Choosing and using a scanner

Although the scanners in multi-function printers can deliver high-quality prints from scanned snapshots, few are able to scan images on film. (Source: Epson.)

Most photo-capable scanners can be used for scanning both prints and images on film. Many can also enable you to improve the images by removing scratches and creases and resurrecting colours that have faded over time.  

Like digital cameras, scanners create a copy of an object constructed from millions of tiny ‘picture elements’ ““ or ‘pixels’. The end result of a scan is virtually indistinguishable from an image file captured by a digital camera. In fact, the  same file formats are used to output the digitised images. This allows scanners to be to be used to ‘photograph’ small, three-dimensional objects, such as jewellery and electronic components.

Many people acquire a scanner when they buy a multi-function printer.  These simple scanners are easy to use and allow you to make prints directly from scans. Most support enlargement up to A4 size (the maximum print size the device can produce).

However, although the higher-priced models can perform acceptably when scanning prints, few are able to scan images on film. Low-cost film scanners are available but many will only scan negatives, most are restricted to 35mm format and the cheapest models scan at relatively low resolutions. It’s usually worthwhile paying a little more for a flatbed scanner that can scan filmstrips and mounted slides.

The most popular flatbed scanners have scanbeds large enough to accommodate A4 (or the slightly larger US ‘letter’ sized) documents. Most A4 photo scanners can accommodate 120 and 220 film strips ““ and the high-end models are often able to scan 4×5-inch and even 8×10 inch negatives and transparencies.

A high-resolution scanner with film-scanning capabilities is ideal for photo enthusiasts and people with lots of old photographs on film.

A4 scanners can also scan smaller documents and prints and many can scan several of these at a time and deliver the scans as separate image files. You can scan larger originals ““ but only segment by segment. And the segments must be joined afterwards in suitable software.

When scanning film images on a flatbed scanner a special ‘transparency unit’ is required. This is a second light source that’s usually built into the lid of the scanner and covered by a slide-out document mat when reflective originals are scanned.

Removing the transparency unit cover to reveal the secondary light source in the transparency window. (Source: Epson.)

Scanning Resolution

Scanning resolution influences the degree to which you can enlarge the original. It should be dictated by the size of the original images and required output dimensions.

Two numbers represent optical resolution in scanner specifications: the number of sensors in the sensor array and how finely the stepper motor can move the sensor across the scanbed. If a scanner claims an optical resolution of 4800 x 9600, the 4800 refers to the number of photosites in the scanning head, while the 9600 figure represents the number of steps by which the head is moved to cover the scanbed.

The resolution of your scans should be dictated by the ways you plan to use the digitised images. For most people it will be one of these alternatives:

1. The image will be used at the same size as the original.

2. The image will be enlarged as it is scanned.

3. The image will be enlarged and edited after scanning.

As a general rule of thumb, the following resolutions provide optimum file sizes and output quality:

1. Set the resolution between 72 dpi and 150 dpi for images that will be emailed, uploaded to the Web or only viewed on-screen.

2. Set the resolution to 300 dpi for images that will be printed up to A4 size.

3. For images that will be enlarged and edited after scanning, increase the resolution by the amount the image will be enlarged.

Most printers have been designed to work with image resolutions of between 180 ppi (pixels/inch) and 300 ppi. For images that will be printed at small sizes, 300 ppi resolution is recommended as these prints will be examined closely and you want to make sure all the image details can be seen.

Images that will be enlarged to A3 size (297 x 420 mm) are viewed from a greater distance so their resolution can be reduced to between 200 ppi and 250 ppi. Photos that will be printed at A2 size (420 x 594 mm) can be scanned at 180 ppi. These figures should be seen as the lowest resolution required for acceptable print quality at the specified image size.

Bit Depth

Bit depth defines the number of hues and tones (colours) a device can reproduce. JPEG files are always 8 bits, in which each pixel can have one of 256 different intensity levels (between black and white). Since all printers work with 8-bit files, this should be enough.

In contrast, raw files can be 12 or 14 bits and raw files can be converted into 16-bit TIFF files for editing. However, each pixel in a 12-bit raw file can have any of 4095 intensity levels, whereas each pixel in a 16-bit TIFF file can cover 65,536 discrete intensity levels. High bit depths naturally lead to large image files.

Unlike digital cameras where the bit depth figures in the A/D conversion specifications correspond with a single colour channel, the bit depth figures quoted for scanners combine the bit depth of the three colour channels: red, green and blue. Even entry-level scanners can reproduce 8 bits (256 colours) for each of the three channels, enabling them to cover 768 hues and tones.

Dedicated photo scanners usually support 48-bit scanning, which combines the data from three 16-bit channels, each covering 65,536 colours, providing total coverage of 196,608 hues and tones. The higher the bit depth, the larger the image files the device can deliver ““ and the greater the scope in these files for editing the images. (Low bit depths are best for images that will be emailed or viewed on-screen.)

Successful scanning results from balancing bit depth against output size. You need a high enough bit depth to deliver enough image data to provide ‘room’ to edit the file yet not so much as to make the file too large to utilise and store.

Density Range

Like a digital camera, a scanner must be able to capture the full tonal gamut of the original without blowing out the brightest highlights and turning the shadows to black. This isn’t as simple as it seems. However, you can find a scanner’s density range by looking at the D-max (maximum density) and D-min (minimum density) figures in the specifications.

Few manufacturers publish D-min figures for scanners. However, it is safe to assume a high-quality scanner will have a D-min of approximately 0.3. To find a scanner’s density range, simply subtract 0.3 (the D-min) from the D-max figure.

The higher the density range, the more detail the scanner can resolve, particularly in shadows. You need a dynamic range of about 2.0 for print scanning, 3.2 for film negatives and 4.0 to 4.2 for slides. This means a D-max of 2.3 for prints, 3.5 for negatives and at least 4.3 for scanning slides.

Scanning Modes

Most scanners provide three scanning modes; Full Auto, Home and Professional. The Full Auto mode is designed for novice users and only provides basic corrections like dust removal and colour restoration. The Home mode lets you adjust brightness and colour and optimise text or line art scanning. The Professional mode provides a full range of adjustments and allows you to scan to a specific output ‘target’ size, such as A4.

The Professional mode provides a full range of adjustments and allows you to set the scanning resolution and output size.

General purpose scanners and scanners in multi-function devices may add an Office Mode for scanning multiple documents of the same size. This mode usually requires an automatic document feeder to be fitted.

Image Correction Tools

Most photo scanners come with facilities for removing dust and minor blemishes, restoring faded colours and boosting shadow details in scans of backlit subjects. In general, these adjustments can improve the appearance of your scans. However, they can’t work miracles so don’t expect all the blemishes to be removed from badly damaged originals ““ or full, lifelike colours to be restored when the originals have faded almost to vanishing.

Dust Removal is mainly used when scanning images on film but can also be used for subduing scratches on the surface of films or prints.

Colour Restoration can be applied to prints and film scans and works mainly by boosting saturation (which is reduced in faded images). Most scanners apply the adjustment globally to the entire image.

The Colour Restoration tool can bring faded and discoloured originals (top) back to life (bottom).

Backlight Correction boosts shadows without affecting highlights and midtones and can be used with prints and slides. Many scanners allow you to adjust the level of correction to obtain natural-looking results.

Descreening provides smoother scans of photographs from books and magazines. It can also suppress the rippled patterns that can occasionally appear in subtly-shaded parts of the image, such as skin tones. However, it may also result in a loss of image sharpness.

Calculating Scanning Resolution

You can calculate the scanning resolution you require by measuring the original picture. Because resolution is normally expressed in inches, calculations are easiest when the dimensions of the original are in inches as well.

A 35mm film frame measures 1.5 x 1.0 inches. If you want to print the scan at snapshot size, the scanning resolution should be 300 ppi x 1.5 x 6 inches, which is 2700 dpi. (Because pixels are square, it’s only necessary to make this calculation for one side of the image.)

To print a scan from a 35mm frame at A4 size (11.7 x 8.3 inches), the calculation is 300 x 1.5 x 11.7 = 5265 dpi. For A3 size (16.5 x 11.7 inches), the scanning resolution must be 7425 dpi, while an A2 enlargement (23.4 x 16.5 inches) requires a scanning resolution of 10,530 dpi.

Scanning at a higher-than-necessary resolution is time-consuming and produces more image data than you need. However, the digitised image can be printed at smaller output sizes without loss of quality.

Image quality is always sacrificed when you exceed the maximum output size for the image resolution. Scanning a 6 x 4 inch snapshot print at 1440 dpi produces an image with 8640 x 5760 pixels, which could be printed at A2 size, although the end result may contain artefacts because of the high degree of enlargement. It would be better to scan at 600 dpi, which produces a 3600 x 2400 pixel image. This allows you to enlarge the image to around A4 size.

 

This article is an excerpt from Mastering Digital Photography 3rd Edition.