主题：D70 VS D100 -- 高头庄

Scanner Features

Bit depth

Its continuous voltage readings are converted to a digital signal for output. This analogue to digital conversion usually produces a signal of between 8 and 16 bits for each of the three channels, 8 giving 256 different values and 16 giving some 65 thousand for each channel. Most current consumer models can distinguish somewhere between this number of levels, producing 10, 12 or 14 bits per channel. So long as the software can make use of these, a greater number of bits means better images.

Density Range

Different photosensitive cells in use differ considerably in the range of light values they can detect. This is measured as the density range or optical density. In general, higher figures make for better scanners, but the methods used to measure these by different manufacturers are different, and comparisons between them are doubtful.

Scanner software

The software supplied by the manufacturer for you to use the scanner is often in the form of a twain or similar driver that will interface with your imaging program. In general manufacturers of consumer scanners seem to put little effort into providing software that enables you to get the best from the scanner.

Some scanners come with third party scanner software that does a better job, or you may be able to buy this yourself. This is generally more necessary if you want to scan film rather than prints. Find out more about this in the last part of this feature - Scanner Software - see box at top right.

Descreening

When scanning pictures from books and other printed material there is a problem because these images are generally printed as dots in a regular pattern. Because the scanner looks at points also in a regular way, there is always some interaction between the scanner and the dot screen, resulting in a pattern effect known as 'moiré'.

This interaction can often be reduced by placing the image so its edges are not parallel to the platen edges, but at perhaps 15 or 30 degrees to it. If you plan on scanning much material from printed sources you should also check that the supplied software has a 'descreeening' option, preferably with settings for different qualities of print.

Flat bed sizes

Most consumer flatbed scanners have a platen that will scan an A4 page. There are some smaller scanners intended for scanning photographs up to 6x4" only.

A3 (and even larger) scanners are also available. There are some which are basically A3 versions of consumer scanners and available at reasonable prices. High quality large flatbed scanners, such as the Creo-Scitex range (at very high prices) are also used to produce repro-quality scans.

Flatbeds and film

Some flatbed scanners can also scan transparent media, such as slides and negatives. This requires an alternative light source, either in the lid of the scanner or in a special 'transparency adaptor' attachment that needs to be fitted. The light shines through the film onto the photosensitive receiver.

The transparency adaptor may limit the size of film that can be scanned. Some are very small and only scan a single frame of 35mm film, while others will take sizes up to 4x5". With some scanners, the whole area of the platen can be used, and in some expensive high-end devices, there is a scanning light-source on one side of the film and a scanning receiver on the other.

With some scanners there is a small area of the scanner that can be used at a higher resolution than the rest of the platen especially for film scanning.

From cheap consumer scanners, the results from 35mm film are generally poor, but may be good enough for low quality images on the web. They are likely not to make very good prints.

If you use medium format film, a suitable transparency adapter can give reasonable prints, but the scans are still generally not up to the level of a good film scanner - but then nor are the costs.

The best film scans I have ever worked with have come from a high-end flatbed scanner. The differences between these and scans from a good drum scanner in a similar price range are immaterial.