I was talking to an author the other day about photos he wanted to put in his book. I looked at the image files he had sent over with his manuscript.
“They look great,” the author told me, “I’ve put them into the book in Microsoft Word, I have it on the screen in front of me, it looks terrific.”
“Uhm, I don’t think these are going to work in your book,” I said.
“What do you mean, they seem fine to me? What’s wrong with them?”
What the author couldn’t see was that I hadn’t even opened the image files. I didn’t need to, all I had to do was look at the size of the files in the email attachment panel. One was 4k, the others were 8k or 10K or even 24k in size.
“Look, I explained, “there’s a big difference in what we use online, when you’re only going to be looking at them on a screen, and what we need when we go to print these same photos. I know it doesn’t seem to make sense, but the images you’re looking at on your screen are 72 dots-per-inch (dpi). We need files that are 300 dpi. And if they aren’t 300 dpi, there’s a good chance the printer will reject your job.”
Using File Size to Quickly Gauge Reproduction Size
I want to show you how to calculate this for yourself, even if you just want to be able to tell from the files you’re looking at if they are likely to work.
Okay, we’re going to do a little math now, but I promise it won’t hurt (too much).
Let’s look at the typical sizes of photographs or other grayscale images you might use in a typical book. I’ve taken a page from a 6″ x 9″ book and divided the space within the margins this way:
- A full page illustration can be as large as 4.25″ x 7.6″ without going into the margins or bleeding off the page.
- A half page is 4.25″ x 3.8″
- A quarter page is 2.2″ x 3.8″
Now, taking what we know—that graphics have to be 300 dots per inch at their reproduction size let’s do some calculating:
- Our full-page illustration will need to have:
4.25″ wide x 300 dots per inch = 1275 dots, or pixels
7.6″ tall x 300 dots per inch = 2280 pixels
The photo that’s 1275 x 2280 will have a total of 2,907,000 pixels. That’s about 3 MB of data.
- Our half-page illustration:
4.25″ wide x 300 dpi = 1275 pixels
3.8″ tall x 300 dpi = 1140 pixels
So the photo will be 1275 x 1140 or a total of 1,453,500 pixels, or about 1.5 MB.
- Our quarter-page illustration:
2.2″ wide x 300 dpi = 660 pixels
3.8″ tall x 300 dpi = 1140 pixels
The photo will be 660 x 1140 or a total of 753,060, or about 800 KB.
Image Compression Makes a Difference
But wait. Most photos are saved as JPG files. Leaving aside for a moment whether this is a good idea (it’s not a good idea if you plan to keep editing the images). One of the reasons we use JPG online is for its ability to compress the files into much smaller sizes.
But it’s good to know what’s actually in the files. Let’s say we’ve saved each of our illustrations in both TIFF format with no compression, and JPG format with the compression set for maximum image quality. Here’s what our final file sizes look like, as reported by the Mac operating system:
- Full page, TIFF: 2,989,336 bytes
Full page, JPG: 841,578 bytes
- Half page, TIFF: 1,560,237 bytes
Half page, JPG: 537,242 bytes
- Quarter page, TIFF: 844,607 bytes
Quarter page, JPG: 351,015 bytes
And now you know why I knew many of this author’s photos were too small (and probably screen grabs at 72 dpi, way smaller than what’s needed for offset printing).
Even for a quarter page photo, saved with compression, the file size ought to be around 350 K. Obviously, photo files showing a file size of 50 K or 100 K will not be usable for a printed book.
By the way, this is another reason it makes sense to create the layouts and artwork needed for print production for your book before addressing ebook versions. You need the high resolution images for print. While it’s easy to reduce the size and resolution of your images, it’s not possible to add detail and data (or resolution) back into the image once it’s gone.
Takeaway: When planning to add photos to your book, make sure you have files with enough resolution to print at the size you want in the finished product.
Image licensed under a Creative Commons Attribution 3.0 License, original work copyright by See Ming Lee, http://www.flickr.com/photos/seeminglee/