A few scanning tips


Scanning film with a TMA

There are two big concerns when scanning film:

Dynamic range and Resolution

Insufficient dynamic range cannot show the detail in the darkest areas of slides. And worse, these dark areas are often noisy (many random red, green, and blue pixels in the dark tones of slides, it can be awful). Better equipment has more dynamic range, meaning deeper reach in the dark tones and lower noise levels. The black tones in film are at the scanner's zero signal level, and zero signal is where the CCD noise is. Better dynamic range is the most expensive feature in a scanner. It lowers the noise floor using better, shielded electronics, and also provides a better CCD to reach into those lower signal levels. Better equipment has better optics too, perhaps six element lenses like a fine camera lens. But dynamic range is what costs the most to provide.

Don't expect much dynamic range in a $200 flatbed. Scanners are getting better, but they have been rather poor in this respect. The dark areas of slides are optically noisy (again, noise is random red, green, or blue pixels in the darkest areas of slides, or inverted to CMY in the highlights of images from negatives). A higher Black Point can hide some of this noise, but it limits dark detail too. Noise is not much of a factor when scanning reflective prints and documents, that reflective media has very little requirement for dynamic range. Low-end flatbeds are a good match for photo prints, this is their designed purpose, but they really are not film scanners. See chapter 19.

This noise is inverted to be in the highlights of images from negatives, where it is much less visible, and therefore cheaper equipment can work fairly well for negatives. There are some low-end $350-$500 film scanners that do a decent job.

The other concern, at least for 35 mm film, is that the film is tiny, and the flatbed scanner with TMA remains at its original resolution. Therefore, even a high quality flatbed cannot enlarge the image from 35 mm film as large as you may want. A real film scanner offering 2700 to 4000 dpi is needed to enlarge 35 mm film to 8x12 inch (or A4) size or larger. Larger film size helps, but it is not a choice if we have 35 mm film to scan.

If this is not obvious yet, it's like this: A 35 mm slide frame is perhaps 1.3 x 0.84 inches if in a cardboard mount. At 600 dpi, you would get no more than a (600 x 1.3) x (600 x 0.84) = 780x506 pixel image from full frame. A 1200 dpi scanner gives images twice that size, maybe 1560x1012 pixels from mounted 35 mm slides, full frame. Either image size might be more than enough for web pages or the screen, but the number of pixels is insufficient for printing large prints.

Said another way, the ratio of (scanning resolution / printing resolution) is the enlargement factor of the film size when printed on paper. A 35 mm film frame is 36x24 mm, and medium film is called 6x6 cm, but the frame is 56 mm square. Using these numbers, and assuming we are scaling to print at 250 dpi, then we can compute this chart:

scanned at
Enlargement if
printed at 250 dpi
Maximum print from
35 mm film (inches)
Maximum print from
6x6 cm film (inches)
300 dpi1.2 x1.1 x 1.72.6 x 2.6
600 dpi2.4 x2.3 x 3.45.3 x 5.3
1200 dpi4.8 x4.5 x 6.810.5 x 10.5
1600 dpi6.4 x6 x 914 x 14
2400 dpi9.6 x9 x 13.621 x 21
2700 dpi10.8 x10.2 x 15.323.8 x 23.8
4000 dpi16 x15.1 x 22.635 x 35

This assumes full frame, and any cropping reduces this size. You can always print at more or less than 250 dpi to alter the printed size that way too. The short side of 35 mm film is normally the limiting factor for printing computations.

Flatbeds do offer 2400 dpi now, which is not quite the exact same concept as film scanners use, see page 21. Every factor always has tradeoffs, pros and cons. This feature allows 2400 dpi flatbeds, and it is a bit more detail than 1200 dpi, and while sharpness may be lower, aliasing problems are reduced (page 157). Many people do find these 2400 dpi flatbeds satisfactory for 35 mm film. In fact, many are very pleased, at least for casual work, but those images will need more sharpening than usual, at least when used at full 2400 dpi. Other flatbeds are conventional 1200 dpi scanners with regular CCD cells, and 1200 dpi is pretty much current state of the art, as high as flatbeds currently go, due to greater noise levels in smaller CCD cells. 1200 dpi can be plenty on larger film, for example, 4x5 inch sheet film needs very little enlargement. A few high-end flatbeds provide two selectable lenses, to scan 8.5 inches width at 1200 dpi, or 4 inches width down the center at 2500 dpi. But the economy models will have low dynamic range for film, and more noise in the images, so this is not yet the best answer for film.

In my opinion, the good answer for printing large full page images from 35 mm film is to invest the TMA money in a film scanner instead. Film scanners typically offer 2700 to 4000 dpi for this reason, because the 35 mm film is both small, and capable of that much detail. These images will be 25 to 60 megabytes. If you want to scan 35 mm film and print it large, then you really need a film scanner.

There are good low-end film scanners down around $400 US now. A good flatbed with TMA for film costs that much too. The top end 35 mm film scanners are about $1500, or about $3000 for medium film. The film scanner offers the greater resolution necessary for the purpose of obtaining larger images for printing. Better dynamic range, no Newton's Rings, and also more convenient to use, batch scanning, and holders to hold the film straight.

Larger photofinishers are offering film scanning now, so if you only need a few frames, you might ask your local Wal-Mart if they can do it. Be sure to inquire about the final image size (pixels).


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

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