One of the most confusing things for a new photographer is understanding image size, resolution, and printing.
So in this article, I’ll explain what these terms mean.
And I’ll show you how to resize your images depending on what you want to do with them.
Let’s get started.
What is resolution in digital cameras?
When talking about digital cameras, resolution refers to the number of megapixels produced by an image sensor.
This, in turn, generally corresponds to the amount of detail a camera can capture.
So if your camera packs 20 megapixels (often written as 20 MP), it captures less detail than a camera with 30 megapixels, which in turn captures less detail than a camera with 40 megapixels.
But what is a megapixel, really? And how does it affect your ability to print and display photos?
Megapixels and photo size
Technically, a megapixel is equal to 1,048,576 pixels; in reality, camera manufacturers round this number to 1,000,000 when stating how large of an image the camera will capture.
So my camera, for example, captures 14.6-megapixel images, which is around 14,600,000 pixels per image (14.6 x 1,000,000). This information tells you nothing about the actual pixel dimensions of the image – it only tells you the total number of pixels that make up the image.
My camera, like most DSLRs, captures images with an aspect ratio of 1.5. So the ratio comparing the number of pixels along the long edge of the image to the short edge of the image is 3:2.
Each of my full-sized RAW images is 4672 x 3104 pixels in dimension. So by multiplying the number of pixels along the image width by those along the image height, we get the actual number of pixels in the image (4672 x 3104 = 14,501,888). You and I might call this 14.5 MP, but camera manufacturers round this up and call it a 14.6 MP camera.
You can check the width and height of an image using your photo editing software. In Photoshop, you can open your image, then choose File > File Info > Camera Data. The image above shows the resulting information dialog box.
Now, a pixel itself is a single picture element – and for our purposes, it’s the smallest element that your photo can be divided up into. A pixel can only be one color, and a photograph is made up of a grid of thousands of pixels, each of the different colors that together make up your image.
You can see these pixels if you open a photo and zoom in until you see single blocks of color (as shown below). Each of these blocks is a pixel:
Why size is important when printing
When you’re printing an image, you may encounter the term PPI or pixels per inch. This literally refers to the number of pixels in an inch-long line of an image.
Most printing services, and indeed your own printer, will require a certain density of pixels in the image (PPI) to be able to render a print that looks good (i.e., with smooth color transitions so you can’t see each individual pixel).
Typical printing PPI values range from 150 to 300 PPI, although some high-end magazines may require images that are 1200 PPI.
So for example, if you want to print a 4 x 6 inch image at 300 PPI, then you need a file that has at least 4 x 300 (1200) pixels along its short side and 6 x 300 (1800) pixels on the long side. In other words, it needs to be at least 1200 x 1800 pixels in size.
To print an 8 x 10 inch image at 300 PPI, use the same math:
Multiply the printed image’s width and height in inches by 300 pixels. The result is 2,400 x 3,000 pixels, which is the image resolution you need to print an 8 x 10 image at 300 PPI.
Therefore, when cropping and sizing an image for printing, you’ll need to know what PPI the image should be. Your printer manual or printing service should be able to tell you this.
Below is a screenshot from the MpixPro.com website, showing their optimal and minimum image sizes for standard print sizes. Their printer outputs at 250 PPI (but can handle 100 PPI images), though other services may differ, so always check before preparing your images.
Use the crop or resize feature in your software to size your image to the desired width and height and the desired PPI resolution.
Here, an image cropped to a size of 3000 x 2400 pixels is being adjusted from 72 PPI to 300 PPI in preparation for printing at 300 PPI. There is no resampling required, as the image is already the correct dimensions and only the resolution requires adjusting.
Photoshop, like other applications, will also crop an image to a fixed size and resolution if you type your desired values into the options bar when you have the crop tool selected (see below). If your image is smaller than the typed dimensions, then the image will be enlarged using the default resampling method. While it isn’t generally advisable to enlarge images, provided the image is already close to the desired size, enlarging it a little generally won’t cause a noticeable loss of quality.
Sizing for the screen
When it comes to displaying images on the screen, you need far fewer pixels than you do for printing.
This is because the density of pixels on the screen is far less than what is required for printing. For example, a typical monitor is 1920 x 1080 pixels in size; to fill the monitor, you only need an image that is 1920 x 1080 pixels in size. That’s about the same size image you need for a 4 x 6 print at 300 PPI – yet the 1920 x 1080 pixel image displays perfectly on a 23-inch monitor.
