What do PPI and DPI mean?
Dots Per Inch = DPI is used when we talk about printed photos. You can think of it as the "accuracy indicator" of the printed picture. DPI describes the amount of ink dots per inch on a printed image. It refers to the resolution value of a physical printer. Printers reproduce an image by spitting out tiny dots, and the number of dots per inch affects the amount of detail and overall quality of the print.
- 72 DPI picture is unsharp when viewed from close distance
- 256 DPI is somewhat sharp
- 305 DPI is sharp
- 400 DPI and more are super sharp even when viewed from close distance
Other numbers are something from between. When we talk about digital photos the only thing that matters is the amount of the pixels. DPI number can tell you how many pixels we want that there is in the photo or the other way around that what size of the print you can get in which accuracy.
256 ja 305 are established DPI-measures because it is easy to count the picture dimensions
- 1 cm @ 256 DPI = 100px
- 1 cm @ 305 DPI = 120px
If you want to make for example A4 -size magasine (21 x 29,7 cm) cover photo with 305 DPI accuracy (1cm = 120px) the picture file must be 21 * 120 px wide and 29,7 * 120px high, so the measures are 2520 x 3564 px.
If again we have 1200 x 1800 pixel sized file, we can count with DPI, in what accuracy you can get in what size when it is printed:
- 305 DPI: 1200px / 120 = 10cm and 1800px / 120 = 15cm = 10 x 15 cm
- 256 DPI: 1200px / 100 = 12cm and 1800px / 100 = 18cm = 12 x 18 cm
so the accuracy decreases as the print size grows
Higher DPI does not always equate to a higher quality print. One manufacturer’s dots might look as good at 1200 DPI as another manufacturer’s dots do at 700 DPI. Books and magazines often use 150 DPI for photographic reproduction, and newspapers often use 85 DPI. Ask the printshop or consult the printer specifications to find the appropriate DPI for your project.
You can define DPI -value to the picture file as the measure the photographer has intended the picture to be printed. But then again the customer can print what ever sizes he or she wants only the accuracy will suffer if they print the size of one meter and the pixels are not enough. Changing the files DPI -value the pixels will not decrease or increase and it does not change the actual content of the file at all. The DPI -value that is in the file is only side information that has no real value. For example the file could have the information about what camera was used to take the photo but it does not have any effect in anything. The only thing that matters is the amount of the pixels in the file.
When we are talking about PPI?
Use PPI whenever you are working with digital images. PPI is most useful in preparing files for printing (though DPI will be used by the physical printer). An image with a higher PPI tends to be higher quality because it has a greater pixel density, but exporting at 300 PPI is generally considered industry standard quality.​
Because increasing the PPI increases the size of your file, you will want to use a high PPI only when necessary. For example, when printing involves many fine details on a a glossy surface, it’s best to consider using a higher resolution. Printing an image on canvas does not require as high a resolution because details get lost in the texture of the material. PPI does not really matter for distribution on the web because the pixel density of your monitor is fixed. A 72 PPI image and a 3000 PPI image will appear the same on your screen. It is the pixel dimensions (the amount of pixels) that will determine the size and detail of your image.