Enligt Wayne Fulton på
http://www.scantips.com/
The first thing to remember is that bit depth and dynamic range are NOT the same thing. It is going to sound much the same, but it's not. That difference will be covered here.
36 bit scanners with good dynamic range can sometimes capture more shadow detail from an image than can a 30 bit scanner. Specifically, dynamic range may add more detail in the shadow tones of images from positive film (slides), and in the highlights of images from negative film. Dynamic range is not a major consideration for scanning photo prints, because prints themselves are very limited, but it is very important when scanning film. We'll try to explain HOW it helps, because it's the subject of many incorrect myths.
Virtually all scanners are 36 bits now, and some are 42 bits. More bits support more dynamic range, but does not ensure it exists. More bits are required to hold numeric values containing better dynamic range, but this one detail does not ensure it. While the two factors are often associated, there is also a second requirement. High-quality low-noise CCD and electronics (i.e., expensive) are needed for better dynamic range. The trend today is that inexpensive scanners are offering 36 or 42 bit A/D conversion (analog to digital), which just means that inexpensive A/D chips are available now. But don't assume that a $200 42 bit scanner can achieve as much dynamic range as a $1000 36 bit scanner can offer.
What is Dynamic Range?
Image density is measured from image brightness with optical densiometers, and ranges from 0 to 4, where 0 is pure white and 4 is very black. More density is less brightness. Density is measured on a logarithmic scale (similar to the Richter Scale for earthquakes). Density of 3.0 is 10 times greater intensity than a density of 2.0. An intensity range of 100:1 is a density range of 2.0, and 1000:1 is a range of 3.0. Density 4.0 is not a theoretical maximum, the math is not limited, but it is a practical maximum of density, because nothing you can scan will reach 4.0.
The minimum and maximum values of density capable of being captured by a specific scanner are called DMin and DMax. If the scanner's DMin were 0.2 and DMax were 3.1, its Dynamic Range would be 2.9. DMax implies unique image tone values are distinguishable, and not hidden by electronic noise. Greater dynamic range can detect greater image detail in dark shadow areas of the photographic image, because the range is extended at the black end.
When I say the "black end", I speak of positives, either prints or slides. When images from negatives are reversed, this effect transfers to the highlight tones. Most literature about 30 or 36 bits just says it improves "shadows and highlights" without making this distinction.
A printed magazine image has a dynamic range well less than 2.0, maybe half of that (1.7). The blackest ink still reflects some light, the white paper is not so bright that it blinds us, and the difference is relatively small. Photographic color prints have a dynamic range of less than 2.0 too. Film negatives might have a range up near 2.8. Slides may be near 3.2. These are not precise numbers.
