I often get asked this question and to answer it, we need to delve a little deeper into the terms used to describe digital images. Here are a few of the main ones -
dpi - dots per inch. This is how the resolution is measured when you print your image at home. It is also used to gauge the detail needed in order for the picture to be clear. True to the name, your printer prints ‘dots’ to form the image.
Pixel - the smallest controllable element of a picture represented on the screen (Wikipedia)
ppi - pixels per inch - the number of pixels in a square inch
Megapixel: one million pixels. This is how the resolution of digital cameras is measured. The larger the megapixel rating, the more information you can capture within an image and the larger you can print the high quality image.
I have deliberately left out a whole host of other terms that we really aren’t interested in for this purpose!
For photos to look good on a screen, the resolution of the photos must be no less than 72 pixels per inch or ‘ppi’. These images are easy to share, download and you can store a huge number on a modest memory card.
Just try to print a photo on paper from the internet and you will quickly discover that 72 ppi is not enough to create a quality print. For that, we need to have 300ppi.
Higher resolution cameras not only allow us to print larger quality images but also make it possible to crop large areas of an image and still retain a large enough image file to print a high quality picture.
Now to the question...stay with me here...this is where we need a bit of thought.
Let’s say I want to print a 16x24 print - what resolution do I need in order to achieve a quality print. On the short edge, we need 16 inches and we already know that we need 300 pixels per inch.
So 16x300=4800 pixels. On the long side it’s 24x300=7200.
For the whole image in total it’s 4,800x7,200 = 30,240,000 or about 30 megapixels.
Another example I hear you say :-)
A 5 x 7 print would need 5 x 300 x 7 x 300 = 3,150,000 pixels or approximately 3 megapixels
Below is a chart that outlines inches on the x and y axis and how many megapixels are needs to print the size with 300ppi. Click on the chart for more information.
Hope this helps a bit...as always, do get in touch if you have any questions or would like to know more.