17 April 2000

Dmax is usually stated along with the specifications of a scanner. This will help you determine if the scanner can capture the full details of your originals, such as prints, slides, and negative films. You can use the approximate Dmax values for other devices as a reference point -- clearly, if you print anything, you will be tossing out information in the original digital image that simply cannot be reproduced on paper. Even a monitor cannot display the full range of colors present in an original film slide.

Usually, Dmin is not stated, but is 0.5 or less. The difference between Dmax and Dmin is the full range of detail that can be captured by the scanner. Naturally, with the assumption of Dmin above, a higher Dmax means a better scanner.

Dmax: the darkest shadows in an image (Dmin: the lightest highlights in an image). Dmax ranges from 0 to 6. Dmax 5 can be considered to be black carbon laser print.

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Calculation of DMax:
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Dmax = Log10 (shades per color RBG)

If you have 24 bits/pixel of color, that's 256 shades per RGB. Thus, log10 256 = Dmax 2.4.

Dmax 2.4 = 256 shades RGB = 24 bits/pixel
Dmax 3.0 = 1000 shades RGB = ~30 bits/pixel
Dmax 3.3 = 2000 shades RGB = ~33 bits/pixel
etc.

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Approximate Dmax scales:
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1.5 - 2.0 - half-tone prints (eg. magazines, publications)
2.4 - 24 bit computer images (eg. 16 million colors displays)
2.8 - 3.0 - Print photographs
3.2 - 3.5 - Slide photographs
4.5 + - Looks identical to reality.