
Consumer class, 1st, 2nd, 3rd tier
P.
Perfection [model name]
Ex.
Expression [model name]
TPU
Transparency unit. "Trk TPU" denotes one with a lamp tracking the scanner carriage, rather than just a lit film strip area.
ADF
Automatic Document Feeder. Duplex capabiliy is noted.
fICE
Digital ICE technology for film. Scans in infrared where film is near-transparent to identify dust, scratches and whatnot, then uses that information to clean up the scan.
pICE
Digital ICE technology for prints. Identifies dust by illuminating the print with two different lamps – the stuff that gives different shadows must be surface dust.
SCSI / USB
There were two versions, one SCSI-only and one USB-only
USB
USB 1.1
USB2
USB 2.0
1394
IEEE-1394, a.k.a. Firewire
PP
Parallel Port (IEEE-1284)
AF
(Automatic) Focus adjustment
DF
Dual Focus (focal point shift for transparencies by inserting a piece of glass in the optical path).
DL
Dual lens –device switches focal point for transparencies (~3 mm above glass vs. ~1 mm or so) by actually using two different lenses in the scanner carriage.
~ 2003B
Model remained available up to (and including) the second half of 2003

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Build notes

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Performances Notes

Perfection 636(U) / GT-7000(S/U)

Perfection 1640SU

Expression 1640XL

Perfection 2450

Perfection 3200

Perfection 3170

Perfection 4870

Perfection 4490

Perfection V600

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Servicing

The most common issue by far appears to be haze on the inside of the scanner glass – usually after several years, but sometimes right from the factory (oops). A scanner inevitably has to be just about airtight in order to keep dust out (unless it's a Plustek OpticPro UT24, that is…), and outgassing from lubricants and plastics eventually produces a film on the glass thick enough to become disturbing. Sure enough, this seems to be more common on "plastic fantastic" models, and virtually standard in e.g. the 2400, 1660 or 1240U. At least the outgassing rate would be expected to be decaying exponentially over time, so with a bit of luck the cleaning would not have to be repeated any time soon.

Lighter vertical lines are another reasonably common problem, it seems. This happens when dust gets onto the white strip in the calibration area that is hidden from view. Again, cleaning fixes this.

I have been able to find service manuals for the following models on the interwebs: Perfection 4870, 3200, 1640SU, 1240U, 1200, 610, 636 (or GT-7000, rather), plus Expression 1640XL, GT-12000 (Ex. 836XL), GT-10000, GT-9600 (Ex. 800), GT-9500 (Ex. 636), GT-5500, and some oldies like GT-8500, GT-5000 (Action Scanner II) and GT-300.

The following opening procedure seems to work for many 1st/2nd-tier consumer models:

  1. Work in a clean environment with no air movement. Wait for dust to settle. Make sure you have ample space around the scanner, as you may need to put the cover on the side. Keep all the required cleaning utensils handy. Get someone to help if feasible.
  2. Remove document cover.
  3. This should expose two screws at the hinges. Some newer models like the V500 have two more lurking on the underside of the unit near the front. (1640SU: 3 at the back, plus two under the front for the front cover that needs removing first.)
  4. Lift the back of the top case and push it towards the front to release some hinges in the front. (Newer models may have a bunch of nasty clips that are all kinds of fun upon reassembly.)
  5. Now you should be able to lift off the top case. There'll be a cable of very finite length going to the buttons, so carefully examine which side the top case could be flipped over to without damage. You may have or want to unplug said cable.
  6. Now you ought to be able to clean the inside of the scanner glass. Do so thoroughly but quickly in order to avoid getting too much dust in. You can try working with dishwashing detergent first and window cleaner (or alcohol + water) second if streaks are stubborn.
  7. Reassemble in reverse.

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Driver Fun with Old Scanners

Epson's support policy is pretty standard for big companies like that – driver support runs for a given time, typically 5 years after discontinuation in this case, and after that they won't move a finger no matter what (though they'll happily sell you a new device, of course). Technical feasibility does not figure in at all, so it's pure bureaucratic randomness. (You could have had even more fun by buying an HP though. They thoroughly wrecked their reputation by shipping lousy scanner software in the mid-2000s, and then there were cases of the warranty having run out before the scanner was even bought. All of which was a bit of a pity since the hardware actually was quite good and really fast in the higher-end models, even if the duplexer in their ADFs had a tendency to jam and make the whole scanner hiccup. Another "fun" company would have to be Microtek and their non-support – I thought they went bust a few years ago, but they still seem to be around.)

So what do you do with an older, well-built, perfectly functional device that just doesn't happen to have any drivers for a more modern operating system that you are using?

Let's focus on the last item. Like I said, support policy is all bureaucracy with no technical background. Now thankfully Epson's command language (at least for the higher-end models) was pretty well-established by about 2000. If you have the common problem of no Windows 7 drivers, it is often possible to coax the driver for a related newer model Into accepting yours by modifying the USB device ID in the respective INF file(s). (The SANE epson2 list of supported devices tends to be helpful here.) For example, people have used Expression 1600 drivers for the Perfection 1640SU, CX3200 for ye olde Perfection 610, Perfection 4990 drivers for the 2450 and Perfection 2400 drivers for 1200U and 1640SU (though the Expression 1600 tends to be a better match for the latter, save for color profile maybe – the lamps are different; if in doubt you could always buy an IT8.7 test target and get LProf and roll your own ICM). Actually 2400 or 3200 would seem closest to 2450 technically, but to each their own.

I have found that when using an Expression 1600 driver for the Perfection 1640SU, it is best to run setup and install the regular Expression 1600 driver first before pointing Device Manager to the modified INF file. Otherwise you'll get the English-language interface only (instead of whatever other language you'd prefer).

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Dmax, Scanning Speed and Pink Sky

You may be wondering what that Dmax spec is good for. Maybe you've heard that a higher value gives better results on slides. We'll try to, err, illuminate this subject a bit.

A scanner like the ones here uses a CCD sensor. Like any optical sensor, this has some limits:

  1. It only accepts a maximum number of photons before saturating. Each time a photon gets through, an electron is being collected in an internal "bucket". Once that is full, output voltage cannot increase any further.
  2. If light intensity gets very low, each cell (subpixel) may only be collecting a very limited number of photons. As you may know from statistics, the exact number of photons becomes very random. So if it's an average of 2.5 photons per cell, one cell might collect 5, another 2, another 1, another 4, and another none at all. You can try that by throwing pebbles into a bunch of buckets. In an image, such randomness is nothing but noise. In addition, the electronics used to read out the CCD and perform analog to digital conversion also contribute noise, and back in the day it would not have been unusual to see it dominate the overall noise level entirely.

Now the scanner manufacturer has to couple this given dynamic range to the amount of light that the sensor receives in a sensible way. Now that depends on:

  1. The light source. Note that a scanner has two different light sources if it is equipped with a transparency unit.
  2. How much light the scanned object either reflects (when using the lamp in the carriage) or transmits (when using the transparency unit). As in many other fields, a logarithmic measure has proven practical – here it's called the optical density D.
    D = -log10 (T) with T being the linear transmissivity (so we're looking at the transparency unit case). D = 1 means that only 1/10 of the light gets through, with D = 2 it's only 1/100, and so on. A typical reflective target might show a minimum D of 0.3 and a maximum D of 2, while a slide might cover almost 0 to 3.0 or even 4.0.
    Converting D to photographics stops (or effective number of ADC bits) is easy, by the way:
    #stops = 10/3 * D
    So D = 3.6 amounts to 12 stops or bits, which means that a real-life 12-bit converter is likely to achieve rather less. (If you look at scanner models with 12 bits per channel, you'll notice that none of them are specified with a Dmax over 3.3 = 11 stops.)
  3. Exposure time. A scanner has no variable aperture like a camera does, but exposure time can be controlled electronically.

You'll probably notice that there is some degree of freedom here. You obviously cannot do much about the kind of things to be scanned, but you do have some choice when it comes to what lamp to install (and what sort of power to run it with), and you can vary exposure time widely, though you probably don't want to go over a few milliseconds, or else it becomes the limiting factor in scanning speed. You're going to want to coordinate these two factors in such a way that the minimum D ever to be encountered just about aligns with the sensor's saturation limit. Otherwise you're either wasting dynamic range and getting more noise in the shadows than necessary, or clipping the highlights, which is even worse. I assume that Epson's Dmax specification assumes this condition, in which case dynamic range = Dmax. Otherwise the dynamic range is given by the difference of Dmax and Dmin. Dmin = 0 can, unfortunately, not be taken for granted, so "cheating" is easily possible.

Have you ever wondered why some scanners produce pink sky on slides? What happens is that the scanner manufacturer apparently messed up sensor dynamic range alignment in transparency mode (or optimized for much less-transparent negatives, or felt like playing the Dmax numbers game), yielding badly overexposed highlights with red, geen and blue channels all being clipped and maxed out. And then automatic white balance comes along and pulls the green channel down (a bit of a green cast is not unusual in flourescent lamps). The result: Pink sky. Yuck.
You can try rectifying this issue by stacking an ND 0.3 (ND2) neutral density filter on top of the slide though; the loss of a polarizer may also do the job. (Noise in the shadows will obviously increase, but unlike blown-out sky it may be inconspicuous or could be addressed with noise reduction software.)

Now how do we get a massive D range equalling 12 bits or more to display on conventional graphics cards and monitors with 8 bits per channel? Simple, nonlinear quantization. IOW, the possible 8-bit values are not equidistant. Ever heard of a "gamma curve"? That defines the nonlinear input/output relation. This makes sense because our eyes do not register light in a linear fashion either. Straight linear 8 bit would give more resolution than needed in bright areas and not enough in dark ones. The nonlinearity permits getting a lot more mileage out of these bits.

Note that it is quite possible to apply gamma within the scanner and output 8 bits per channel only, giving a speed advantage if interface throughput is the bottleneck. (This, however, means that for best results you must set approximate black and white levels and gamma in the scanning software, and cannot rely on doing it all afterwards.) Obviously for this to work properly, the scanner's (linear) ADC must provide a fair bit more resolution, typically at least 12 to 14 bits depending on CCD.

We were talking about speed earlier. What are the limiting factors here?

  1. Exposure time. High-resolution sensors at full resolution are usually light-starved. Can you guess why the Expression models, Perfection 4990 and V7x0 use a tracking light source in the transparency unit now? An A3-sized scanner would hold an advantage here, by the way, since it can scan an A4 document sideways (reducing the number of lines to scan). Reducing exposure time at lower resolutions seems to be a common tradeoff – the line of thought being that you'd only be using these for reflective documents anyway, so some more (but still irrelevant) noise wouldn't matter, while speed would.
  2. Image processing speed. ADC sample rate and processor power tend to be rather finite.
  3. External interface throughput. My Expression 1640XL, for example, could churn out data almost twice as fast as its measly USB 1.1 interface will allow when using, say, 300 dpi. (The Perfection 1640SU still is about 25% faster than USB 1.1.) And if you do the math on the monstrous GT-30000's real-life 8.57 seconds for a 600 dpi A3 scan, you'll know why that one came with an UW SCSI interface… (Hint: That's 199 MiB per scan.)
  4. Maximum stepper motor speed. That's usually the limit for very low resolutions, like in prescan.

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Focusing and Sharpness

Most scanners use fixed-focus optics, with a focal point just slightly above the scanner glass (maybe 1 mm). That's perfectly fine at normal resolutions like 300 or 600 dpi, but once you are getting into 1600 dpi and higher territory and you need that kind of resolution because you want to scan negatives or slides, the range of optimum sharpness shrinks and shrinks. Now the negative holders you got with your scanner may or may not position the material at an optimum distance, as calibration in consumer scanners does not tend to be that precise. Some experimentation is definitely worth a shot. (I like the coin method. Australian 5 cent coins appear to be 1.3 mm thick, so this 4870's focus in transparency mode must be about 5 mm above the glass, rather higher than it should be.) Flatness of the material also becomes increasingly critical, of course.

Epson's "dual focus" models have two selectable focal points, one at about 1 mm above the scanner glass for reflective targets and the other at about 3 mm for transparencies. This works by inserting a piece of glass in the optical path (and thereby lengthening the latter) for one setting. Having continually adjustable focus (like the Expression models and other high-res pro scanners do) is even better, of course, sparing you from the potentially tedious process of tweaking height.

What manufacturers don't tell you is that basic depth of field (DoF) not only is much greater in CCD-based scanners when compared to their CIS cousins (all the scanners listed here use CCD sensors, which admittedly have other drawbacks like potential geometry issues and limited maximum sharpness introduced by their optics), there also is some variation among CCD units. Many of them seem to range around 3 to 7 mm at moderate resolutions, but I have also seen values of several cm quoted.

You may remember that DoF in cameras is tied to focal length and aperture, and scanners are no different. Now for a given focal length, aperture also determines how much light gets to the sensor, which brings us to the dynamic range vs. speed tradeoffs that I mentioned in the previous section. If you want to make a speedy high resolution scanner with high dynamic range, you'll need enough light for that, so you'd want a larger aperture, but that in turn reduces DoF and potentially the resolution of the optics as well (see typical effect of stopping down in lenses), so your chances of actually being able to use the high sensor resolution drop. Has anyone ever thought of producing variable-aperture scanner optics?

If you need to make super sharp scans with absolutely no geometric distortion, CIS actually is your best bet – but only if your original can be quite literally pressed against the scanner glass.

Did you realize that a 1200 dpi scan of a 35mm slide (36x24 mm) only gives a 2 megapixel image? Now granted, those would count about as much as 4 area sensor (camera) megapixels, but still, we're a bit past that sort of resolution already. (Some people apparently scan their slides at 600 dpi and are happy with that!) Good film on a sharp shot may give about 100 lp/mm, that's around 5000 dpi. Since flatbeds have a really hard time getting anywhere near their nominal resolution even with focus being bang on (a Perfection V700 might reach the resolution equivalent of around 2500 "real" ppi), this should explain why film scanners stil hold a resolution advantage. Flatbeds are handy for medium format though.

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Created: 2014-08-27
Last modified: 2015-09-11