Dynamic Range


About Slide Scanning Dynamic Range

These pictures compare a DpsDave slide scan (on the left) with a scan produced on a Canon 9000F flatbed scanner. The Canon scanner is perhaps the best commercially available scanner for slides, but is clearly not able to produce results comparable to DpsDave due to its limited dynamic range.

DspDave @ 6,000 dpi
Dynamic range: 500,000 to 1

Canon 9000F @ 4,800 dpi
Dynamic range: 65,000 to 1

Slide Scanning Dynamic Range – DpsDave Slide Scanning Services

Our eyes have two types of light sensing cells, those that are sensitive to how much light there is (rods), and those that can tell what color the light is (cones). The rods outnumber the cones by 20 to 1, making our eyes much more sensitive to how much light there is than to what color. The sharpness and detail comes from the black-and-white part of our vision. Scientists and artists refer to the part of the image which comes from the black-and-white part of our vision as contrast, and it is perhaps the most important thing concerning slide scanning.

A good example is shown above. For under- or over-exposed images, the difference is the greatest, and the images shown came from a rather dark slide. As you can see, the image to the left (recorded with a 500,000:1 dynamic range) is much more aesthetically pleasing than the image on the right (recorded with a 65,000:1 dynamic range). The haze and grain in the images on the right comes from the electronics trying to see in the dark.

Comparing the original slide (viewed under a microscope at 40x) with the DpsDave-scanned image, the original appears to be grainy, and have more detail in the shadows and bright areas. The graininess comes from the grain of the film, and the DpsDave-scanned images are smoothed to remove this grain while retaining detail. The additional detail that can be seen in the shadows and highlights come from two factors: our eyes are twice as sensitive as the scanning system, and the microscope is a better display than even the best computer screens.

Computer screens, as good as they are, cannot show us the entire dynamic range that was recorded when we at DpsDave scanned the image. In fact, the only a small percentage of the full dynamic range can be displayed on a computer display. This is due to the digital nature of LCD displays.

This can be a big problem, as all the dynamic range we at DpsDave worked so hard to capture doesn’t make the digital image any better. However, we use intelligent software to distinguish between the too bright and too dark areas of the picture. These areas usually have detail in them, but due to the limitations of digital displays, the detail is hidden. You can bring out the detail in the dark areas by increasing the brightness of the image, but then the rest of the image is all washed out. The intelligent software we use can separate areas that are too dark from areas that are OK, and so can brighten only the too dark areas, and darken the too bright areas. This results in a digital image where you can see the detail in the very dark and light areas that would normally be hidden from view.

After the intelligent software gets done with an image, a digital image displayed on a computer screen compares much more favorably with the original slide viewed under a microscope. There is still some detail that can be seen under the microscope that is hard to make out on the computer screen, but this difference is slight. It is comforting to know that humans are still better than computers!

Slide scanners that list a dynamic range in their specifications are rare, but most report the number of bits in the A/D converters, and this is directly related to the dynamic range of the scanner. Scanners with 16 bits of A/D conversion have a dynamic range of 64,000 to 1. However, since there are no standards for how to determine dynamic range, the numerical rating can be abused. DpsDave’s claim of 500,000 to 1 dynamic range is real, not marketing hype. This would imply that we have A/D converters with 19 bits, and all you electrical engineers out there know there is no such animal. Exactly how DpsDave gets this high of a dynamic range is a trade secret, but in general we combine electronics, optics, mechanics and software in a novel way to produce these results.

You may be wondering, “what happened to the grain in the original slide?” This grain was erased by using a multiple step software process where we treat it as noise using a signature analysis technique. As a result, skin tones appear consistent, smooth areas (such as the sky) look like they should, and small details are retained. It works like magic, but it’s really just high power computers performing hundreds of millions of operations on the image. This computing power is what makes a slide scanned by DpsDave a truly superior digital image.

For more examples of slide scanning dynamic range differences, call DpsDave at 866-935-1361!