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Book Designer & Self Publishing coach

File Resolution


March 21, 2021

A case where size matters.

Often people ask me, “It looks good on my computer. Why are you telling me my file is too small?”  File resolution is one of the biggest problems I encounter when designing indy books. And it comes up most often with the author’s back cover photo. And for that, I blame Facebook. It seems that people naturally gravitate to their Facebook profile picture when they need a back cover photograph. There can be many problems with using your profile pic, but today I will focus on resolution.  The main problem is that the image resolution on a computer monitor is much lower than print resolution. And images off of Facebook are reduced to a computer monitor resolution. The original photo may or may not have been shot at an adequate resolution, but once it is uploaded to Facebook, that image is no longer suitable for print.

File resolution never used to be an issue because my clients would provide prints to scan, but now with everything digital, print resolution is a reoccurring problem. People will often give me images off Facebook, their website, or phone photos shot at low rez. These images may look terrific on the screen, but when printed, they look terrible.

What is file resolution?

Resolution determines the clarity of and image. The more detail in the photo, the clearer and crisper the image will reproduce. Resolution is measured in PPI pixels per inch in digital media and DPI dots per inch in print. Digital resolution for online images is typically pretty low, 72 PPI. In comparison, print resolution ranges from 85 dpi for some newspapers to 2400 DPI for high-end printers. 

For most printing, you will be safe to be at 300 DPI without noticeable degradation. But it is always good to ask your printer what DPI they will be using. You can always go higher, but it can make your files big and cumbersome to upload if you have loads of images. But when you go lower, it can affect print quality and sometimes dramatically.

Simply put, more pixels per inch or dots per inch = better resolution, fewer pixels per inch = lower resolution—higher resolution = better print quality. 

visual examples of file resolution

The image at the left shows a crisp, full-resolution image.
The image in the center shows a pixellated and fuzzy, low-resolution image.
The image on the right shows a low rez image with a photoshop noise filter added.

Typically most book printers use 300 dpi as their standard. So I aim to have my photographic images 300 dpi or higher at the size they will print. That means if I want the image to be 4″wide in my book, then my digital photograph needs to be 4″ wide at 300 DPI. If the art is line art solid black and white, such as a map or flow chart with no shading, I shoot for 600 dpi at the size it will print. I’ll explain my reasoning for this in another article.

So how do I know if my file resolution is good enough for print?

First, if you are working with a designer, you may just want to send them your files and ask how big they can reproduce.  But if you want to know before enrolling a designer, here are three ways you can tell.

One: By file size

If you know how many megabytes (MBs) the file is, you can get a general idea of how large the image can reproduce.

Below I am using my iPhone image size and proportions (4×3) to illustrate, but you can use these MB file sizes to get a general idea of how big your image can print. 

Off my phone, my resolution setting default is 4032px x 3024px at 34.9 MB.  Your default settings may be different, so if you are using your phone as a camera, I recommend using the highest setting available and test before you commit.

At my phone’s 4 x 3 image proportions:

    • a 34.9 MB image can print no larger than 13” x 10”;
    • a 6.95 MB image can print no larger than 6”x 4.5”;
    • a 3.09 MB image can print no larger than 4” x 3”;
    • a 1.74 MB image can print no larger than 3” x 2.25”. This image is pretty darn small;
    • if an image is under 1 MB, e.g., 700 K, it is likely, not suitable for print.

Two: By Image Dimension 

Most cameras record their images at 72 dpi, but they may still be big files. If your photo is at 72 dpi, but large they still may be suitable for print.

For instance:

    • An image that is 56” x 42” at 72 PPI can print 13” x 10” at 300dpi  (34.9 MB)
    • An image that is 13” x 10” at 72 PPI can print at  3.2” x 2.4” at 300dpi  (1.98 MB)

Three: By Resolution

If you know the pixel dimensions, you can calculate how large the image can reproduce in print by dividing the pixel width and height by 300.

To convert pixels to inches, divide the pixel dimensions by the resolution. The resolution for print is usually 300 dpi.

So for example a 4032px x 3024px image ÷ 300 = 13.44” x 10.08”

What can you do if your image file resolution is too low?

Unfortunately, there is not much you can do. The data is just not there. Even if your software allows it, you cannot boost resolution by digitally adding more dots per inch—the information is not there. Multiplying anything by zero gets you nowhere.

But there are a couple of workarounds if you have a low rez photograph you want to print. You may be able to get a pretty clean photographic print of the image at a drugstore photo kiosk or maybe even a desktop inkjet printer on photo paper. I have found that photo prints can look crisper even at a low resolution.  Then you can scan that print at a higher resolution. It won’t be perfect, but the distortion will be less, and if printed in black and white, it may not be noticeable at all. 

I have also increased the dpi and used noise filters on low-resolution images. The noise filters can create a uniform distortion and can look more like grainy photos rather than distorted. They can work well as background images on cover designs or section dividers. The filters can minimize the appearance of computer distortion and make your image look more illustrative (see example above).

I know all these numbers can be confusing, but basically, if your image is under 2 MB, it is probably too small, and if you want your image to fill the cover, you will likely need an image that is 10 MB or higher. Check out my article Effective Book Covers for more. And if you have any doubts, send your image files to your designer, and they can tell you how big your images can print without distortion.

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