What is it?
Drum scanning is the ultimate pursuit of quality in the realm of film photography. It is the last scan you will ever need, since there is nothing that will give you the kind of resolution, detail, sharpness and dynamic range that drum scanning can give you.
PMT vs CCD/CMOS
CCD and CMOS scanners are basically all consumer, prosumer, and many high end scanners, that are not drum scanners. The difference here is essentially that it uses the same technology that digital camera's use (a chip) that is used to record an image through a lens (often one that is highly inferior to the one you shot the image with). Essentially, with a CCD or CMOS scanner, you are taking a picture of your image with a digital camera.
How a drum scanner differs is that it uses PMT's (Photo Multiplier Tubes). These are basically vacuum tubes (one for each RGB color) that absorb the image pixel by pixel. It's a technology that was derived from nuclear physics.
Why compromise on quality when you can get the best scans that are out there? National Geographic, Vogue, pretty much every big publishing house used to have drum scanners which cost up to $65K or more when new and could scan at up to 12,000 dpi. For these publications, for advertising agencies, for anyone that wanted to have the ultimate quality and/or print big, there was no compromise.
PMT's are ultra sensitive to light and therefore outperform CCD and CMOS sensors in their dynamic range. The stated D-Max on most CCD and CMOS scanners is highly inflated. Many of the consumer and prosumer manufacturers do this to show that their specs are higher than drum scanners, when in reality those specs are not very accurate.
Wet-mounting is the act of sandwiching the film on the drum between a sheet of mylar and the drum surface. Having the film in this fluid creates an optical effect that increases dynamic range, color vividness, renders fine details more refined, hides many of the defects (scratches, etc...) and reduces film grain. You can't emulate, simulate or fake this optical effect in digital post processing - it's pure physics.
Unlike flatbed or prosumer scanners the scanning surfaces on the drum create almost no side-reflections from neighbouring areas. This drum design gives the best possible flatness to the film.
I use the highest grade mylar and oil based fluid made by Scanscience. This fluid is the safest for the environemnt and myself, as well as for the drum and the film. Most labs will use much cheaper fluids that have hydrofluoric carbons such as N-hexane, whilst ours are the safest in the industry. These fluids are completely neutral to the negatives and positives and do no harm to the film. In fact, the mounting fluid evaporates fairly quickly after dismounting the mylar sheet. Wet mounting is a lot of time-consuming handwork but the results are well worth it.
Resolution and Color Rendition
Drum scanners use each of the red, green, and blue photo multiplier tubes to scan the image pixel by pixel. Drum scanning is slower but a lot more precise reading the film point-by-point when compared to a CCD or CMOS scanner that scans an image much like taking it with a digital camera. The drumscan therefore has the upper hand in optical density, resolution and color rendition.
The scanner I use is a Danish made Scanview Scanmate 5000 which offers up to 5000 DPI in optical scanning resolution. Scans like these have been used to print extremely big prints at art galleries and even billboard sized prints at 400dpi. The larger formats make digital look like it's years behind in resolution.
Drum scanning is a lot of hard work due to wet mounting compared to regular consumer or prosumer CCD-scanning. Drum scanners are high-end, high-precision mechanical devices that need a lot of care, regular maintenance and most of all - a skillful operator. It takes months to learn the skills and nuances needed to be able to wet mount and scan well.
Drum scanners need a lot of care, regular maintenance, and often big investments to replace expensive high-end electronic or mechanical parts to keep them performing to the very best of their ability. However, they are overall very bulletproof and built to last many years, even decades of use with proper maintenance.
For those seeking second rate quality, this may not be something you would be interested in. Drum scans are not as cheap as a local lab's CCD scan, but they are truly the best and purest path to the ultimate quality possible. It's better to scan less but scan better images that will really wow people.
The costs of running a drum scanner is never as cheap as a CCD or CMOS. The costs may add up - from the chemicals, tape and mylar needed for wet mounting to the maintenance and power usage. However, the value is worth it when you compare the quality and only want to ever have to scan an image once. It may be a time consuming process but drum scanners were actually built for volume scanning, so even though the actual mounting of the drum may take a while, scanning many images at once is easier than with many CCD or CMOS scanners that allow you to preview only one image or strip at a time.
My goal is to make the highest quality scan of your negatives and slides with a 16 bit tiff output. The high resolution and detail is something that will be future proof for many years to come. In fact, for larger negatives the resolution is still years ahead of digital. Analog photography is something that is pure and it is my goal to keep this art form alive by helping people digitize their images for their own use.