Post-Processing Workflow in GIMP

Finishing touches

Two final things – resizing and sharpening.

Resize

All manipulations (except sharpening) should be done before resizing, especially rotation and perspective correction. This is also the time to make sure your image has the right aspect ratio. For example, the typical photo paper aspect ratio is 3:2. If you would like to print a standard size photo, you should crop your image to match this ratio.

The size of the picture can be changed in Image -> Scale Image.

The size of a digital image is always converted to pixels and setting pixels is straightforward.

However, when it comes to printing, image resolution is also important. It is measured in dots per inch or pixels per inch. To make a decent print, the resolution should be 300 by 300 pixels per inch, which is about 1200 by 1200 dpi. So, to make 15x10 cm prints with 300x300 ppi resolution, just set it in image size settings as seen above. It seems that a 2MP camera should be enough to produce small prints of acceptable quality.

If I wanted to print a 600x600 ppi image in A4 format (many home printers are capable of this, at least "on paper"), the image size would have to be 5000x7000 pixels, which would require a 645D camera.

Sharpen

Sharpening is usually done with the 'Unsharp mask' tool, which is found in Filters -> Enhance menu. This method is well covered elsewhere on the Internet. Incidentally, I have noticed that Pentax 35mm F/2.8 (an awesome lens) needs far less sharpening than Sigma 28-200mm (a sub-standard lens).