A few scanning tips


Color Negatives

Color negatives are very different than scanning positive slides or prints. Color negatives have an overall orange mask, designed to aid photo printing of negatives onto regular photographic paper. The orange mask makes scanning color negatives difficult. Its presence seriously interferes with "automatic" correction of any real color casts. Color casts often remain, and must be manually removed. The photofinisher did this for us in prints, but it's our job now. Digital color balance is easier than darkroom work, but color balance is harder for negatives than for prints or slides.

Negatives require the image be inverted, easy enough for B&W, but color negatives also require that the orange mask color be balanced out (it is a strong greenish blue when inverted to positive). The film scanner software has a Negative mode that does this, but the shade of orange varies among film brands, and even in different films from one manufacturer. Most film scanners offer a few film types to aid this.

Negatives can capture perhaps 3 more f/stops of scenic range than can slides, so there is less loss of shadow or highlight detail. This greater range is compressed to fit "in the film" so to speak. Slides have dramatic contrast, but not very good shadow or highlight detail. Negatives capture greater scenic range, but store it compressed in less range on the film. Negative film itself actually has lower overall dynamic range scanning requirements than slides, which makes life easier for inexpensive scanners.

Images from both slides and negatives can only span tone values 0 to 255 in the 24 bit image. But because this greater range of scenic detail is compressed into that span on the negative, it offers more possibilities to expand selected regions, to favor the shadows or favor the highlights, to show maximum detail. We cannot favor both ends, but we can do either, by various brightness adjustments, and negative film offers more opportunities for photo manipulation. Chapters 20-23 have details.

Film grain (or more likely its effect) can sometimes be visible in the image from negatives, particularly for fast film or B&W film. The effect is seen as a grainy or sandy appearance in smooth areas, such as skies. The answer is to use slower finer grained film. What we perceive as grain is larger than the actual grain, and is often the effects of digital aliasing, new false data created by the interaction with the smaller grain size, smaller than the scanner's resolution can resolve (digital sampling works very differently than analog optics).

Sometimes slight defocusing of the scanner can help hide this "grain", and one direction of misfocus may be more effective than the other. Subsequent resharpening is necessary. Scanning large at full optical resolution, and then resampling smaller helps hide it. Adequate scanner exposure can help smooth this too.

On some scanners, the scanning Exposure Time control is extremely important when scanning film. This is the principle difference in actual procedure (see Chapter 20). But more and more, the Exposure control is implemented as an automatic feature. The photofinisher did this adjustment for us in the prints.

Scanning at integer divisors of maximum optical resolution (Chapter 9) may be a little less convenient with film, because there are fewer practical choices. With a 2400 dpi scanner, the choices are 1200 or 2400 dpi, but you may want a size corresponding to 1600 or 2000 dpi.

Dropping a 600 dpi flatbed back to 300 or 150 dpi is often exactly where we wanted to be anyway. Dropping a film scanner back to half resolution is fine for images on video screens, but is usually too small for printing at large size. So rather than scan at say 1800 dpi, you more likely will scan at maximum optical resolution, and then adjust later in an external program. Bite the bullet and buy more memory.

Scanning film on a flatbed TMA often has limitations of minimal resolution for small film and minimal dynamic range for film. Inexpensive flatbeds are not suitable for critical results from film. And Newton's Rings can be a problem. See Chapter 17.

Copyright © 2000-2010 by Wayne Fulton - All rights are reserved.

Previous Main Next