Scanning

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Scanning is the process of digitising negatives, slides or photographic prints so that they can be stored and possibly postprocessed in a computer system.

Scanners are available cheaply, and are often provided free within computer packages, or fitted as part of all in one printers. However, the quality achievable from cheap flat-bed scanners is vastly inferior to that from high-end equipment.

Contents 1 Key issues in scanning 1.1 Original Image Quality 1.2 Generation of the original 1.3 Fragility of the original 1.4 Mechanical and optical quality of the scanner 1.5 Ability of the scanner to scan the original evenly 1.6 Resolution of the scanner 1.7 Bit density of the scanner 1.8 Image density 1.9 Software compensation

Key issues in scanning

The key issues in scanning are:

1. The quality of the original
2. The generation of the original (first generation, second generation, etc)
3. The fragility of the original
4. Mechanical and optical quality and focus of the scanner components
5. The ability of the scanner to scan the original evenly
6. The resolution of the scanner, in dots per inch
7. The bit density of the scanner (8, 10, 12, 14 or 16 bit, typically)
8. The density range of the scanner (DMax minus DMin, range is 0 to 4)
9. The ability of software to compensate for problems or limitations earlier in the process

Original Image Quality

Larger negatives or transparencies, lower ISO film, better exposure and overall better condition (absence of dust, scratches, other marks) dramatically improves the quality of the final scan.

Generation of the original

Ideally, negatives or transparencies should be scanned rather than prints, as they have only been through a lens once before the reach the scanner. Flatbed Scanners are the best choice for scanning prints, but flatbeds with a hood or other attachment for scanning negatives or transparencies never compare well to dedicated negative or transparency scanners. These include Nikon Coolscans, Imacons and drum scanners.

Fragility of the original

Very fragile negatives or transparencies cannot be drum scanned. Both drum scanners and Imacon scanners require 35 mm transparencies to be removed from their mounts before scanning. Nikon Coolscans pull the transparency or negative into the scanner on a mount. All scanners heat the image to some extent.

Mechanical and optical quality of the scanner

The scanner must in some way move its scanning head across the original, or move the original across the scan head. Irregularities in the mechanical movement produce distortions in the final image. The scanner must also focus the image. In principal, the optics for a flatbed scanner can be far less sophisticated than those in a negative or transparency scanner because they are scanning a much larger area. However, a print has already been through the optics of an enlarger, which means that the imperfections of the scanner optics are multiplied by those of the enlarger. In high end scanners such as Coolscans and Imacons, the optics are specified (Nikon ED glass for Coolscans, Rodenstock for Imacons), as this is a crucial piece of information in selecting a scanner.

Ability of the scanner to scan the original evenly

Nikon Coolscans suffer from being unable to correct any curvature on the negative or transparency, which is typical for most 35mm slides and negative strips. Both Imacons and drum scanners are able to correct this.

Resolution of the scanner

The optical resolution of the scanner is the primary indication of how well it will capture fine details. However, many consumer scanners are quoted for their interpolated resolution. Typically, flatbed scanners have a native resolution of around 300 dpi, whereas negative and transparency scanners will have a native resolution between 2400 and 8000 dpi. Clearly, details which are finer than the grain of the negative or transparency cannot be captured in any event. However, in most cases, scanning is able to resolve details which are never evident on prints.

Bit density of the scanner

8 bit per channel scanning produces a colour-degraded image (also known as 24 bit, because with RGB scanning 8 bits per channel is 24 bit in total). 14 bit or 16 bit scanning will generally capture all of the resolvable colours within the colour space of the scanner. 10 bit or 12 bit scanning will still produce a 16 bit image, because TIFF and JPEG can only save 8 or 16 bits, so it is important to check what the scanner is actually recording, rather than merely the format of the image produced.

Image density

High end scanners can produce an image density of 3.9 or above, whereas cheaper scanners may only be achieving a density of 3.0. The image density refers to the dynamic range captured. 0 is pure white and 4 very black. The dynamic range is the scanner's DMax minus its DMin, and the scale is logarithmic. Photographic colour prints have a dynamic range of less than 2.0, so, even if a flatbed scanner has a rated density of better than 3.0, it will still only be capturing 2.0 if you are scanning prints. Negatives have a density of 2.8 or better, and slides 3.2. It is still essential to have greater dynamic range in the scanner than in the original.

Software compensation

The software can instruct the scanner to operate multiple passes, which can increase the resolution or bit density, though, if the scanner moves its components or the original mechanically, this can actually degrade quality if the registration is not exact. Software can also compensate for poor tonal range and for dust and scratches.