A few scanning tips

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Dynamic range

24 bit or 36 bit color depth in scanners

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.

All scanners are at least 36 bits now, and most are 42 or 48 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 48 bit A/D conversion (analog to digital), which just means that inexpensive A/D chips are available now. But don't assume that a $100 48 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.

Slides have less photographic range than negatives. Slides have perhaps about 5 or 6 f-stops of total scenic range, compared to perhaps 9 or 10 f-stops for negatives. Expose a slide half a stop off and the results are objectionable. Do that with negatives, and you never even realize it.

But slide film itself has more contrast, a steeper gamma curve, and while the captured scenic tonal range may be less, the density extremes on the film can be greater. The extremes of slides are more likely to be clear or black, and contain greater dynamic range as seen at the scanner. Color negative film has the orange mask (helps reversal color balance) which also limits DMin and the overall scanning range. Images from negatives invert dark noise to be in the highlights, less noticeable there. Slides are more difficult than negatives to capture the shadow detail, and slides need a film scanner with greater dynamic range.

24 bit scanners might have a dynamic range specifications near 2.4, needed for photo prints. 30 bit scanners might be near 3.0, needed for negatives. The best 36 bit scanners might approach 3.6, better for slides. For sure, they can't be more. Only rotating drum scanners can approach 4.0 (these use Photo Multiplier Tubes, PMT, extremely expensive). And of course, all scanners are not equal, some will have higher dynamic range than others because their electronics have less noise. Price is definitely a factor.

The greater dynamic range extends the signal into low black levels where the noise is. To be effective, the electronics must be improved greatly to reduce the noise. I would not expect a dynamic range of 3.4 from a $100 48 bit scanner, not this year. The hardest problem for the scanner is the black end, density values beyond 3.0. One good reason is low level signal and noise. But the big problem is the necessary logarithmic mapping explained below.

Caution again: It is easy at first to assume that more bits will yield data with more dynamic range. Superficial reading might even think I said that in places. But it is NOT true at all. More bits are the container needed simply to store data with high dynamic range, but the bits do not create dynamic range, they only allow it to be stored. In the same way, perhaps a large wallet is needed to hold great sums of money, but having a large wallet does not necessarily imply the money is present. Many wallets are not exactly full.

Why is 36 bits better than 24 bits?

24 bit color is three 8-bit bytes, one byte for each of the Red, Green, and Blue CCD channels, to describe the color of each pixel in the image. 30 bit color uses 10 bits for each of the three primary RGB colors. In binary numbers, each bit is a power of 2, meaning that each additional bit doubles the maximum size of the numbers that can be stored. Same concept in decimal numbers using powers of 10, each digit there allows numbers 10 times larger to be represented.

Scanners detect light intensity corresponding to the density of the original. More film density lets less light come through, or more print density reflects less light. The CCD sensor measures that resulting light intensity. The image RGB numbers stored are proportional to intensity values in the original.

Basically the human eye responds to brightness in a logarithmic manner. The human eye does not perceive twice the intensity as being twice as bright. For a common example, photographers use their light meters on a "standard gray card" made to reflect 18% of the light falling on it. Metering from that card is used to calibrate middle gray (50% to our eye) in the hypothetical "average" scene. We see that 18% intensity as apparent 50% brightness.

The 12 bit scanner divides the scanned density range into smaller steps, 4096 steps in 12 bits instead of 256 steps in 8 bits, and therefore can show slightly more unique detail in the shadow areas, for a couple of reasons. Tiny variations that might be the same one color value at 8 bits could be 4 slightly different shades at 10 bits, or 16 slightly different shades at 12 bits, or 64 slightly different shades at 14 bits. Tiny differences, and it is really only significant at the black end, but that's more detail. And the possibility of larger numbers provides an opportunity for a better CCD to extend the dynamic range a little way into the next "10 times" logarithmic density interval. The better CCD is required to capture this detail in the darkest film, and more bits are required to store the numbers representing more range..

Continued


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.


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