by Joseph T. Sinclair
To start a review of images to be used in Android and Apple book apps, you need to understand certain basic information.
- Android highest screen resolution is 2560 pixels x 1600 pixels.
- iOS highest resolution (Retina) is 2048 x 1530.
- Mobile device screen resolutions range from 240 x 320 all the way to 2560 x 1600.
- Commercial color printing is 240 dots per inch (dpi).
- Ultra high quality color printing is 300 dpi.
- Dpi can be considered the same as pixels per inch (ppi), pixel density.
- The latest mobile devices have pixel density from 264 ppi to 330 ppi, but legacy devices have pixel density as low as 130 ppi.
- JPEG images are compressed with a loss of quality. The resulting quality is rated on a scale of 1-12 with 8 being the highest without a significant loss of quality.
- A 1600 x 1000 JPEG image (8 on the JPEG scale) is an image file in the range of 250K to 750K. It depends on what’s in the image.
- Apple allows 60MB per app and Android allows 50MB per app.
There are certain realities you must keep in mind:
- If an image is too small to span the width of the screen, a mobile device may display it at its true size but most likely will enlarge it to display larger, or even full width.
- If an image is larger than the width of the screen, a device will decrease its size to fit on the width of the screen.
- If adjustment is necessary, a device adjusts an image to the width of its screen, not its height.
- Decreasing image size doesn’t diminish image quality much. Enlarging the size creates degradation (lesser quality), something you want to avoid as much as possible.
- Mobile devices vary in size, screen resolution, pixel density, and processing capability.
All of the above directly affect how a device displays an image. In addition, there are other variables that affect how an image is seen in a digital book.
- Device operating system (OS)
- Graphics processing
- Browser software
- App programming (which can change anything and everything)
- Distance of a user’s eyes from the screen
- In some software a tap (click) on an image will open it in a new full-screen window at it’s true size, or more likely a size that fits the width of the new window.
Can you satisfy all these variables? No. You can only do something that maximizes your chances of an image appearing in high quality on most devices.
To continue this analysis, the basic quality considered will be 1600 x 2560 resolution and a JPEG score of 8. This Android resolution is currently the highest. And a JPEG with a rating of 8 is high quality for digital color.
To determine what size images to use in creating a book app, you have to make certain assumptions. The first assumption for a digital book is that users will read it in the portrait (vertical) mode. At the highest resolution (Android), that means the screen is 1600 x 2560 pixels (vertical).
The width determines the size of the image you can put in the vertical page. Because the width of the vertical screen at the highest resolution currently available is 1600, a 1600-wide image will fill the width of the page in the vertical mode without any enlargement. A device will reduce the image’s size for lesser screen resolutions.
Note that in landscape (horizontal) mode, the 1600-wide image will not fill the screen (2560 wide), but you ignore the horizontal mode because it’s not appropriate for reading a normal book. The exception to this observation is reading on a phone. Some users will read with the phone in the horizontal mode. However, images are so small physically on a phone that a moderate degradation in image quality is not likely to be noticeable. Another exception is when a user turns the device to the horizontal mode just to see the image display larger. A 1600-wide image will be noticeably lower in quality than a 2560-wide image. If the highest quality is important for your book and you never want to have any degradation in image quality, then you need to make all your images at least 2560 wide.
How high can the image be? In the vertical mode an image 1600 wide can be 2560 high and still be fully visible on the screen in a high-resolution device without enlargement.
Thus, 1600 x 2560 (a vertical image) is the highest quality for the vertical mode (portrait book mode).
In the vertical mode all images less than 1600 wide will be displayed at their true size, or more likely enlarged by the device to be larger, often as large as full-width.
Some aspect ratios and usable resolutions for images:
- HDTV 16:9 – 1600 x 900
- Camera 4:3 – 1600 x 1200
- Camera 3:2 – 1600 x 1066
Let’s say you want to display multiple 1600 x 900 images, which fall in the file-size range of 250K to 750K each. These are large image files. Suppose the text in your book is 1MB. Android allows a 50-MB app. iOS allows a 60-MB app. So the practical cross-platform limit is 50 MB. If you subtract 1MB for text, you have 49MB left for 1600 x 900 images at an average of 500K each. That means you can include about 98 images, assuming you use no additional media in your book app.
Unfortunately, using images at the maximum size will limit the number of images you can use due to the app limit of 50 MB. If you go with smaller images at a lower resolution, however, a mobile device will likely enlarge the size of many of your images with the grave likelihood of loss of quality.
But let’s put things in perspective. If you’re using images that don’t require high definition such as charts, graphs, cartoons, some drawings (not all), etc, you can use such images in a low resolution without a noticeable loss of quality when a device enlarges their size.
Photos are different though. They require high definition; that is, high resolution. With the above information in mind for digital books, I recommend that you keep your photos (and other high definition images) to 1600 wide.
Unfortunately, it isn’t enough to just follow a standard specifications that you set for your work. You need to experiment. You need to experiment with phones, tablets, and other digital devices that a significant percentage of your customers will use to view your digital book. That means trying out your digital book on both low-resolution and high-resolution phones, 7-inch tablets, 8-inch tablets, and 10-inch tablets. If the book is to be read on computer monitors, you need to test a selection of monitors too.
And you have to do all this for Android, iOS, Microsoft, EPUB, and MOBI in regard to mobile devices. In regard to computers, you need to test your monitors on Windows and the Apple OS.
A daunting task to say the least! Obviously you’re not going to do all that testing, but you do need to pick and chose the testing that will cover most of the devices that your customers use.
Keep in mind that this article is generic in that it generalizes in regard to all software that runs on phone and tablet devices. In creating a book app or ebook, you will use specific software. You need to analyze exactly how that software handles images. If it handles them in such a way that you can use images of smaller resolution without the software automatically enlarging them, by all means do so. Less memory devoted to images means more memory that you can use for other media or simply to keep download times lower.
Indeed, you have enough data in this article to help you make an analysis of any software you might use. The major point to be made is that if you don’t pay attention to image size, you are likely to suffer consequences that you don’t want. If the images you use are larger than necessary, they will take up more memory than necessary. If the images you use are smaller than necessary, the automatic processes that such software uses uses to handle images may enlarge the images causing lower image quality than you might deem acceptable.
Summary For the highest quality image for a device with the highest screen resolution (2560 x 1600), the image must be 2560 wide. This will prevent the device from enlarging the size of the image with a corresponding loss of quality. For a digital book, however, a user will read the book in portrait (vertical) mode, that is, 1600 x 2560. Therefore, you may be able to get by with an image only 1600 wide so long as you are willing to ignore the fact that some users will turn the device to the landscape (horizontal) mode to see the image as it displays larger.
The author of this article, Joseph T. Sinclair, is the author of twenty How To books published by national publishers.
For low-cost non-exclusive reprints rights for this article, contact sales@AuthorsAndPublishersDigitalReview.
©2014 Joseph T. Sinclair. All rights reserved.