Author: Peter Martin Jones 

All digital images are made from grids of tiny squares (pixels) with most cameras using anything from 2 to 12 million (or 2 to 12 Megapixels). When you print your images, the most important thing is that the pixels must be so small that they are invisible to the naked eye. This prevents the print looking like a pattern of tiny squares giving the dreaded "fuzzy" or "pixellated" appearance but just how small do the pixels need to be?

Print Resolution is measured using pixels or points per inch (ppi). The higher the ppi, the smaller the individual pixels in the image and the less likely you are to see them and the sharper the print will be.

A 100 ppi print has 10,000 pixels per square inch, which means that there are 100 ppi horizontally, and 100 ppi vertically. The consensus is that for true photographic quality you need to be printing at around 200 ppi, which corresponds to 200 x 200 = 40,000 pixels per square inch. This guide is based on viewing the prints from a distance of around 18 inches - the average viewing distance. If you are going to hang a print on the wall however, you can get away with much lower resolutions as you will be viewing it from a greater distance.

Images taken with a digital camera have a fixed, maximum number of pixels dependent on the CCD sensor chip of the camera. If your camera has a 5MP (5 million pixels) model you can easily work out the maximum print size by using the relationship:

Print size = number of pixels divided by the print resolution

So for a 5 million-pixel image at 200ppi (40,000 pixels per square inch) the maximum print area is:

• 5 Mp = 5,000,000 divided by 40,000 = 125 square inches, or roughly 10 x12 inches.

Similarly, we can calculate other sizes

• 3Mp = 3,000,000 divided by 40,000 = 75 square inches or roughly 10 x 8 inches

• 4 Mp = 4,000,000 divided by 40,000 = 100 square inches or roughly 10 x 10 inches.

• 6 Mp = 6,000,000 divided by 40,000 = 150 square inches or roughly 10 x 15 inches

• 8 Mp =8,000,000 divided by 40,000 = 200 square inches or roughly 12 x 16 inches.

If you have Adobe Photoshop or Photoshop Elements or a similar program you can see all this worked out for you. Click on "Image" from the main menu and then on "resize image" and make sure the 'resample image' option is not ticked (click on it if it is ticked) and you will be shown the maximum print size that corresponds to your image.

Many people wonder why everyone quotes a recommended print resolution of 150-300 ppi when their inkjet printer has a maximum resolution of 2880 dpi or even higher. The reason is a simple one. Inkjet printers use tiny dots of ink in three, four, six or more colours to reproduce all the colours necessary to achieve a vast range of colours using complex 'dithering' patterns.

These dots are incredibly small. For example at 2880 dpi, there are over eight million dots in each square inch of the print. However, more than one dot is required to represent a single pixel, which is why printer resolution is dots per inch and print resolution is pixels per inch.

Now that you know how it all works, you will know how to determine the maximum print size you can achieve without running the risk of producing "fuzzy" prints.

If you like being able to make very large prints, it will also help you to decide the Megapixel value of your next camera. So if you like to regularly produce 12 x 16 inch prints that look fabulous, you may decide to purchase an 8 Megapixel model.