Image Resolution: Inches, Pixels, and dpi

31 Mar

When adding images to your book (or to anything you plan to print), you want to make sure you have the largest, highest resolution files possible. Talking about resolution can get confusing pretty fast—so I’ll try to break it down to the basics.

Especially if you’re searching online for an image, it’s important to check the dpi or the dimensions in pixels if they are noted. To get the best quality, you’ll want to make sure the images you use are at least 300dpi (dots per inch) at the finished size.

However, if you can’t figure out the dpi or can’t get a version of the image that is at least 300dpi, you’re not out of options. While we recommend 300dpi, it’s often true that 200dpi is good enough.

When previewing an image, zoom in to about 200% of the image’s size. This will give you an idea of how it may look when printed. If you’re OK with the way it looks at 200%, chances are you’ll be happy with the way it prints. (Though we still recommend printing out a proof copy before printing the full run of a book, just to double check!)

If you find yourself stretching or enlarging an image a considerable amount, chances are it’s not going to print well.

You can find out what resolution your image is in most photo editing programs like Adobe Photoshop. However, you can also find it easily by (on a PC) right-clicking on the file and selecting Properties.

The properties window will open. Select the Details tab along the top of the window.

image
Here’s where you’ll find all of the info you’ll need about the image, including the dimensions and the dpi.

image
On a Mac, you can right-click on the file and select Get Info.

image
A window will pop up with information about the image including its dimensions in pixels.

image
This image is only 72 dpi, but I still really want to use it. Luckily I can change the dpi by changing the size of the image. This is when I’ll take a look at the pixels.

The image is 240 x 135 pixels (I found this information in the Properties/Get Info boxes above).

The width in pixels divided by the desired resolution will give me the maximum width in inches before the image becomes low resolution. (Pixels/desired dpi=maximum size in inches).

So, I take my 240 pixel width and divide this by the 200dpi quality I’d like. This gives me 1.2 inches. Basically, this means my photo will be blurry or low resolution if it’s larger than 1.2 inches. At 1.2 inches wide, it’s high-res and ready to print.

image

I know what you’re thinking—this is way too small! No worries.

If you know that you need an image to be a specific size (for example, 5 inches wide), you can find out what resolution it will be using a similar formula. The width in pixels divided by the desired width in inches will give you the resolution (pixels/inches=dpi).

So, I take my 240 pixel width and divide this by the 5 inch width I want. This gives me a dpi of 48, which is even worse than the original 72 dpi. This means if I’m enlarging the image; it will look pretty blurry printed and even looks blurry on screen:

image

If you have access to a larger or higher resolution version of an image, you have a better chance of getting it to look the way you want. Luckily, I found a version of the same image that is 1024×576 pixels. This could work.

1024 pixels divided by 200 dpi (the lowest dpi I can go before I risk losing quality) = 5.12 inches. This means I can make my image 5.12 inches wide or smaller and it will (most likely) look good when printed.
image


I’m much happier with the way this image looks, even though it’s still not the recommended 300dpi.

If I wanted to ensure beyond all doubt that the picture would print perfectly without having to re-size it, I’d need to find a version of the image that was at least 1500 pixels wide. (If you’re catching on, you’ll know that I found this by multiplying my 5 inch width by my desired 300dpi resolution!)

NOTE: even though it seems like it may work, going into Photoshop and simply changing the dpi to 300 will NOT improve the way your photo prints. If only it were that easy!

For those of you who (like me) are not mathematically inclined… here are those formulas again:

Pixels/inches=dpi

Inches x dpi=pixels needed

Pixels/dpi=maximum dimension in inches

Crunching the numbers and trying to figure out image resolution can be a real headache, but it’s an important step in getting your book to look the way you want. High resolution images make your book look much more professional, and it’s worth the extra time and effort to get a great looking final product!

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One Response to “Image Resolution: Inches, Pixels, and dpi”

Trackbacks/Pingbacks

  1. Book Cover Design Dos and Don’ts « 48Hr Books - April 28, 2011

    […] won’t print well. If you want to learn about image resolution, I go into more detail on that here. Likewise, make sure your text doesn’t look blurry or […]

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