Resizing and Output Sharpening

Suppose you have captured, and edited, and now want to print an uncropped digital image that was made with a 12MP camera. The image is 5760 pixels wide X 3840 pixels high.

Assuming you have followed the advice about maintaining maximum image quality and maximum colour information, and your image looks on the screen as you would want it, there are critical final steps in ensuring that your photograph may become a maximum quality print.

Resizing and resampling:

The most straight-forward and accurate print is one where each pixel in the image is sent to an exactly corresponding ‘dot’ on the print.

For the purposes of fine-art printing, Canon (and HP) inkjet printers can be considered to have a 'native' resolution of 300 dots (of ink) per inch,1 while Epson printers have a 'native' resolution of 360 dots per inch (dpi). (Where do these numbers come from?)

In practice, an inkjet photograph smaller than about 24" X 16", which is printed to be hung in a gallery or used in a competition, should be printed at 300 dpi on a Canon (or HP) printer, and 360 dpi on an Epson inkjet printer.2

The challenge with making a quality inkjet print is that the photographer almost always has an exact size in mind for a print (say 12" X 8" or A3, etc), which dictates how many dots must be printed.

For the sake of this discussion let's assume you're printing on a Canon inkjet printer at 300 dpi. You are aiming to make a 21" X 14" print to frame and sell.

A 21" x 14" photograph will be printed using 6300 X 4200 ink dots.3

But your digital image has only 5760 X 3840 pixels!

For best quality you need to resize and resample the image so that it will have 6300 X 4200 pixels. Then, when you print, 1 pixel ➝ 1 ink dot.

Here you can see the process for resampling using Photoshop Elements. Most applications will be similar: You will need to specify the print size and the print dot resolution (300 dpi for Canon or HP, 360 dpi for Epson)

Output sharpening

Sadly, having been resized and resampled even choosing the best resampling methods, the image may have been made 'soft': edges may have lost their definition. In most cases it is necessary to apply a certain amount of final (output) sharpening to the image to counteract the softening caused by resampling. This sharpening ionly needs to be carried out if the image has been resampled and, of course, must be carried out after resampling — just about the last thing done to the image.

The amount of output sharpening required depends on both the nature and extent of the resampling that was done and the nature and dot resolution of the output medium (paper or screen).

It turns out that once those are known the amount of output shaprpening required is pretty much the same regardless of the image content, so it can be standardised easily. Some photo applications recognise this and are very helpful — others not so much!