When using functionally-rich image editing software such as Adobe Photoshop Elements, it is important to understand the principles of resolution (dpi) and size (dimension) as well as how these relate to print, web and screen images. This will help avoid confusion when, for example, images are printed too small or huge photographs are displayed on the screen.
What is the DPI of an Image?
DPI is a fundamental concept in dealing with images both online and in print. DPI stands for Dots Per Inch and is an approximate measure of the resolution of an image. The key ideas can be explained simply as follows:
- Imagine a square one inch long. The dpi measurement would be the number of dots that fit along the edge of the square. It is possible to draw the edge of the square with many or few dots and still recognize the edge.
- Imagine an image printed on stretchy material. The actual image will stay the same, whether the material is stretched or not, but the density or resolution of the image will change. If the image was printed on loose material and then the material was stretched a long, long way, then the quality (resolution) of the image would decrease as it would be harder to see the details. On the other hand, if an image is printed onto a stretched-out piece of material and then the material is released, the resolution of the image would be higher and quality would look the same or improved.
- On a computer screen, everything is displayed at 72 dpi – that means that for every inch on the screen there are 72 dots making up the image.
- High-quality printing typically takes place at 300 dpi. So for every inch, 300 dots are printed.
A photograph, which may be scanned into a computer at 300 dpi, ready for printing will display on a 72 dpi screen much larger than the real-life photograph. Similarly, when creating an image in Photoshop Elements at 72 dpi, it may display at an expected size on the screen, but when printed at 300 dpi it will look much smaller on paper. This is due to a resolution.
Dimension (Length) Measured in Inches, cm, mm and Pixels
An image can have fixed dimensions such as length, width or height. For example, 10cm, 100mm or 8 inches.
Photoshop Elements allows all images to be manipulated based on both dimension (lengths such as mm, cm or pixels) and resolution (dpi).
The dpi dot is often referred to as the pixel and images can also be set to a length of a certain number of pixels.
Changing Image Size
A common requirement is to change the size of an image and in Photoshop Elements this can happen in a number of ways.
An understanding of the resolution and dimension of an image are important when completing many tasks in Photoshop Elements. A typical task could be taking a high quality photograph and then displaying it on a web page. The photograph may have been taken on a digital camera and stored on the computer at 300 dpi (or more) and be too large for a web page. All screens display at 72 dpi so there is no point having a huge, high quality file on the web page. The resolution needs to be set to 72 dpi and the picture dimensions need to be reduced to fit the design of the web page.
In Photoshop Elements 6.0, once the file has been opened and the photograph is displayed, the following selections should be made:
- From the menu, ‘Image’
- ‘Image size’
The resolution should then be set to 72 pixels/inch – the dimension of the image (in pixels) will be automatically updated to keep the dimension the same. The dimensions can be changed if required, either by changing the number of pixels or by changing the length in cm, mm, etc.
If a change to a photograph has resulted in the image on screen looking much larger or much smaller than expected, it is probably due to the incorrect resolution being used.
An understanding of image size in terms of resolution and dimension will help users to exploit Adobe Photoshop Elements functionality.