Pixels and Printing


Summary

How many pixels do you need when printing a photograph?

This worked example is based on a 10 megapixel camera.


Pixel Count

The pixels are laid out on the camera sensor in a grid.


The proportions are usually about 3600 x 2700 giving a 4 : 3 format image.


You can also crop this to a 35mm traditional 3 : 2 format using an image editor.


You just keep the 3600 pixels and crop the other side to 2400 pixels.


Printing

Printers usually print at 300 dots per inch (300 dpi).


300 dpi gives excellent results on paper.


The print will be 12 inches long because we are printing at 300 dpi and have a 3600 pixel image.


Printing Bigger

What if you want a 24 inch print from the same image?


The printer will print it out at 150 dpi giving a decent quality print.


For a 24 inch print at 300 dpi you need to upsize the image in your imaging editor:


24 inches at 300 dpi = 7200 pixels required for printing.


If you do upsize then do it repeatedly in steps of around 1000 pixels each time until you are there. This gives better quality.


Image Resizing

Use an image editor to resize the image in steps to 7200 pixels and accept the default settings for everything else.


Image resizing is very easy but looks complicated. All you need to do is type in the number of pixels for the largest dimension of the image and the editor will adjust the other dimension automatically while preserving the image proportions.


The upsized image is saved under a different filename so that the original image in unchanged.


Quality Concerns

After upsizing always zoom in to 100% magnification and check that the quality is acceptable.


You may notice that the upsized image appears softer than the original but this can be used to your advantage especially if you are intending to create a painterly effect with further processing.


You can also sharpen the image slightly to lose some of the blur.


Finally

You just need to know how many pixels you have along the largest side of the image.

Knowing that you need 300 dpi for a print of excellent quality will tell you roughly how much you need to upsize.

You can upsize more than twice but eventually you will reach a limit where the loss of printing quality becomes unacceptable