Digital photography printing - understanding resolution

To get good results in digital photography printing it helps to know how pictures are made up. Every digital image is made up of individual dots called pixels.

This is the same process as many printed images, televisions and even the monitor you are looking at right now. Look closely at that monitor screen, closer, closer still . . . you’ll see the screen is made up of lots of tiny dots.

As you move away from the screen your eye can no longer see the individual dots. They blend together to form a complete picture. Think of each dot on your computer monitor as one pixel.

Click to jump to the table showing how many megapixels are needed for good quality digital photography printing.

NB: For the purists, dots are associated with actual printing of pictures. Pixels are the actual digital data recorded in an electronic image.

On this page I explain how resolution works. It’s fairly straightforward and I’ll be as jargon free as I can. It sounds boring but a basic understanding will help you improve your digital photography printing, I promise! is updated regularly. To learn out about the updates as they happen subscribe to the digital photography blog, and this site’s RSS feed.

. . . ok, back to your computer screen . . .

The resolution of any image is a way of saying how many pixels go across, and how many pixels go from top to bottom. If you were to count the dots (or “pixels” as we’re now calling them!) from left to right on your monitor and find there are 1024 of them, then you count how many are up and down and you find there are 768 pixels from top to bottom.

Now for a little bit of maths. The resolution of your screen is:
number of horizontal pixels x vertical pixels

in our example above it is:

1024 x 768 = 786,432

- This is then divided by 1 million (makes 0.786432)
     - rounded up (makes 0.8)
          - and expressed as megapixels (makes 0.8 megapixels)!

So that's megapixels, but what's "ppi" all about?

Resolution is often expressed in another way – by stating how many pixels there are per inch (or "ppi"). This is important in digital photography printing because it determines print quality. The more pixels you can cram into every inch, the better the print will be.

Lets go back to our computer monitor. If your monitor has 1024 pixels across, and the width of the screen is 12 inches (that’s a standard 15” monitor – 15” is the diagonal screen size). The maths:

1024 ÷ 12 = 85ppi

The more ppi you have, the smaller the dots are. Lets say we make each pixel on our screen half size. We can then fit twice as many of them into the same space:
2048 ÷ 12 = 170ppi

The picture on the screen would look much sharper and you would have to get a lot closer to your monitor screen before you could make them out individually.

NB: this is what High Definition TV does
– smaller pixels, more of them = clearer picture.

How many ppi is enough?

The holy grail for good digital photography printing is 300ppi. With 300 pixels crammed into every inch of your image the naked eye cannot make out the individual pixels. The image appears beautifully crisp!

Suppose you’re getting into digital photography and buy a digital camera with 0.8 megapixels. It would be perfect for taking pictures to view on your monitor. But what about printing . . . hmmm sadly not enough ppi. The biggest print you would be able to produce would be (maths again - last time, I promise!):

1024 ÷ 300ppi = 3.4”

So our print would be 3.4 inches wide. Not very big, is it?

My recommendation for the absolute minimum number of pixels needed for a good digital photography print is 2 megapixels. These cameras have sensors with around 1600 pixels across. They can produce reasonably good quality digital photography printing up to six inches wide – that’s a standard snapshot print size.

The table below shows how many pixels you will need for good quality digital photography printing (for information on actually buying a digital camera read my digital camera comparison guide):

Megapixels: (MP) Quality prints up to this size:
minimum of 2MP 6” x 4”
minimum of 3MP 7” x 5”
minimum of 6MP 10” x 8”
minimum of 8MP 12” x 10”

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NB: Prints are determined by the number of “Dots Per Inch”, or “DPI”. Printers use small dots of ink to make their pictures up. The difference between dots and pixels is meaningless. For now just consider a “dot” to be the same as a “pixel”.

Before you get really stuck into digital photography printing, click to find out more on preparing images in Photoshop to ensure good quality digital photography printing results.

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