When is it a Dot, When is it a Pixel?
Overview of handling photographic files
You will use an application like Photoshop, Photoshop Elements, or iPhoto to work with your picture files. Read their manuals, but those books often describe features beyond our needs. Our recommendation is to stick to the elementary tasks outlined on these four pages.
- File formats (uses, benefits, and conversion of different file formats)
- File formats revisited (dots, pixels, resolution, and file conversion)
- Scanning photographs (using a scanner and generating a master file)
- Retouching photographs (alignment, cropping, color modes, light and contrast)
Whether a picture is on paper or monitor, the picture itself is made up of discrete points. Obviously these points have to be so small that they become itty-bitty pinpoints of color or shades of gray in a smooth visual (you have seen this effect in television ads for digital cameras and Hewlett-Packard equipment).
These small pinpoints are variously called dots (and dots per inch [dpi]) or pixels (and pixels per inch [ppi]). Both terms represent small discrete elements that together constitute a picture.
Evolution of the dot
A person laying out a newspaper in the last century placed the type and left holes for art that would then be sized and etched in place as a pattern of dots (the individual dots would be black; undotted areas would be white; and partially dotted areas would be gray). These early newspapers may have use 60 dots per linear inch.
The practice in printing art, therefore, has been to state the size and separately specify the resolution.
Evolution of the pixel
Computer monitors use extremely small lightbulbs (light-emitting diodes, or LEDs) to put color on the screen. Each LED generates a pixel of color, and the number of LEDs is part of the monitor — the standard for most monitors is currently 96 pixels per inch.
To place electronic art, you specify the size of the art in pixels and the “resolution” of that art is automatically modified by the specification for the number of pixels.
The file size of an 8 x 10 photograph will be large because of its immense number of pinpoints (dots or pixels). If what is desired is only a wallet-sized picture or a small picture on the monitor, the sheer size of the entire file will slow the printing or display — it is better to reduce the file size to one that suits the final use.
To size your picture for use on a computer monitor, use the pixel dimensions section, typically at the top of the Image Size window:
A web designer defines the size of a picture in pixels, but monitors generally have around 96 pixels per inch. Thus, the final actual size showing on the monitor is determined by the designer’s specs and the number of pixels per inch in the viewer’s monitor.
Typically, designs call for widths of 100-800 pixels with no height specification. To resize a piece of art, open the Image Size window and check both Constrain Proportions (keeping the height-to-width ratio constant) and Resample Image (allowing pixels to be discarded). Insert the designer’s value in the width window of Pixel Dimensions (in this case, replacing 2049) and click OK.
Your computer’s application will reallocate pixels to the desired width, and give you a more efficient file for your final use.
To size your picture for printing on paper, use the document size portion, typically in the middle of the Image Size window:
To print the picture on paper (including photographic paper), the unstated standard is for 300 dots per inch and the designer will provide a spec for width in inches. Go to the Image Size window, check Constrain Proportions (keeping the ratio of height to width constant), BUT DO NOT check Resampling Image.
Insert the desired width and click OK. The height will be automatically filled in and the resolution should be above 300dpi. If the resolution is less than 300, ask the designer to use a smaller picture box; if the resolution is over 600, check Resample Image, reduce the resolution to 400, leave the width at the correct size, and click OK. This will create a smaller file but will still provide a quality printing job.
Saving the file to the correct format
Your last step with this file is to save it in the correct file format.
- If the file will be used on a monitor, save it as a JPG in the Save As window (left, below):
- If the file will be printed, save it as a TIF in the Save As window (right below) WITH ONE EXCEPTION:
- If the file is to be used for making copies on photographic paper, save it as a JPG (left, below).
This article is one of several to help you document local history. Other articles will help you convert your interviews, documents, pictures, and artifacts into documentation of your local history that can be shared with your community.