Digital Philatelic Workshop

Fran Adams

Building a digital philatelic study relies heavily on the use of graphics. Some of our readers may not be not familiar with terms used to describe graphic images or their properties, so let’s define some of these image terms in plain English.

• Image Format
  Photographic images with colors/shades which blend into each other are best displayed in the ‘joint photographic experts group’ (jpg) format. ‘Tagged image file format’ (tiff) is good for photographic images as well, but the file sizes are very large and in most cases prohibitive. The older format of ‘device independent bitmap’ (bmp or dib) is seldom used as the sizes of the files are large, similar to tiff.

   

  Illustrations using line art or large single color areas are best displayed in ‘graphics interchange format’ (gif) format. This format presents the best option for small file sizes for this type of image. A newer format is ‘portable network graphics’ (png) which was designed for images on the internet and allows transparency.

  A new type of file stored by many modern digital cameras is a ‘raw image file’. This image retains all of the data the camera lens captures and does not compress or throw out anything. It’s essentially a digital negative. There are any number of abbreviations for these images as each camera manufacturer has their own system for storing the files. Graphic manipulation / editing programs such as Photoshop will read these files, but some of the programs with more limited functionality may not.

• Image Resolution
  Resolution is a measurement of how many dots are contained within a single square inch commonly referred to as dots per inch (dpi).

   

  Computer monitors typically display images at 72 or 96 dpi as a standard. If you magnify a standard dpi image on the computer screen, the image tends to become a bit fuzzy. That’s the result of the white space between the dots becoming more visible to the human eye. Make the image smaller and the dots squeeze closer together, making the image appear clearer.

  If an image is prepared at 150 dpi, it will hold its clarity better than the standard dpi image when magnified slightly. The bottom line is: higher dpi levels allow greater levels of magnification while maintaining clarity. This is also a factor in file size as higher dpi images are larger and require more disc storage space than lower dpi images.

• Physical Size
  Physical size of the image is simple. How many inches (or centimeters) wide or high is the image? This is not file size.


Size of image displayed in various values

• File Size
  File size is how much disc space the image uses when stored on a device like a hard drive and is usually measured in megabytes (mb) these days. This is not the physical size of an image.
• Image Optimization
  Images can be optimized to produce the best image possible for their file size. That means it takes less disc storage space, appears quicker on the viewer’s computer monitor and maintains a reasonable level of quality. Optimizing is done through compression of the image. The process essentially discards some of the image data to reduce file size while keeping the most important data as much as possible.

   

  There is always a trade-off when optimizing images – file size versus image quality. We should opt for a slightly larger file size but we’ll get higher quality images that can be magnified slightly and provide the viewer a closer look at image detail.

We’ll discuss focus and color in the next article Defining Graphic Image Terms Part 2.