Digital Photography -
The two most important scanner parameters are resolution and Dmax value. The resolution of a scanner indicate how much you can enlarge from a photograph. People often think that you should buy a scanner of equal or higher resolution than you printer. This is fundamentally a total waste of time and money. In a later segment of this document I will discuss COLOR and B&W printing and what parameters are needed from a scanner to achieve good results.
For now we'll look at resolution in terms of enlargement, and how to apply it to photographs. Remember that film has higher resolution than desktop scanners and digital photos. You can apply the Dots per Inch (DPI) scale available in scanners to enlarge a small portion of a photograph because you can selectively use higher resolution while scanning one small area of the origial.
Rescanning an original produces new pixels with whatever detail the original photograph provides. The result can be used to enlarge the image if viewing on a monitor, or to produce more detail if printing.
If the film photo were created with 35 mm film, it would contain about 6000x4800 total pixels. A 5"x 4" print would thus be equal to about a 1200 dpi scan. (6000 toal pixels dived by 5") Any scanning resolution used below 1200 dpi should thus be a scan of original pixel information. It would neither add nor subtract data from the resultant image. Scans at higher resolution would be capturing more data than was theoretically available on the original.
Scanning a one inch segment of the original photograph at 1200 dpi does not enlarge or reduce the resolution. It could then be converted to 75 dpi, 150 dpi or 300 dpi and still produce images without changing any real pixels, only the size.
The 75 dpi image would produce a 16" original image. (16" x 75 dpi = 1200 pixels.) The table above summarizes this, and shows the various size images that could be produced without any loss in resolution. They range from 4X to 16X enlargement.
An original digital photograph is always created @75 dpi, and hence a portion cannot be enlarged without creating new pixels.
A 4X enlargement of a 1" portion means that the new image will have 75/4 or about 19 pixels of resolution per inch. Each pixel will become very blocky and the image will become pixelated.
In the example photograph above (bird), the digital image is enlarged between 2X and 3X, and pixelation is already quite obvious.
The idea of being able to enlarge by scanning a portion of an original photograph is the main use for higher resolution scanners. (The second use being for print purposes discussed later.) The more resolution you buy the more enlarging you can do up to the quality of your original content.
The act of enlarging a real film negative to a photo print will actually involve some loss in quality (resolution). Each time a print is made from a negative it becomes a second generation image, and with each generation there is always some degree of loss. The best quality from original film is accomplished by using a film or negative scanner (slide scanner). Here the image is created directly from the original media, and film/slide scanners have a resolution of 2700 dpi or higher.
Don't expect to get that kind of quality from a flatbed scanner. You can get good results from a flat bed, but never as good as that obtained using a film scanner.
Flatbed scanner specifications my show two sets of resolutions, OPTICAL and INTERPOLATED. Optical is real resolution and defines what the scanner can do, interpolated resolution is like digital zooming verses real zooming. It uses math to create information, and does not actually collect new information. It produces softer images, and does not produce sharp original content. Always compare optical resolution to optical resolution when comparing two scanners.
Now lets look at Dmax and color depth.