DPI and Image Resolution
DPI can sometimes be tricky business when trying to figure out how to export your file, or choose a size to print. The best thing to do when thinking about DPI is to make sure it matches the resolution that the camera captured in the image.
DPI is a sliding scale based off of the number of pixels in the image (i.e. the resolution captured by the camera, a fixed number) and the intended physical sizing of the print in inches (not fixed). When you choose a smaller size, the fixed pixels in the image will be condensed over a smaller surface area, providing a higher density of pixels per inch, and a theoretically clearer image. If you choose a larger size, that same number of pixels is spread out over a larger surface area, therefore creating a print with fewer pixels per inch. So, if a high DPI print is what you are after, the thing to do in order to achieve this, is choose the appropriate print size for the resolution of the image, as created by the camera.
When an image is captured with a camera, a finite number of pixels are used to digitally store the image. That set number of pixels is all the visual data stored in the file, and is what makes up the photo. There is no more data or image beyond that set number of pixels.
For example, if your camera is 24MP, it will produce a file that is about 6000X4000 pixels. To get a 300DPI print out of this file, you will want to choose a size that is about 12X18 or smaller. This will maintain the distribution of pixels over the print area at a density greater than 300 pixels per inch. This may sound surprising as we offer some very large print sizes, and still get great results, but the fact of the matter is that 300DPI is not necessary for a nice clear print in normal viewing circumstances. The goal of 300DPI is for razor sharp clarity at extreme, magnified viewing (like under a loup). If the print is on the wall and the viewer is standing a foot or a few feet away, an image of about 150DPI or higher will still look great! That same 24MP image, at a range around 150DPI will be able to produce nice clear print results in sizes like 24X36 or 28X42 without issue.
When printing images, every photographer hears that 300DPI is what to aim for. Now, although this is true for ideal printing, you can still achieve great clear results with images printed at a size that makes them less than 300DPI. If you have your heart set on getting a print in 300DPI, the real trick is choosing the appropriate print size, and not increasing the resolution of your file.