Film Scanning
Digitizing transparent media using a common desktop scanner.
This section is dedicated to scanning photo film and transparent media in general, onto a computer using a common desktop scanner. Mine is a Microtek Scanmaker 5600. There are several sites by others with similar interest, although not oriented to formal research. |
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Scanning film onto a desktop scanner is easy. Scanning film onto a desktop in the highest level of quality
is more difficult.
To begin with, a camera has a set of controls to control the exposure of the film to light.
Shutter speed and aperture diameter. The aperture controls light intensity.
The shutter controls the cumulative exposure to light over time.
A scanner has no aperture and no shutter speed.
What a scanner does have is an open glass bed to hold the film, and a constant scan speed.
Several things could be done. A box over the bed with a pinhole aperture
would turn the scanner into a "pinhole camera". The pinhole could be replaced by a rotating
wheel with several holes of differing diameters. The duration of the light source could be controlled
manually. If the "pinhole camera" approach is undesirable, try something else. The open bed could be
exposed to a light source with an intensity control such as a rheostat used with light dimmers. This still
leaves the nature of the source undetermined. The light source could be moved closer or further away to
control exopsure. The stronger the source, the greater the distance the light must move to change exposure
levels, e.g. 15 feet. After choosing a method, the light source must be determined. Commercial scanner
lights have been cold cathode fluorescent lamps. Other emerging technologies include Halogen and LED lamps.
If only a few images are being scanned, you may want to try natural daylight. On a sunny day, the color and
intensity are unmatched. Daylight is the standard to which photographers defer. Artificial light sources are judged
by their similarity to daylight. Any method that controls the accumulated light on a scanner bed will work with
daylight.
One fact of interest to some is that incandescent lights will create a scratch layer with some film types sensitive to infrared light. Some 620 B&W film will, when exposed to incandescent light, show up all the scratches, dust, smudges and surface defects in general. The light must be held off to one side to avoid hot spots and to create the shadows needed to highlight the defects. Useful with post-process repairs and corrections. Such a layer could be used as a layer in a program like Photoshop, to correct a fluorescent illuminated layer. Finally, in case the analogy hasn't occurred to you, , the scanner bed is the equivalent of a sheet of film in a large format camera. Some issues in scans include noise. There are several types of noise. One type is removed by simple averaging of multiple scans. Expensive in terms of time and disk space. If a large dpi is used, 1200 or larger, simple blurring will help and will have a minimal effect on sharpness. Ambient (room) temperature effects the amount of noise. Some scanners make some attempt to compensate for temperature. I placed a scanner in a cooler half full of ice and closed the lid for half an hour. A series of images was made with the scanner still in the cooler. There was a noticeable effect. There is a low level noise that always exists and is effectively removed simply by increasing the light intensity. Electrical noise contributes periodic noise. Nearby space heaters, some fluorescent lamps and any unshielded electrical device will generate noise. Both the device and the computer need to be shielded. Most computers are shielded just for that reason. Scanners are not. If you see periodic noise in your images, turn off everything nearby except the light source. If the noise persists, turn off the llight source and make an available light scan. If the noise disappears, your light source is the problem. Some fluorescent lamps have neon lamps and transformers inside that generate high frequency noise. A diffusion layer is usually required. There are two types, reflective and transmissive. If the scanner's internal lamp is used by covering the bed with a white box, then a reflective diffuser is used. The white box reflects the light back into the scanner in a diffuse manner. If an external light is used, then a sheet of semi-opaque plastic is typically used. A lid from a plastic kitchen container works well. White plastic of any kind will work. At various times I have used :
One of the problems in light source selection is the color content. The spectra of types of sources is very different. Natural daylight is considered the ultimate reference. Fluorescent is the best source for scans but is terrible when a color comparison with daylight is made. Several companies produce lights just for color reproduction accuracy in color work. A couple are Gretag-Macbeth and Just Normlicht. High costs are associated with manufactured resources in this area. A device to measure color can cost several thousands of dollars. For film scanning, such equipment is not needed. Choose the best light source (I still recommend daylight) and exposure control method that is best for you. Format considerations must deal with the final form the film must take. For this I have based my choice on my experience. High dynamic range imaging is ideal for this situation. Prof.Paul Debevec developed a computer algorithm to combine several images, taken in a specific fashion, into a single high dynamic range image. The software to do this, and the subject are both discussed elsewhere on this site. There are several output file formats available. The best seem to be Tiff( logluv from Greg Larson), Exr (Industrial Light and Magic), and raw floating point data. The image series mentioned must be taken with a constant aperture and a varied shutter speed. The reason the aperture remains constant is to remove the effect of a changing depth of field which occurs in cameras with a change in aperture. In this application depth of field is not a problem and the effect on light of a shutter speed is replaced by a different light control method, say lamp distance. So, a series of scans is made with the lamp varying in distance from the scanner bed. The resulting images are mathematically combine using the computer algorithm and stored in a chosen file format. The last major issue is the choice of a compressor. Before explaining the purpose of a compressor, I will discuss the nature of an HDR. The advantage of an HDR is the range of light brightnesses representable in the image. Current computer technology is limited to between 256 and 65,635 levels of brightness in file storage and 256 levels in monitor display. Real world brightnesses are not even approximated by the monitor. The nature of the HDR means viewing on an ordinary monitor is not possible. For this reason, compressors were invented. A compressor selectively reduces the number of levels of brightness of an HDR to a number compatible with computer monitor viewing. The design of a compressor is based on the criterion and philosophy of the designer. Some compressors are designed to imitate the human visual system (Greg Ward Larson), some reproduce real world methods established by traditional photograpy (Eric Reinhard/Ansel Adams), yet others attempt to include all details in a scene in the final image, something that does not occur otherwise. Some details remain hidden in the shadows if a bright illumination is used. Some images have detail in bright areas that is lost if a low illumination level is used. Without modification such as the "dodge and burn" method used by traditional photographers, some detail is usally lost anytime a print is made from a film negative or positive. The compressor produces the equivalent of a digital print from a digital negative, hence the same problems must be dealt with. The final methodology will remain undisclosed pending getting an acceptable sponsor and research funding. If a sponsor is not found or funding is not found, the project will remain undisclosed unless patents and copyrights are established. |
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Here are some links : More information: abstract concrete works.com at archive.org Fluorecent Flashlight : http://www.afn.org/~afn11300/index.html on archive.org THE FOLLOWING LINKS ARE DEAD How To Scan Film Negatives ; Slides, General Computing by Helen Bradley February 2001 Vol.12 Issue 2 http://www.smartcomputing.com/editorial/ |