Image resolution is one of the most important items on any designer’s checklist as we prepare files for print or web. Raster images, also known as continuous tone or photographs, are composed of tiny squares (pixels) that are distributed across a given amount of space. Resolution is the number of pixels per inch (PPI). The term “PPI” is often confused with “DPI” (dots per inch). But it is “PPI” that quantifies an image’s resolution, while “DPI” refers to the resolution output of a printer, such as inkjet, laser, and high-end devices.
Raster images are resolution-dependent, meaning their output quality depends on their resolution. Vector images are “resolution-independent” because they are composed of connected lines and curves, which can be scaled up or down without losing quality. So, with raster images, you must pay attention to the resolution and how it changes when you scale it up or down in size. The actual resolution is what exists in the image file before any scaling is applied. The effective resolution is the resulting PPI after scaling is applied; divide the actual resolution by the scale to determine effective resolution.
In the print world, 300 PPI at 100% of image size is the ideal specification. On the web, 72 PPI (Mac) or 96 PPI (PC) at 100% is best. Without any scaling, the actual resolution equals the effective resolution. Here is an image with 300 PPI at 100% size:
A print vendor should be able to provide you with their “tolerance” level of scaling. Usually, you can scale up to approximately 120% of image size and retain very good quality for reproduction. That would result in 250 PPI of EFFECTIVE RESOLUTION (300 / 120% = 250). Here is the same image enlarged 120% (there are no noticeable differences):
If you scaled this same image up even more, to 200%, the effective resolution would be 150 PPI (300 / 200% = 150), and you’ll see degradation of quality:
Sometimes it’s possible to improve resolution by reducing the size of the placed image. Let’s say the actual resolution is 72 PPI and the physical size of the file is 11 inches. If the placed image is used at 50%, the effective resolution will increase to 144 PPI. If it’s used at 25%, the effective resolution increases to 288 PPI.
Ways to Determine the Effective Resolution
Aside from calculating effective resolution manually, there are 2 other simple options:
Open the LINKS window and select an image to see the actual vs. effective resolution. In the sample below, the actual PPI = 300. Since the placed image is reduced 42.4%, the effective PPI = 707, which is very good for print. I just have to change the color space from RGB to CMYK for this image to be “print-ready”.
Under the IMAGE menu, scroll down to IMAGE SIZE (CMMD-OPTION-i/Mac; CNTRL-OPTION-i/PC). In the pop-up window that appears, de-select RESAMPLE IMAGE, then change the Document Size by using Percent. The resolution changes from actual to effective.
Notes About Image Format and Digital Cameras
It’s a good idea to avoid placing JPGs into files that will be printed. JPGs are a “lossy” format, and thus lose data each time they are saved. Repeatedly re-saving a JPG will result in more and more data being lost. Recommended formats for printed images are TIFF, PSD, PDF, and EPS. For details, see my blog article Image File Formats: print vs. web.
The default setting on digital cameras tends to be 72 PPI. If you use digital photos from your camera for printing, you need to use the highest pixel count setting your camera offers. The camera will be able to store fewer images, but they will be of higher quality.
Resolution and Text
When setting text in Photoshop, many printers advise that you use 400 PPI. The type will be vector in PSD and EPS files, but gets rasterized once it’s in the production process at the print vendor. The 400 PPI will ensure that the text stays crisp.
Print images should be 300 PPI in the size that most closely matches your placed image size. Increasing the resolution via resampling in Photoshop won’t work: it merely makes the low-resolution image look like a bad high-resolution one. Scaling the size down will increase the effective resolution, but make sure it’s enough to improve the quality of the printed image.