When you start with digital processing of your photos, one of the first terms you will hear is DPI. DPI stands for Dots Per Inch and it is related to the quality of your photos. Generally, the higher the DPI of an image, the better its quality because the image resolution is higher and the image is more detailed but high quality comes at a price.
There are many cases when high image quality is not necessary. For instance, the DPI of Web images is typically 72 DPI. This is very low but for the Web it is acceptable because even though the images look far from perfect, they are small in size and download faster. 96 DPI is also used for Web images. 96 DPI slightly better but still it has nothing to do with the resolutions for printed images.
There are many resolutions for printed images. 300 DPI is acceptable for printed images but 600 DPI is better. The DPI of your image is largely dependent on the printer (or plotter) where it will be printed. There are printers and plotters that can print at resolutions of 2400 and above. So, your DPI largely depends on the purpose of the image. You can have the same image at several DPIs and use each copy in a different case.
Changing image DPI can be done easily, when you downgrade the DPI (i.e. when you convert a 300 DPI image to 72 DPI). In fact this is the most frequent DPI change the majority of people make. For instance, if you have some digital photographs taken with a more powerful camera (5 megapixels or more) and you want to publish them on your blog, you might have to drastically decrease their DPI and/or their size but you can print them on paper without having to make any DPI changes.
When changing the DPI, it is important to keep in mind some standard resolutions. As already mentioned, 72 and 96 DPI is for Web. 150 DPI gives printed images of fair quality but if you want something nice, you should use at least 300 DPI (or even 600). Sometimes you might want to go for a lower DPI because all equal, the image size (both as MBs and as height/width) is smaller when the DPI is lower.
You can also change the DPI without having to change the image height and width. Keeping the height and width of an image is possible only when you are decreasing the DPI. What you in fact do by decreasing the image DPI and keeping the original size and proportions is that you just remove some of the pixels in the image – you don’t change the image itself, you just make it less detailed.
Changing image DPI is done via software programs like ImageConverter and in many cases all the work is done automatically for you, so you don’t have to worry about the technical details. You just select the image(s), select the target DPI and the program does the rest for you.
There are programs that allow you to choose from preselected resolutions (i.e. 72 DPI, 300 DPI, etc.) but there are also programs which allow to enter the DPI manually and you can have your image at any DPI you want. If you insist on using a program that allows choosing any DPI, have in mind the standard DPI recommendations in this article. Also, though you can select different horizontal and vertical DPI, don’t do it, unless you have a reason.