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Image standards and formats


There are many different views as to the most appropriate size, type and resolution for digital images. Below is an outline of the standards used for images in the FARNE archive:



Size, Resolution and Format

For the purposes of the FARNE project it was decided to create two types of image file for each item: A Digital Master file (an archival quality, high resolution replica of the original) and a Web Delivery file (a low resolution copy of the original for displaying on the Web site).

Master copies – There are two main issues surrounding the creation of a master image. These are resolution and file size. - The digital size of an image is measured in kilobytes (K), megabytes (MB), or gigabytes (GB). File size is proportional to the pixel dimensions of the image. Images with more pixels may produce more detail, but they require more disk space to store and may be slower to edit and print. Image resolution thus becomes a compromise between image quality (capturing all the data you need) and file size.

It was decided to scan all master images for the FARNE archive at 25 MB where possible. A number of institutions have found this far more useful than setting the resolution at which an item is scanned. For example, if a postage stamp is scanned at 600 dpi/ppi (dots/pixels per inch) and then printed to A3, the resulting quality would be far less than if say a large poster were scanned at the same resolution and then printed to the same size. A file size of 25MB should ensure that image resolution is always of a high enough quality for any future requirements such as printing etc. All master files for FARNE were saved in tiff format, to avoid any compression methods involved with other file formats.



Web Delivery Images – With regards to files produced for the purposes of the Web, size and quality are equally important. Due to varying compression methods used by GIF, JPEG, and PNG file formats, file sizes can vary considerably for the same pixel dimensions. Similarly, color bit-depth and the number of layers and channels in an image affect file size.

FARNE web images are saved as JPGS at a set pixel width. This ensures that the images are not only small enough to load quickly, but that they are the correct size on screen. FARNE decided to provide three web images per item:

  • A full screen shot of the image, set at 600 pixels wide. – This size ensures that the image will not only fill the screen but that the user will only ever have to scroll down the image, rather than across. (According to user groups, images are easier to view when scrolling vertically, rather than horizontally).

  • Two thumbnail images. The first is set at 150 pixels wide for landscape images and 150 pixels high for portrait images. In this way we can ensure that the thumbnails, which are displayed with accompanying text, never take up an area of more than 150 pixels squared. In this way the layout of a Web page can be kept uniform no matter what shape an image is. The second thumbnail is created using the the same spec. at 200 pixels high/wide. This thumbnail accompanies the full page caption in the FARNE archive.


    Saving three different images for each item may seem extremely time consuming, especially when you consider that there are over 4000 items in the FARNE archives. There are, however, processes which can dramatically reduce the time it takes to resize and save images. Many of the actions involved can be written to a script using Adobe Photoshop, allowing batch processing of image files.

  • To create a script or ‘action’ click ‘Window’ and select ‘Actions’. Go to ‘Create New Action’ and name your action (e.g.“Save as Full screen” or “Save as Thumbnail”.) Chose ‘Record’ and carry out the processes you wish to include in your script on a sample image. Remember to click stop when you have carried out all the processes you wish to include.

  • To apply your script to a batch of images click File, Automate, Batch. Chose the script you want to use and the group of files you wish to apply the script to (you can apply scripts to specific folders on your PC or to files you already have open in Adobe photoshop). Then chose the destination of the edited images. (Images can be saved and closed, left open in Adobe or sent to a specific folder on your PC.) When you have selected all the options you require click ‘OK’ and the batch process will start.

    Tips

  • It is worth remembering that each time a file is saved as a jpg some degree of compression occurs. For this reason it is always best to create Web delivery images from high resolution tiffs.

  • If using batch processes to produce thumbnails, remember to write separate scripts for portrait and landscape images. – Remember you want to set the height of a portrait thumbnail and the width of the landscape thumbnail.







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    FEATURED STORIES



    Selecting a method of image capture
    Selecting a method of image capture
    Issues surrounding appropriate image capture for archival items

    Carrying out image capture
    Carrying out image capture
    Methods of image capture.

    Editing digital images
    Editing digital images
    Processes and practices involved in digital editing.

    Image standards and formats
    Image standards and formats
    Creating master copies and web delivery files.


    libraries@gateshead.gov.uk

    Gateshead Central Library
    Prince Consort Road, Gateshead, NE8 4LN
    Tel: 0191 433 8430