A coded language
Fortunately, you don't need to know advanced math or be a secret agent to understand these two terms
Let us first begin by DEFINING them:
DPI = Dots Per Inch. This term refers to the number of dots required to create an image.
PPI = Pixels Per Inch. This term refers to printing screens.
What we are talking about here is the resolution, or the capacity to render the minutest details of an image. The term DPI is that most often used for digital imaging, while PPI is more commonly employed in the printing industry.
The same DPI measure must now be divided into two groups, one used to designate the image capture resolution and the other to refer to the print resolution.
Image Capture (camera and digitizing)
Most of the time, digital camera resolutions are indicated in "megapixels". For example, a 5 megapixel camera has a resolution of 2560 dpi x 1920 dpi, which adds up to 5 million pixels.
The higher the resolution, the smaller the pixel size, and the finer the recorded details will be.
See the two diagrams below for a demonstration of this formulation :
600 dpi image
3000 dpi image
In this case, DPIs have very little to do with the quality of the finished image. A thermal printer with a resolution of 312 dpi will give results that are as good as those obtained with an ink jet printer of 1400 dpi!
Here DPIs only indicate different print qualities for the same type of printer.
To obtain quality prints with an ink jet printer, a more important factor is the diameter of droplets in PICTOLITRES. The higher the number of pictolitres (i.e. 4 pictolitres), the more precise the rendering will be. The number of base colors used (4, 6 or 7) is also an important aspect to consider when it comes to the quality of the final image.
So What Resolution Should I Use?
At this point, you're probably saying, "Now, I'm really confused! How will I get quality photographs? How many megapixels (DPIs) do I need?"
ARE YOU READY, What follows is amazingly simple !
To obtain a "photo" quality image, that is, equivalent to a conventional photograph, there is a very simple formula. For example, if you usually print or have your images printed in a 5" x 7" format, the calculation is the following :
5'' x 300 dpi * = 1500 dpi
7'' x 300 dpi * = 2100 dpi
1500 dpi x 2100 = 3,2 megapixels (approximately)
(*) 300 dpi being the print quality (number of dots seen by the eye).
Thus a 3.2 megapixel camera will yield results that are equivalent or very close to those obtained with a conventional camera. A 2 megapixel camera, however, will also give very satisfying results.
Should you wish to reach higher plateaus and admire your images in a 16" x 20" format, for example, then you would do better to choose a 5-megapixel camera.
In choosing a camera, the desired results should be your first consideration. Would you like to make souvenir photos, enlargements or Web images? Your choice is closely tied to the number of megapixels of your camera.
Go for it, and Happy Shooting
With the cooperation of Jacques Bourdage, PFE.