Image acquisition for archives - part 1 - General guidance

March 12, 2015  •  Leave a Comment

This series of posts provides examples of appropriate technical specifications and workflow for the capture and processing of images for archives and future editing. I'll be adding some example images over the next few days.

General guidance for image acquisition

New images should be captured at the highest possible quality, the captured images can then processed and exported in a format and at a quality appropriate for the purpose.

The most appropriate method of image capture will depend on the source material. The two most commonly used methods are digital camera and flatbed scanner. A good quality flatbed scanner should always the preferred choice for reflective material and is the only practical choice for transparent film.

In circumstances where the use of a flatbed scanner is not possible then a high resolution digital camera such as a mid to high-end digital SLR with a good quality lens can be used. There are some suggestion for choosing appropriate equipment later in this document.

The capture resolution should allow for enough detail to resolve the smallest useful component of the source material. For example if scanning for digital restoration, archive or reproduction the resolution should be high enough to capture film detail down to the grain, in the case of letters and manuscripts the paper fibre should be discernable. If photographing rather than scanning the same rule applies, capture enough detail to resolve the smallest useful element. Larger items may out of necessity need to be scanned or photographed in several sections but again you should be working to resolve the smallest detail that may be of relevance, such as individual brush strokes or the weave of the canvas.

When scanning reflective or film sources the resolution, bit depth and colour space used for capture are highly dependent on the nature and size of the source material and the intended purpose. For example a simple line drawing may only need to be scanned as a grey scale image at 150dpi, whereas a negative film or positive slide may need to be scanned as a 64bit HDR image at 6400dpi or higher. Some types of media need specialist hardware or software, Kodachrome film is a good example as it has a pronounced blue colour cast when scanned that is almost impossible to correct accurately without specialist software and an IT8 Kodachrome target.

If using a camera for image capture the camera should always be set to the highest quality settings available; should be mounted on a tripod or copy stand; the sensor should be parallel to the source and should be aligned with the centre of the area to be captured.

When using any form of digital capture it is essential that the equipment is profiled appropriately to ensure faithful reproduction of tone and colour. For scanners it is recommended that profiling is carried out periodically using IT8 targets and for cameras the profiling should be carried out with a colour chart from a reputable manufacturer for each change in shooting conditions for each camera/lens combination used. It is also worth considering acquiring a resolution chart for scanners to determine the optimum scanning resolutions. If relying on auto-focus (which is not recommended) a lens calibration tool for cameras that support focus adjustment is essential.

That's it for now, part 2 concentrates on file formats.


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