Choosing a Photo Scanner

In a way you can call scanners a bridge between standard film based photography and digital photography, because they convert your old photographs (and in some cases; negatives) into the same compilations of bits and bytes that digital cameras use. This means that you can carry out the same processes on the scanned images, such as enlarging them using your printer, or touching them up in imaging software.

Though scanners are by no means as confusing as digital cameras, they can still be a bit daunting if you don't understand the technical terms used, and what they mean for your images. But not to worry, that's what this article is for! Read on to learn all about what matters in a scanner...

Please note that this article will focus on the photography aspects of a scanner... i.e. to scan in photographs. Scanners can do a lot more than that, but seeing as this is a guide about photography, that is what this will mainly be focussing on.

Resolution

A scanner's resolution is what will decide how much detail of the image the scanner can record. Unlike with digital cameras, scanners measure resolution as the amount of pixels it can record per inch, rather than in the whole image. The unit is called "dots per inch", and is more commonly reffered to as "dpi". If you have a scanner that can record 1200dpi, that means that for every inch of an image, the scanner can make 1200 pixels of the computerised image. So, with a 1200dpi scanner, a 6 inch by 4 inch photograph would end up as a file with 7200 pixels by 4800 pixels.

Knowing what resolution you need can be a tricky thing... even some professionals have a hard time working out the resolution needed for some things. As a general guide though, remember that it is generally accepted that an output of 240dpi is the minimum needed for a great-looking print... so if you have a 6 inch by four inch photo that you want to enlarge to 12 by 8 inches, and you know that you need to have 240dpi for each inch of your enlarged image, you would realise that you would therefore need a scanning resolution of 2x 240dpi, which is 480dpi. Of course, remember that in terms of resolution, more is nearly always better.. though don't overdo it, because if you do you'll have huge files that you don't have space for!

For a more detailed description of resolution, please see the article on choosing a digital camera.

150dpi 300dpi 600dpi 1200dpi

Optical v. Interpolated Resolution

The issue of resolution doesn't end with choosing dpi... we now have to deal with optical v. interpolated resolution. Sometimes, scanner manufacturers mention a really, really high numbers as the scanner's highest resolution. The catch? It's only interpolated. This means that all the scanner is doing is taking, say, four pixels, then stuffing some more that it's guessed inbetween those pixels. The true resolution of a scanner is its optical resolution: that's a measurement of how many pixels the scanner can actually read per inch, not just guess at. For instance, I once had a scanner that advertised a resolution of 9600dpi! However, the real, optical resolution was only 600dpi. Never be fooled by interpolated resolution: the photo-editing software that comes with your scanner will do a much better job of enlarging by "guessing" the pixels anyway!

Bit depth

As you may know, computers store data using things called "bits". A bit is one binary digit. As you may also know, binary is a number system that only has two digits: 0 and 1. Therefore, one bit can only represent two different values: 0 or 1. Now, "bit depth" is a measurement of how many bits, or 1s and 0s, that the scanner uses to represent each pixel of your image. If you had, for example, a one-bit scanner, that would mean that for each pixel that the scanner could scan in, it would only be able to detect two different colours: white or black. Now, if you had a two-bit scanner, that means that each pixel of the image would be represented by two 1s or 0s. This means each pixel could be represented by either of these combinations: 00, 01, 10, 11. As you can see, that's a total of four combinations. This means that, with a two-bit scanner, images would be scanned in using four different colours: black, dark grey, light grey and white. Now, in real life, scanners have the ability to scan in many more bits than that: proper photo scanners can scan in 48 bits... which equates to a whopping 281 trillion, 474 billion, 976 million, 710 thousand and 6 hundred and 56 possible colours for each pixel... is that enough for you?

What all this means in short terms is, that a scanner with a higher bit depth will be able to recreate more different colours than scanners with a lower bit depth, thus giving the final computer image files a smoother, more graduating, and overall more accurately-coloured appearance. Below is a selection of of images to demonstrate this:

Optical Density (aka. "Dynamic Range")

This specification of a scanner can be quite hard to come by, but it is very important for photo scanners nonetheless. It is a measure of how good the scanner is at recording different amounts of light i.e. lighter and darker parts of an image. It is on a scale of 1 to 4, where 1 means bright, pure white and 4 means dark, pure black. Scanners tend to be in the range of about 2.8 to 3.4. The higher the OD, the larger range of brightnesses the scanner can record, thus providing you with images with a greater tonal depth. Scanning negatives requires the highest dynamic range, because negatives are very dark, meaning that to successfully scan a negative, the scanner must be able to distinguish between lots of very similar dark light levels.

Below is an (albeit very simplified) example of how dynamic range can affect your images:

Good Dynamic Range > > Poor Dynamic Range

Types of Scanner

Now, though your local computer store may not lead you to believe so, there are actually two (or even more, but we won't cover those here) very different types of scanner: flatbed scanners and negative scanners. Flatbed scanners are by far the most common type of scanner; they're the ones that look like a small photocopier, with a bed of glass which you place your image on, and a lid that then goes over that bed. These are capable of scanning basically anything... which isn't the case for film scanners. As their name suggests, they only scan film negatives. However, they do a much better job at scanning negatives than a film scanner could hope to ever do. This is mainly because that have very high optical resolutions and bit depths, as well as dust removal technology (though admittedly, there is one flatbed scanner on the market with this technology as well).

Types of Flatbed Scanner

Yep, there's more! You see, though all flatbed scanners will scan a photograph in for you (though with varying degrees of success), only some more expensive ones have the ability to scan in negatives. These scanners are often denoted by the word "photo" somewhere in their name, and you'll find a mention of an "integrated transparency unit (TPU)" somewhere in the features or specifications. A TPU is basically a light in the lid of the scanner, which shines light though the negatives. This is because to see negatives, as I'm sure you know, you need to let light pass through them, rather than reflect off them as you do for normal photographs. Without a TPU you have absolutely no chance of scanning in negatives... so make sure you remember to check your scanner has a TPU before you buy it if you do want to scan negatives.

Connectivity Interface

There are several different ways by which a scanner can connect to your computer: USB 1.1, USB 2 (a much faster version of USB 1.1), or FireWire (a contender of USB 2). In fact, there's one scanner you can buy that doesn't even need to be connected to a computer at all to scan images! That aside, it is important that if you intend to do alot of scanning, you buy a scanner that is up to the job: if you're going to be archiving hundreds of high-resolution negatives, there's no way a USB 1,1 scanner will keep up: you'll find yourself waiting quite alot just for the scanner to send its data to the computer. In these situations USB 2, or even better - FireWire, are a much better alternative. You'll find your scanner will be able to operate much faster if using one of these interfaces, because it won't need to wait while it sends all the data to the computer before continuing with the next part of an image.

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