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Guides to Photo and Document Preservation
and Sharing for Families and Community Historical Societies
Copyright © 2009 - Historical Images of New England LLC - All Rights Reserved.
Understanding Digital Image Terminology and
Technology
A solid understanding of the terminology and concepts associated with digital imaging can improve the results of your digital image preservation and sharing activities.
Digital Imaging Activities
To understand digital image technology, it useful to consider four distinct categories of activities.
Capturing - Digital images are typically captured by scanning an existing image, or by creating a new one with a camera. In both cases, sensors record image information at an adjustable level of detail for such image attributes as color, hue, and contrast.
Editing
- With the use of digital image editing and management software,
one can make useful and quick adjustments to color, contrast, brightness
and other image attributes. In addition to these adjustments,
non-professionals often use image editing software such as Adobe Photoshop Elements 7
Filing - The captured image information must then be formatted and filed digitally for archiving or future use. This entails a choice of digital image formats and labeling/indexing schemes.
Sharing / Displaying - The image information previously captured and stored is then available to be displayed and shared in a relatively broad range of ways. Sharing activities include email attachments, web display, and printing. Further possibilities include online albums and share sites, photo books, and self managed websites.
There are distinct relationships between these activities, and decisions made in one activity will impact the range of options available in other activities.
For example, a decision to set a scanner at a particular resolution will impact the amount of image information to be saved and the level of detail available for print enlargements. The selection of a file format will impact the required storage space necessary for the related file and the quality of the information. All of the prior decisions will impact the maximum size image you may print at a "high quality" level of print resolution.
This article is focused on de-mystifying some basic terminology, and providing a sense for how the activities relate. Specific "How To" guides can be accessed via the menu to the left.
Digital Image Capture Basics
Digital images are most often captured/created by cameras or scanners as tiny squares of image information called pixels (short for picture elements). The technology of these sensors is not important to us, but an understanding of the form and shape of the information captured is useful.
Digital image information is recorded as a grid consisting of columns and rows of pixels. The information about each pixel is recorded as a shade and hue of the colors red, green, and blue (RGB). The number of rows and columns of squares (total pixels) recorded is a function of the maximum pixel capability of the equipment used and the settings selected by the user.
Typically, the price of equipment increases with higher pixel capture and resolution ratings for cameras or scanners. However, as discussed in other articles, much of the equipment available today provides capacity well beyond our typical needs.
To better understand the impact of pixel counts, look for a nearby object of roughly 10" by 10" and try to picture it within a grid of 100 squares consisting of 10 rows and 10 columns. If color, hue, and other information for each 1 inch square were to be averaged and recorded, a reconstruction of the image information would be vague and blurred.
On the other hand, if the same 10" x 10" image information is recorded in a grid of 1,000 rows by 1,000 columns, the info would now be reflected as information about 1 million picture elements (pixels), and the reconstruction of the image would reflect considerably more detail / resolution.
In the case of a digital camera, you are allowed to set the number of pixels/squares of information of detail to be captured for a given image, up to the maximum megapixel (million pixels) rating of the camera. In the case of a scanner, you are allowed to set the pixels per inch (PPI) of the actual object to be scanned, up to the maximum PPI rating of the scanner.
Knowing that you have control over the number of pixels to be captured, and the math of how this will impact your digital files and potential uses of the images is important to planning your image preservation and sharing projects.
Cameras are rated and marketed based on the number of megapixels (million pixels) that the camera's sensor can capture at the highest setting. Compact digital cameras normally capture pixels in a height to width ratio of 1:33, while digital SLR cameras normally capture images in an aspect ratio of 1:5. Multiplying pixel height by width (columns by rows) provides a total pixel count, and industry practice is to round to the nearest 1 million pixels to indicate a megapixel rating. Camera cost normally increases with megapixels. However, some argue that there is a point of diminishing returns beyond 5 megapixels for the non-professional, since files become very large and most non-professional uses do not require digital files in excess of 5 megapixels.
When scanning images, we are asked to select a pixels per inch (PPI) resolution at which to capture the image of the object to be scanned. For example, we may be scanning an old 4" x 6" print. Based on our intended use to print a certain size enlargement, we may have decided that we should capture the image information at 600 PPI. The pixel count for the scanned image will be (600 PPI x 4") or 2,400 pixels of height x (600 PPI x 6") or 3,600 pixels of width = 8,640,000 pixels = 8.64 megapixels.
Tip / Hint: You will often read articles, equipment manuals, and software settings that use PPI (pixels per inch) and DPI (dots per inch) interchangeably. This is the source of considerable confusion. It can eliminate a lot of confusion if you always think of image capture and display resolution in terms of pixels and pixels per inch. Only think of dots and dots per inch as related to a printer's mechanical ability to place dots of ink on paper. Pixels are not dots of ink. Pixels are best thought of as squares of image information. Dots per inch are seldom useful in our thinking, as the capability of current "photo quality printers" is well beyond our ability to discern a dots per inch difference. On the other hand, the number of pixels we have to work with in displaying an image on a monitor or in a paper print can make a big difference in our perception of the resolution quality of the image.
Deciding on settings for image capture pixel resolution will impact the size and manageability of your digital files, and will impact the ways in which you will be able to use and share the captured images.
Digital Image Files and Formats
Once image information is captured in pixels, the information must be formatted and saved as a digital file containing bits and bytes of data reflecting information about each image pixel.
Cameras, scanners, and image editing software often offer us a choice of digital image file formats to save and store our captured image information. In this section, it is recommended that Jpeg and Gif file formats be your typical choices for preservation and sharing needs. As described below, the choice will depend on the nature of the image. In this section, we briefly describe the alternative uses of these formats and the basic reasons for our recommendation. For a broader discussion of file formats, see our article titled Digital File Formats and Recommendations.
When to Use Jpeg - The Jpeg file format is recommended to record information about full color or subtle gray-scale realistic images, such as photographs. The Jpeg format can distinguish between blends of 256 shades of red, 256 shades of green, and 256 shades of blue that result in millions of color variations.
Jpeg has two major advantages. First, it is the most commonly used format for non-professionals. Therefore, it is the easiest format to use to share and transfer images. Secondly, the Jpeg format will compress the image file at a compression level largely determined by selectable settings on cameras, scanners, and available in image editing software.
Tip / Hint: Terminology used to set compression levels for Jpeg files varies. Simply keep in mind that a low compression level equates to keeping a saved image at a high quality. It is generally agreed that compression levels of up to 10:1 (compressed files 1/10th of uncompressed size) will retain a sufficient quality of detail for most non-professional needs. Jpeg compression algorithms compress information on each and every save. Each time a file is re-saved, even modest re-compression will result in some additional lost quality. Therefore, it is a good practice when working with Jpeg files to work with a copy of the original Jpeg file for each use, in order to avoid multiple re-saves of the original Jpeg image file.
When to Use Gif - Gif is the recommended format when working with mages of limited colors and/or including sharp lines. These images would include line drawings, images such as cartoons, text documents, heavily captioned images, some maps, etc.
Although, Gif is a lossless digital image file format (no pixel information is eliminated in compression process), it is effective in consolidating stored information about areas of the same color. Therefore, depending upon the nature and complexity of an image, Gif will often result in a much smaller file than Jpeg for the types of images recommended for Gif.
Digital Image Sharing and Display
With pixel information for photos and documents safely stored in Jpeg or Gif format, the possibilities for sharing, displaying, and printing the images has expanded dramatically.
Many of these possibilities are described in "How To" Guides and project descriptions. In this section, we will provide some basic concepts related to common uses of images for sharing and display.
An important concept to understand is how the number of image pixels we have captured and saved relates to our intended image uses. Two of the most common uses are to display an image on a monitor (as an email attachment or as a website image), and to print the image in one of several available sizes. We will focus on printing concepts as high quality printing objectives will often dictate the minimum number of pixels we need to capture.
Tip / Hint:
When possible, visualize and capture the maximum number of pixels you
need to meet your defined project objective. Capturing more
pixels than you need will result in excessively large files, and
capturing fewer than needed pixels will limit how you can use the files.
Having defined your pixel resolution requirements, you can confidently
set your camera or scanner for appropriate results. If future
uses are unclear, it is better to error on the side of more pixels.
You can
quite easily and effectively downsize / resample a file's pixel
dimensions to meet reduced pixel needs for certain types of projects. However, success is not nearly as predictable when
attempting to increase an image's pixel dimensions with
interpolative software.
Printing Considerations
To understand how digital image pixels relate to printing quality, it is important to differentiate clearly between dots per inch (DPI) and pixels per inch (PPI), since these terms are often confused and used as interchangeable in articles on the subject.
As described on page 2 of this article, dots per inch (DPI) only relates to a given printer's ability to place a certain number of "dots" of ink on an inch of paper. With the DPI capability of current printers, DPI is normally not a consideration, since the non-professional is hard pressed to discern printer differences over about 600 DPI for text and 1200 DPI for photo realistic images, and current printers exceed this level.
On the other hand, a digital image contains a finite number of pixels in columns and rows, and the size of the print will dictate how densely the printer can represent these pixels of image information on an inch of paper. This density of pixels is expressed as pixels per inch or PPI.
How many pixels we have in our digital image file and how far we spread them will mathematically result in how many pixels per inch (PPI) will be reflected in the print. For example, if we scanned a 4" x 6" photo at a scanner capture resolution setting of 600 PPI, our file will reflect pixel dimensions of 2,400 pixels by 3,600 pixels. If we spread this pixel information over an 8" x 12" print size, we could have a professional photo quality print pixel resolution of 300 PPI (2,400 pixels / 8 = 300 and 3,600 pixels / 12 = 300). If we chose to print a larger 16" x 24" print, we have enough pixels to have 150 pixels per inch of image resolution, which would provide a quality print, but not as high of a quality as at 300 PPI.
Tip / Hint: To help you understand how PPI resolution impacts
image print quality, it is generally agreed that:
300 PPI =
Professional Photo Quality
[generally agreed, the best the eye can do]
240+ PPI = Near Professional Photo Quality
[difficult for a non-professional to discern a difference from 300 PPI]
200+ PPI = High Quality
[very good images and high quality capture of subtleties}
150+ PPI = Quality
[good images in this range for routine non-professional uses]
100+ PPI = Acceptable Quality
[depending on nature of image and purpose]
100< PPI = Poor Quality
[may be acceptable for some larger prints at a distance]
Digital Image Web and Monitor Display Basics
Images are often shared and displayed on a monitor as an email attachment, or as an image within a website or a photo gallery. Captured image pixels are produced on a monitor very differently than on printed paper.
Compared to printed images, monitors require considerably fewer pixels per inch of image information to display what we consider a quality and distinguishable image. This is the case since a monitor is capable of representing millions of colors directly from digital pixel information. On the other hand, a high quality paper print requires the overlapping and visual blending of a limited number of ink colors available to the printer to produce a good visual quality image. This printer color overlapping and blending process requires more pixels per inch of image information in order smooth the color transitions and develop a high quality printed image.
Monitors vary in resolution settings available, and monitor resolution settings can usually be adjusted to accommodate individual preferences. Regardless, most monitors are set to display between 74 and 96 pixels per inch of screen surface. This PPI resolution can produce a good quality representation on the monitor, but this PPI resolution in a printed image would typically be considered of poor quality.
Another major difference in how our pixels get used in web and monitor display is how the pixels get distributed. In the case of printing, you have the ability to increase or decrease the number of pixels that you want to place within an inch of printed output. In the case of monitor/web display, the pixels per inch are fixed based on the monitor settings, and the thing that changes with the number of pixels is the size of the image on the screen.
For example, if you are starting with a 2,400 pixel by 3,600 pixel scanned image (our 4" x 6" scan at 600 PPI scan resolution), you would need a monitor set at 94 pixels per inch to be roughly 25.5" x 38.3" in order to display the entire image.
Fortunately, many software programs will automatically reduce the pixel size of "large" images (typically by re-sizing / re-sampling the image) for the intended monitor display use it detects. Whether or not you are using software that automatically detects a necessary resizing, it is useful to understand how to better control the process and to make your own decisions.
For example, in website design, it is most likely that someone made a very conscious decision about the size and placement of each and every image you see in the website.
The photo above started out as a 3" x 5" original. The Photo was scanned at a 600 PPI scanner resolution. This resulted in a 5.4 megapixel image and an uncompressed Jpeg file of 16.2 megabytes. The pixel width of the scan was 3,000 pixels, and the height was 1,800 pixels, much too big for monitor display and insertion in the website.
In Photoshop Elements, the image was resized / re-sampled to a specified image pixel size of 300 x 180 pixels, by indicating a width of 300 pixels and by constraining proportions. The picture width of 300 pixels was selected to be 3/8 of the specified 800 pixel width of the display area of this website.
Copyright © 2009 - Historical Images of New England LLC - All Rights Reserved.