Image acquisition - part 3 - Hardware

April 18, 2015  •  Leave a Comment

Hardware

So far we have covered some general guidelines and file formats, this time it's the hardware. There are two basic choices, scanner or camera. First up it's the scanner.

The scanner:

A good quality scanner with full bed transparency scanning is essential. Some low end scanners only support transparency scanning of film strips and slides. An optical resolution of 6400 dpi is recommended to achieve sufficient detail when scanning 35mm film and slides. 4800 dpi is more than sufficient for most reflective media. A minimum optical density of 4.0 DMax is required to ensure the accurate capture of the full tonal range.

The Epson Perfection V700 or V750 Photo scanner with the optional fluid mount is a perfectly decent mid-range scanner that will serve all but the most specialist jobs.

Suggested settings:

35mm film/slides:

  • Minimum resolution 6400 dpi
  • 48bit (with optional IR (infra-red)) or 64bit HDRi – HDRi is High Dynamic Range with infra-red, the IR channel is used for removal of physical imperfections. The scan should not implement any automatic removal of imperfections, the IR data should just be captured for future processing.
  • Multi-exposure or Multi-pass sampling – reduces noise and increases dynamic range and should be used if available.
  • Higher ISO films (ISO/ASA over 800) can have a sufficiently large grain that you may be able to get away with a lower resolution.

Kodachrome film:

  • As above but corrected with a Kodachrome profile created with a Kodachrome IT8 target.

NOTE FOR FILM AND SLIDES: an appropriate film holder for the make and model of the scanner MUST be used in order to ensure accurate results. Flatbed scanners will be calibrated to focus on a plane at a specific distance from the glass, this distance from the glass is normally specific to the film holder supplied. Placing 35mm film directly on glass will generally deliver very poor results due to the curl present in most film, variable focus across the frame and Newton Rings are the primary issues.

Film and glass plates:

  • As above but with a reduction in resolution if file size is an issue, also some older medium and large format films may have a sufficiently large grain that you can get away with a lower resolution.

NOTE FOR MEDIUM/LARGE FORMAT FILM: The scanner must be set for scanning transparencies directly on the glass to avoid focus issues. It may also be necessary to use a wet scanning process to prevent the appearance of Newton Rings. Glass plates generally need to be emulsion side down to avoid focus issues.

Photographic prints:

General recommendations for photographic prints are difficult because of the huge variety in prints and paper types. However if you start at 300 dpi and compare the results with the original print using an appropriate loupe you should be able to establish a benchmark for each batch of prints on the same paper type. Some papers can require very specific processes. For example textured gloss prints can produce scans covered with bright speckled highlights that result from the light of the scanner lamp reflecting off the gloss on all the peaks and troughs in the surface texture. Some of the techniques and processes are fairly complex so advice will not be included here, but look out for a future article.

Artwork:

  • As with above for photographic prints.

NOTE FOR PHOTOGRPHIC PRINTS AND ARTWORKS: A trial and error approach is required to establish a baseline for each media type, though with experience it will become easier to anticipate appropriate settings for each object being scanned.

The camera

The camera body:

The best camera for digitising work is a flatbed scanner, however if you can’t use a scanner because of the size and/or location of the original, or not being able to flatten the original material, a camera will do.

Generally speaking for photographing plates, prints, artwork etc. you should be looking to a digital SLR with an APS-C or larger sensor or a medium format camera with a digital back. The principal reason for choosing a camera with at least an APS-C or larger sensor is that the size of the individual photosites on the sensor is critical to image quality, a 12 megapixel DSLR will produce a superior image compared to a 12megapixel compact camera. This is because compacts tend to use much smaller sensors, so to get the same resolution output as a DSLR the individual photosites have to be much smaller on the compact. Smaller, usually cheaper, sensors tend to produce more noise and deliver less contrast and dynamic range than their larger, usually higher quality counterparts.

If choosing a DSLR with an APS-C sensor the sensor should be at least 12 megapixels. Less than 12 megapixels will potentially require taking two shots (one of each half of the page) then stitching them together in order to achieve the desired detail. The upper useful limit is generally accepted to be about 18 megapixels in an APS-C sensor. Beyond 18 megapixels there is a very rapidly diminishing return on detail and contrast delivered by APS-C sensors.

Full frame sensors, those with a size equivalent to a traditional 35mm film, will invariably deliver better results because of their larger size can accommodate more pixels at the same or lower densities than APS-C sensors. Full frame sensors will continue to deliver improvements in detail up to ~30 megapixels.

These are generalisations that do not always translate into real life but are useful as a general guide.

The lens:

At least as important as the camera/sensor is the lens used. The ideal lens would be one with zero distortion, zero chromatic aberrations and uniform sharpness across the frame. Unfortunately no such lens exists so there is always some compromise. All lenses introduce some distortion, are slightly softer towards the corners of the frame and all produce some degree of chromatic aberration, usually more as you get closer to the edge of the frame.

Close-up or macro lenses with a fixed focal length are usually the best contenders while the ‘kit’ lens supplied with the camera is likely to be very poor, especially on lower end consumer bodies. If buying new the best option is usually to buy a body only and then the lenses you require. There are some very good medium to wide angle zoom lenses that are perfectly good for imaging artwork but they do tend to more expensive than fixed focal length lenses that deliver a similar image quality.

An important consideration, especially if using a copy stand or working in a confined space, is the field of view or FoV of the lens. The FoV will depend on the camera body the lens is going to be used on, the smaller the sensor the narrower the FoV. For example a 50mm macro positioned 1m away from the subject will cover an area of 45cm x 30 cm on a typical APS-C sensor or 72cm x 48cm on a ‘full frame’ sensor. One positive aspect of the smaller FoV delivered by a smaller sensor is that when used with lenses designed for a 35mm or full frame sensor the sensor only captures light from the centre portion or sweet spot of the lens where there is less softening, distortion and chromatic aberration. This means that the is less degradation of the image towards the edges of the frame, this in turn means you may get away with a cheaper lens on a camera with a smaller sensor without any apparent loss of quality.

If you need to photograph a lot of bound material that cannot be opened flat another option may be a tilt-shift or perspective control lens. This is a type of lens that can be configured to correct geometric distortion caused by not being able to align the camera perpendicular to the centre of the material being photographed. In practise this allows you to photograph material at an angle and still capture an image that is not affected by perspective distortion. Tilt-shift lenses also allow greater control of depth of field and tend to deliver very good image quality across the whole frame. However, tilt-shift lenses are complex pieces of equipment to use and in most cases specialist perspective control software applications will deliver perfectly acceptable results.

That probably enough for anyone in one sitting but if you want more you'll have to come back later for part 4 which look at some of the software you'll need.

Cheers for now,
Adrian


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