Choosing a Good Scanner

Some Easy Tips

There are nowadays a lot of scanners available to choose from - they have gone from being a very specialized device to a common accessory.  Hopefully the comments below will help you to evaluate and select a suitable scanner for yourself.

Cheaper isn't the same as better

The availability of scanners for as little as $50 is both good news and bad news.  It is wonderful that anyone can now afford such a device, but it is a deceptive illusion to believe that you're going to get good quality scans out of a $50 unit.

Yes, I know, the $50 scanner you just bought says on the box '2400 dpi equivalent resolution' or some other amazingly impressive big number, and of course, the bigger the number, the better - right?  Well, that is a bit like saying 'my car is better than yours because it has a bigger engine'.  The resolution is only one factor that goes into determining the quality and capabilities of the scanner.  But it is the most commonly quoted one, so lets talk about it first.

Scanning Resolution

Resolution can be quoted one of two different ways - as 'optical' resolution, or as some other type of resolution - 'equivalent' or 'interpolated' or some other term might be used.  The only number you want to look at is the 'optical' resolution - the other numbers use trickery to seemingly increase the resolution but which you can probably do better within an image editing program.  Optical resolution is the only true type of resolution measure.  Have a look at the bottom two samples on the page showing different resolution scans to see the difference between 600 dpi optical and the same scanner using 600 dpi optical but interpolating up to 1200 dpi - sure, the image is bigger, but it isn't any sharper, quite the opposite, it is a bit blurry, and no more detail can be seen.  The higher resolution is just a waste of file and screen space accordingly.

Optical resolution will either be quoted as one single number, or as two numbers - for example '1200 dpi resolution' or maybe '1200x600 dpi resolution'.  If it shows only one number, it probably means that the scanner is capable of the same resolution both horizontally and vertically; if it shows two numbers it is showing you the different resolution in both horizontal and vertical planes.

You should get a scanner that is capable of at least 600 dpi resolution, although a 'good' scanner at 300 dpi will almost always give you a better final image than a 'bad' scanner at 600 dpi.  Most of the time, your scanning will be at 300 dpi and lower resolutions in any case.  Most scanners today (May 03) seem to have at least 600 dpi optical resolution, with 1200 dpi being common and 1600 or even 2400 appearing in some units.

Actual Resolution

A scanner might boast a very high optical resolution, but if the scanner mechanism isn't excellent, it won't actually give an accurate image at that resolution.  For example, at 1000 dpi the scanner mechanism is moving in increments of one thousandth of an inch - if the mechanism has even a one half thousandth of an inch imprecision in its exact consistent movement, then a 1000 dpi scan will be blurred and unclear.

Pixel Depth

This tells you the number of different shades of color that the scanner can distinguish between.  A good scanner must be capable of at least 8 bits per color channel (ie 24 bits in total for the three channels Red, Green and Blue); better scanners might go up to 10 bits or even 12 bits (ie 30 or 36 bits total).  High end scanners go up to 16 bits/channel.

Most image editing programs and printers can only handle 8 bits, however, so anything over 8 bits rapidly becomes not very useful, although for subtle reasons, there is some extra value in having more bits of data to start with.

Dynamic Range

This is one of the most important measures of a scanner - how many different shades of brightness can it distinguish between full white and full black.  Unfortunately - and perhaps because it is such an important measure - it is close to impossible to find this data on any normal scanners.

However, if you can find the number, the bigger the number, the better, and because it is a logarithmic measure, even a very small difference in number makes a big difference in visible image quality.

A good scanner will score over 3.1.  A great scanner (not one that any of us could afford, alas) would score over 4.

Dynamic range is sometimes referred to as D(max) on a specification sheet.

Color Consistency and Random Noise

If a scanner is scanning a solid color, each pixel of the scan should have exactly the same color value as the other ones in the solid color shape.  Some scanners are better at giving consistent values than others - a poor scanner gives an image a bit like what you might get with a weak signal on a television set, or the image on a copy of a copy of a VHS tape, whereas a good scanner gives a clear image more akin to what you'd get from a laser disk or DVD.

This is, alas, another thing that the scanner statistics rarely tell you, but it is something to look for when evaluating scanners if you have a chance to actually inspect a scan in Photoshop or another image editing program (use the sample tool and move it around a solid shape while seeing in the information palette the changing values for RGB).

Interface Type

There are three main types of scanner interface (ie how it connects to your computer).

The cheapest and worst is a parallel (printer) interface that just plugs into the parallel port, maybe sharing it with a printer (and maybe having some problems in the process).  This type of interface is indicative of an overall cheap scanner, and is the slowest for transferring scanning data.

The other two interfaces are becoming increasingly comparable.  It used to be that the best (and for a while, the only) type of scanner interface was a SCSI type interface; and nowadays there are three different types of SCSI - SCSI 1, SCSI 2 and SCSI 3, getting successively faster and generally better.  Most pc's don't come with a SCSI interface as standard, and so this means as well as buying the scanner, you have to buy a SCSI interface card (although some scanners come complete with an interface card as well).

The latest type of interface is the USB interface.  The newest implementation of USB (2.0) is almost as fast as SCSI and is much more 'plug and play' than SCSI.  SCSCI often has somewhat complicated issues not only regarding simple seeming things like plug types but also termination and logical address as well; by contrast, usually with USB all you need to do is plug the device in and the computer recognises it.  Lovely and easy for those of us that don't enjoy technical challenges!  The earlier 1.1 version USB is more common on computers, but 2.0 devices are backwards compatible with 1.1 computer ports so this isn't a problem.

Scanning Speed

How fast does the scanner scan a page?  That question, by itself, is a bit like saying 'how high is up' because for most scanners, the scanning speed depends on the resolution you have selected.  It might even seem obvious that a 300 dpi scan should be twice the speed of a 600 dpi scan - but, if that seems obvious to you, be warned - it is likely to be closer to four times faster (or slower - depending on which way you're comparing) because the resolution measurements are in two dimensions not one.

Scanning speed depends not only on resolution but also on the selected image area that you are scanning - and in this case, there is no trick.  Scan twice as much area, take twice as long.

I've seen scanners advertised with claimed scan speeds as fast as 4 seconds, but I've never seen the definition of what size object was scanned in 4 seconds, or at what resolution.  To give you a real world benchmark, my HP 6200C will scan a regular sheet of letter paper (8.5"x11" - about the same as A4) in 24 bit color mode and at 300 dpi in 30 seconds, timed from when I click on the 'start scan' button to when the scan is complete.  At 150 dpi, the same scan takes 14 seconds, and at 600 dpi it takes 195 seconds.

It is interesting that the theory and reality diverge so much, and there are some technical reasons why (if anyone wants to know, email me and I'll add some more detail accordingly).  For now, it is simplest to just observe the reality of the numbers seen which you can use to benchmark other scanners.  If they scan no more than twice as slow as this, it is probably acceptably fast, and if they scan faster, then they are definitely very good indeed.

Maximum Scanning Area

Low end units scan a maximum of a regular letter sheet of paper - 8.5"x11", or perhaps a bit more (out to about 11.7", which allows it to scan both letter sized and international A4 size as well).  Better scanners will have a larger area - for example legal (8.5"x14") or (uncommonly) larger still.  You might think that an ordinary stamp is usually small, and so a letter sized scanner would be more than sufficient.  However, it is possible you might occasionally want to scan something larger such as a sheet of stamps, and then you might find yourself benefiting from the extra size of the legal sized scanner.

Flat Bed or?

Most scanners are what are called a 'flat bed' scanner - think of it the same as a photocopier, where you place something on a piece of (flat) glass, close the lid, and then push a button and the copy (scan) happens.

Some scanners require you to feed a piece of paper into a unit, more like how a fax machine works (a fax machine is, after all, just a low resolution scanner).  These are not nearly as good for most philatelic purposes.

Note also that the flat bed of the scanner should not move.  Instead, there should be a moving mirror underneath the glass that does the moving, while the body of the scanner itself stays stationary.

The lid should also be able to come off, or in some other way accommodate thick books if you're trying to scan pages from books.

Back when flat bed scanners were ten and twenty times more expensive than they are now, 'hand scanners' were also sold as an inexpensive way of scanning.  These are dreadful units and it is close to impossible to easily get good accurate scans from them.  If you have one - throw it away and replace it with even the cheapest modern flatbed scanner!  You'll be amazed at the improvement in quality and improvement in convenience.

Summary

As with most things, there is a correlation between price and quality.  Try not to get the cheapest unit available, because scanning stamps is actually a fairly demanding application for a scanner - you're dealing with very fine detail on small items that sometimes you'll want to magnify (ie scan at a high resolution).

I've owned several different Hewlett Packard scanners over the years, and never had a bad one.  Epson, Canon and Nikon also make good scanners, but there are also a lot of low end scanners out there with brand names that I've never heard of.  Spend a few dollars more and get a brand name you recognise and trust.