To transfer negatives or slides to a digital format, a special device is used - a scanner for film. It differs from a conventional scanner in that it is designed to process small transparent images that have high resolution. Although many flatbed scanners are equipped with special modules that allow you to scan slides, but the product obtained as a result is of low quality.
Only CCD (CCD) scan elements can provide the required
depth of field for high resolution images. Therefore, all film scanners are built using them. Some models have one CCD line. In this case, the conversion to digital format requires a triple passage of the
negative. This prolongs the scan, but does not affect its result. Basically, the scanner for the film has a CCD, and the image is digitized in one pass. Some models use a reusable pass to reduce errors in the final image.
An important parameter that you should pay attention to when choosing a scanner is optical resolution. The width of the most common film is 35 mm, and the image itself is even smaller. Therefore, the optical resolution must be at least 2400 dpi (dots per inch). There are scanners that provide 4800 and 5400 dpi. And although the current level of technology allows reaching even greater values, it is impractical - the grain sizes of even a fine-grained film will be much larger than a pixel.
Particular attention should be paid to the dynamic range or optical density. The higher the value of this parameter, the better the scanner for negatives can transmit halftones and smooth color transitions. For high-quality film processing, the optical density should be in the range from 3.2 D to 3.6 D. It makes no sense to purchase models with a higher optical density, since the vast majority of films have just such values.
The digitization quality is also affected by the bit depth of the light representation, which characterizes the color rendering. A modern film scanner can have a 42 or 48 bit color representation, but processing in this format is used only inside the scanner and serves to reduce the "noise" of the conversion. The resulting image has a standard 24-bit color coding for computers.
The slide scanner in most cases connects to a computer via a USB interface. More expensive models can connect via SCSI-2 and IEEE 1394 (FireWire). In this case, quite often there is a board with this controller in the kit.
A film scanner almost always has hardware for image enhancement. This is Digital ICE, which allows you to remove dust particles and scratches from the image without affecting the main image, and Digital GEM, which allows you to remove graininess, and Digital ROC, which allows you to restore colors on faded photos, etc. Quite often, all these tools are combined in one package Digital ICE4 Advanced. Using these technologies significantly prolongs the scan, but the result is excellent. For similar transformations in Photoshop, it will take much more time, and the result is by no means guaranteed.