Ask a Photo Nerd: What’s the Deal with Image Size and Resolution?

Ask a Photo Nerd: What’s the Deal with Image Size and Resolution?

There is one way, and only one way, to specify the size of an image. That is: the number of pixels that make up that image. How many pixels wide it is by how many pixels high.

Learn how to save images at the right size and resolution for print or the web. We've got the scoop on selecting image size for optimum display quality.

Zoom in enough and an image become a collection of squares: those squares are pixels.

Now, you might be inclined to ask, “What about megabytes, Nerd?” To which I’d reply, “Yes! Yes, you are correct. Megabytes are a way to measure the ‘size’ of the image file”. An answer that would immediately be followed by, “NOW GET OUT!” in my very best Maya Rudolph as Donatella Versace voice.

We’re talking about how big your digital image needs to be to make a print or display it on the web.

No matter what you are planning for your image, it’s fairly important to save your photo at the resolution you intend to display it. Let’s start off with saving images for print.

How to create the right size image for printing

Unless you are printing huge (a poster, tradeshow banner, or billboard) you will almost certainly be printing on a 300 DPI (dots per inch) printer, simply because this is the resolution of most printers.

300 DPI means the printer is going to spit out 300 little dots of ink every inch, on every row of your photograph. It is recommended that your image file contains the same number of pixels per inch that your printer is going to be printing per inch.

When somebody asks you for a “300 DPI file,” what they are really asking for is enough pixels to print that image on a 300 DPI printer.

Getting a “300 DPI” file

Since we know the printer is going to print at 300 dots per inch, all we need to do is figure out how many pixels to give the printer. That number is achieved by multiplying 300 by the number of inches the print is to be.

For a 4 x 6 photographic quality print, on a 300 DPI printer, you want 1200 x 1800 pixels.

Secretly, too much figurin’ gives me the math-sweats. If you suffer a similar plight, perhaps this chart will save you some frantic squirting of stain remover on the pits of your favorite shirt.

Learn how to save images at the right size and resolution for print or the web. We've got the scoop on selecting image size for optimum display quality.

Size vs Shape

Your image needs to be the same aspect ratio (shape) as the paper you are printing on, otherwise things get either distorted or cut off. If your image is already the same shape as your paper, you can make this adjustment with the Resize tool.

More often your image is a different shape than the print you plan on making. In that case, you’ll be looking for the Crop tool, rather than Resize.

Displaying images on a screen

Monitors and screens use pixels much more sparingly than printers. Where a printer gobbles up 300 of your hard-earned pixels per inch, the miserly monitor might only use as few as 72.

That means that a 2400 x 3000 pixel image used to create an 8 x 10 print would display at about 2 ¾ feet (yes, we’re talking feet here, folks, not inches) on a 72 PPI monitor.

Sizing for the web would be pretty easy if all screens were 72 PPI. Trouble is, they aren’t; not by a long shot. They used to be, and 72 ppi is still used as a reference for designers, but most modern PC monitors are 96 PPI. Retina displays can be around 220 PPI. This complicates selecting an image size for the web.

You may have noticed when talking about the resolution of printers the term DPI was used, and now when talking about monitors we’ve switched to PPI (pixels per inch). Good eyes! …and yes, that’s right. Monitors don’t deal in dots of ink, they have pixels that light up to create your image.

Sizing images for sharing on the web

When saving images for a website or blog, it is often the existing site layout that will dictate how many pixels high and wide you need to size your image.

On our blog, the images within the posts are around 600 pixels wide. Header images that span the entire page might be larger, in the neighborhood of 1200 pixels, which leaves it up to the browser to stretch it if the browser window is larger.

If you over-accommodate large displays, you’ll end up serving images that slow your site down. It’s a balancing act. You want the smallest file that works for your purpose.

When not matching an image to a particular layout, you can be fairly loose with the size. What I consider a “high resolution” share on the internet would be in the neighborhood of 1200 – 1600 pixels wide.

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Anthony Procissi
Photo Nerd Anthony Procissi is a product manager at PicMonkey and a self-appointed photography know-it-all with a penchant for speaking in laymen's terms. He lives in Seattle with his beloved wife and adored daughter.