One of the most confusing things for a new photographer is understanding image size, resolution, and printing. I’ll try and explain what these things mean, and how to make the best choices depending on what you want to do with your photos.
Megapixels and photo size
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As a photographer you will already have confronted the term megapixel when you first purchased your camera. While 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 an image the camera will capture.
So, my camera, for example, is stated to capture 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 comprise 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 is 3:2. Each full size raw image is 4672 x 3104 pixels in dimension. So, by multiplying the number of pixels along the width by those of the height (4672 x 3104 = 14,501,888) we get the actual number of pixels in the image. You and I might call this 14.5 MP but the camera manufacturer rounds this up and calls it a 14.6 MP camera.
You can check the width and height of an image using your photo editing software. In Photoshop, with the image open, choose File > File Info > Camera Data. The image above shows this information dialogue box.
A pixel itself is a single picture element, and for our purposes it’s the smallest element that your photo can be divided into. A pixel can be only one color, and a photograph is made up of a grid of thousands of pixels, each of varying colors that together make up your image. You can see these pixels if you open a photo inside a photo editing program and zoom in until you see single blocks of color (like below). Each of these 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. Most printing services, and indeed your own printer, will require a certain density of pixels in the image (ppi) to be able to render an print that looks good, 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 which are 1200 ppi.
So, for example, if you want to print an image 4 x 6 inches 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. So, it needs to be at least 1200 x 1800 pixels in size.
To print an 8 x 10 inches at 300 ppi use the same math – multiply the printed image width and height in inches each by 300 pixels. The result is 2,400 x 3,000 pixels, which is the size image you need to print an 8 x 10 at 300 ppi.
When cropping and sizing an image for printing, you’ll need to know what ppi the image should be – your printer manual or the printing service should be able to tell you this. This 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) – other services may differ – so always check before preparing your images:
Use the crop or resize feature in your software to size the image to the desired width and height, and the 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 dimension and only the resolution requires adjusting.
Photoshop, like other applications, will also crop an image to a fixed size and resolution if you type these value into the tool 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 as it is cropped. While it isn’t generally advisable to enlarge images – provided the image is already close in size to the desired size, enlarging it a little bit generally won’t cause a noticeable loss of quality.
Sizing for screen
When it comes to displaying images on the screen you need far less 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. So, for example a typical monitor is 1920 by 1080 pixels in size so, to fill the monitor you only need an image that is 1920 by 1080 pixels in size. That’s about the same size image you need for a 4 x 6 print at 300 ppi, yet this size image displays perfectly on a 23 inch diagonal monitor.
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