in Production
Image resolution for print
Whether an image will print sharply depends on its pixels and its printed size, not on a number in its file settings. Here is how to check.
A printed book needs images with enough detail to look sharp at the size they appear. With too little, they print soft, blocky or jagged. The usual rule is simple, but it is often misunderstood.
Pixels and printed size
A digital image is a grid of pixels. What matters for print is how many pixels there are and how large the image will be printed. Resolution, measured in pixels per inch, or ppi, is the relationship between the two.
The same image can have a high or low resolution depending on how it is used. An image 1,800 pixels wide, printed 6 inches wide, has 300 ppi. Printed 12 inches wide, the same image has 150 ppi.
The ppi value stored in an image file, often 72 for images made for screens, is only a label. It does not change the pixels. What counts is the pixel dimensions divided by the printed size.
The usual target
For colour and greyscale images, the common target is around 300 ppi at the final printed size. It is the figure most printers ask for, and at normal reading distance it gives a result that looks sharp.
Somewhat less can be acceptable for some images, such as soft-focus photographs, background textures or large images seen from a distance. Much less usually shows. Printers and preflight tools often flag images below a threshold, and the printer's specification says what theirs is.
Line art, such as black-and-white drawings with no grey tones, is a different case when it is saved as a pure one-bit image. It needs a much higher resolution, often 1,200 ppi, to keep its edges smooth. Saved as greyscale, it can be treated like other greyscale images.
Enlarging does not add detail
An image that is too small cannot be fixed by increasing its size in image-editing software. Resampling creates new pixels by estimating from the existing ones, and the result is a larger file with no more real detail. The ppi figure goes up; the sharpness does not.
Some upscaling tools produce more convincing results, but they generate plausible detail rather than recovering real detail, and the output needs careful checking at full size. The more reliable fix is to find a larger original.
The usual culprits are images downloaded from websites, images copied out of documents or emails, screenshots, and images that have been heavily cropped. Cropping discards pixels, so an image that was large enough as a whole may not be large enough once only part of it is used.
Images found online also raise questions of rights. Being visible on a website does not mean an image can be printed in a book.
Checking before sending
Layout software usually shows the effective resolution of each placed image, its ppi at the size it is used, in the links or preflight panel. Check every image at its final size, including any scaling applied in the layout.
For covers, include the bleed in the calculation. An image running to the edge has to cover the trim size plus bleed, not just the trim size.
A useful check is to print the page, or part of it, at actual size on an ordinary office printer. Softness that is easy to miss on a screen is often obvious on paper.