Not long ago, there weren't many options for increasing resolution other than using your editing software's built-in resampling methods. Today, we have good software and techniques.
Resizing vs. Resampling
Resizing an image involves changing the print dimensions without changing the total pixel dimensions. As the resolution is increased, the print size becomes smaller, and vice versa. When you increase resolution without changing pixel dimensions, there is no loss in quality, but you must sacrifice print size.
Resizing an image using resampling, however, involves changing the pixel dimensions and will always introduce a loss in quality. That's because resampling uses a process called interpolation for increasing the size of an image. The interpolation process estimates the values of the pixels the software needs to create based on the existing pixels in the image. Resampling via interpolation results in serious blurring of the resized image, especially in areas where there are sharp lines and distinct changes in color.
Common Interpolation Methods
Photo editing software generally offers a few different interpolation methods for calculating new pixels when an image us upsampled:
Bicubic, Bilinea and Nearest Neighbor.
Gradual Interpolation
You can get excellent results when upsampling by increasing the image size in several small increments rather than one extreme step. One advantage to using this technique is that it will work on 16-bit mode images and it requires no additional software other than a standard photo editor.
This technique is simple: rather than using the image size command to go directly from 100% to 400%, you would use only increase the size by 10% at a time. Then you would repeat the command as many times as it takes to get to the size you want.
How to Resample
an Image with little loss of Quality
