How to interpolate digital files?

Interpolation can be achieved with varying degrees of success depending upon the image, the techniques employed, and the amount of interpolation required. Moderation is key, as interpolation results in loss of detail and image sharpness. It is important to know that interpolation does not give more actual resolution, it merely creates a larger image by repeating existing data. It does this by averaging adjacent data to create new pixels and by blurring the edges between the existing adjacent pixels. The more an image is interpolated, the more false data it contains. If a 6" x 9" image at 300 dpi is interpolated up to 12" x 18" at 300dpi, only 25% of the resulting image data is real.

Interpolation or resampling is sometimes necessary to change a file's dimensions to reach the desired size. Many different ways exist to do this. The easiest most efficient method is to resize the image in Photoshop using Image-->Image Size, and plugging in the necessary dimensions. When sizing up the image, it is usually most beneficial to use the Bicubic Smoother setting in the Image Size dialog box. Using the Bicubic Smoother feature will make sure no sharpening is applied. Also, if you know you will be sizing up an image, try to avoid applying any sharpening in the raw processing stage.

In the past it has been recommended that if you are resampling in Photoshop, you do it in steps. 5%-10% at a time until you reach the desired file size. However, Photoshop's resizing algorithms have evolved to a point where it is of little use to do this. It takes a lot of time and processing power, and the differences are negligible.