Image acquisition - part 2 - File formats

March 24, 2015  •  Leave a Comment

File formats

In the last post about image acquisition for archives we looked at some general guidelines, now it time for file formats.

Each stage in the process of capturing and processing an image should utilise an appropriate file format that retains as much information as possible. There are three primary stages during image capture and processing each of which have particular file format requirements:

1.       Capture: the capture file is the result of the output of the imaging equipment used and is processed to create the source file.

2.       Source: the source file is the high quality go-to file that is used as the basis for future edits and processing for different applications.

3.       Output: these are the files you create from the source that will be used in electronic documents, posters, web sites etc.

Capture files

The capture file format i.e. the format used when carrying out the initial image acquisition, should be as close to the raw capture data as possible.

Scanners: For consumer level scanners this will usually be 24 bit TIFF saved at the same resolution as the initial capture. When it comes to high end scanners with appropriate software the file format should be a 48 bit TIFF for reflective scans or in the case of transmissive scans a 64 bit 'RAW' file. A RAW file from a scanner is typically a 48 bit TIFF with an additional 16 bit channel containing infra-red data.

Cameras: Most reasonable consumer level cameras and all medium to high end ‘professional’ cameras will output RAW image files which are representative of the raw data captured by the sensor, camera generated JPEG files should not be used as they have a great deal of in-camera processing applied to them. Many raw data formats are proprietary and require specialist software to process. Raw data files are generally not suitable for viewing or editing with standard image editing applications.

The RAW files captured by either a scanner or camera should be archived either in their native format (especially if including an infra-red channel) or as DNG or 48 bit TIFF.

Capture files may be discarded after being processed to create the source files, though it is not recommended. If capture files need to be discarded they should be archived to removable media and retained until you are comfortable with the processing techniques and are confident that you are able to get the best quality output possible.

Source files

A source file is created as a result of processing the capture file. Source files should be processed to match the original material as closely as possible without any creative embellishments. These files will become the basis of the image archive and once created should be set to read only. The only edits that should be carried out when creating source files are; capture sharpening, noise reduction, straighten/crop, colour and tonal adjustments to match the original material and the embedding of metadata.

If significant creative edits or reconstruction/restoration are required these should be carried out on a copy of the source file. The primary source file should always be representative of the original material.

Source files should be saved as high resolution TIFF with a bit depth at least equal to the capture file, in most cases 48 bit. TIFF is generally chosen for digital archives because it is a lossless format that is well established and widely supported. It also provides a reasonable degree of flexibility through its support for embedded meta data, multiple layers and a wide range of colour spaces.

Output files

Output files are edited and created for a specific use. The format of output files is entirely dependent on how they will be used. All suggestions regarding specific settings are based on the assumption that images are being exported from Lightroom.

On screen display:

If the images are being displayed on a screen e.g. on a web page, they need to be standard JPEG files. The relationship between the pixel dimensions of the image and the size the image is displayed on screen is 1:1, as such there is no point in the pixel dimensions being larger than the resolution of the display. For example an image being displayed full screen on a monitor with a resolution of 1920x1200 only need to be a maximum of 1920x1200 pixels, if the image is only going to occupy ¼ of the screen then its pixel dimensions only need to be 960x600.

Printed material:

Images that are destined for offset lithographic or digital print require a much higher resolution, typically of 240 to 600 dpi depending on the print technology used and the quality of the final output. Usually you will be asked to provide TIFF files with a minimum resolution. The requirements for print ready image files should be discussed with the printing company employed.

Photo printing:

Images that will be reproduced as photographic prints using either inkjet or sublimation printers or sent to a lab for traditional photographic printing will need to be at least 140 dpi sRGB JPEG files saved at a suitably high quality.

JPEG images should be exported with a quality setting not lower than 80, there is a diminishing return as the quality setting increases so quality settings above 92 will significantly increase the file size without any noticeable improvement in image quality.

Some print companies may accept TIFF files with colour spaces other than sRGB, such as Adobe RGB, but always check before submitting anything other than sRGB jpeg files.

I think that's enough on file formats so until part 3 when we'll look at the hardware, goodbye! 


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