Have you ever had a print job done and was disappointed with the quality of the images? This could have been the result of low image resolution of the photos you provided. Using low resolution images is one of the most common errors designers make when creating designs for print. But first, let us clarify some of the terminology before we explain further.
What is image resolution?
Image Resolution is the fineness or detail in a bitmap image and is measured in pixels per inch (ppi). The more pixels per inch, the greater the resolution. Generally, an image with a higher resolution produces a better quality printed image. A lower resolution image will be fuzzy, and less detailed (see illustrations below).
300 ppi image at 100% size
300 ppi image at 300% size
150 ppi image at 100% size
150 ppi image at 300% size
72 ppi image at 100% size
72 ppi image at 300% sizeWhat is printer resolution?
Printer resolution is measured in dots per inch (dpi) and refers to the smallest size dot a printing device can print. Generally, the more dots per inch, the finer the printed output you’ll get. However, image resolution (ppi) and screen frequency (lpi) generally affects the quality of the print project more than printer resolution (dpi) does. Most consumer grade laser and inkjet printers print at 300 dpi to 1,200 dpi. High-end inkjet, laser printers and imagesetters print at 1,200 to 2,400 dpi. Printer resolution is different from, but related to image resolution.
What is screen frequency or line screen?
Screen frequency (also known as screen ruling or line screen) is the number of printer dots or halftone dots per inch used to print images. Screen frequency is measured in lines per inch (lpi.) The higher the resolution of the output device (printer), the higher (finer) screen frequency you can use. A typical 300 dpi laser printer will produce halftones at about 50 lpi. Newspapers print about 85 lpi and most magazines print runs at 133 lpi to 150 lpi.
Halftone image at 50 lpi
Halftone image at 85 lpi
Halftone image at 133 lpi
Continuous tone imageOffset and digital printing
For offset printing and digital printing, the general rule of thumb is to have halftone images with a resolution of roughly 2 times the line screen (also known as the halftoning factor).
image resolution = image line screen x 2
If you are printing at 150 lpi, you will need an image that is 300 ppi (150 lpi x 2 = 300 ppi). For printing at 200 lpi, you will need an image that is 400 ppi (200 lpi x 2 = 400 ppi). Normally, type should not be embedded inside non-vector graphics but when this is unavoidable, the image resolution should be set at or above 600 ppi. Line art should be set at 1,200 ppi.
If you are going to scale your photos, keep in mind that they will lose quality as you enlarge them. Image resolution is inversely proportional to size. A 300 ppi image enlarged by 200% becomes 150 ppi. So the same formula can be expanded to:
image resolution = (image line screen x 2) x magnification
So, if you are printing at 150 lpi and you are enlarging the image 200%, you will need a graphic that is 600 ppi ([150 lpi x 2] x 2 = 600 ppi). If you enlarge the same image by 150%, you will need a graphic that is 450 ppi ([150 lpi x 2] x 1.5 = 450 ppi).
As a general rule, make sure that the photos you use are at least 300 ppi at 100% of the final size for general print services. Offset presses at higher screen frequencies require higher resolution bitmapped images.
Large format printing
For large format print marketing materials like banners, posters, window graphics etc. that are meant to be viewed at more than an arm’s length away, we don’t quite need the same image resolution. You should be able to get great results as long as the images are at least 150 ppi at 100% size. Typically, the further the viewing distance is, the lower the image resolution needs to be. A billboard is typically printed at 10 ppi to 30 ppi. Bear in mind though that image quality is not just measured by image resolution – exposure, sharpness, noise, dynamic range, contrast, colour accuracy, etc. also play a part in creating high qualityprinted materials.
Help! My image resolution is too low!
If you try to use an image from a website (typically saved as 72 ppi) for printing, you will be vastly disappointed with the results. You will need to get the original image (before it was downsized and compressed for the web) and resize it to the right size with the necessary image resolution. Should you up-sample the image (increase the resolution of the image) in Photoshop? Sometimes, you just don’t have a choice but to increase the resolution of an image to try and improve the print quality. Photoshop’s Bicubic interpolation works well for digital images with low noise. However, normal enlargement will usually cause a loss of detail or sharpness and introduce artifacts. To up-sample images with low resolution, we employ state-of-the-art software technologies that uses fractal-based algorithms that optimizes clarity and detail of the enlarged image.
Resolution of Digital Photos
How do you get good quality images from your digital camera? Set your camera’s image quality to the highest setting possible with the least amount of compression and use the largest image size. Save your photos as Raw files or digital negatives (e.g. Nikon uses NEF, Canon uses CRW) to retain the most image information. You can also save your images as TIFF, which is a lossless file format (as opposed to JPEG, which is a lossy format – meaning that every time you save a JPEG file, the file is compressed by throwing away image information). To print a 8″ x 10″ photo at 300 dpi, you will need an image that is 2,400 x 3,000 pixels or a total area of 7,200,000 pixels (7.2 million pixels or megapixels). Most digital cameras and mobile phones can at shoot more than 7 megapixels these days.
Still confused?
Still confused about PPI, DPI, LPI? Give us a call at 604.683.6991 or leave a comment below.
Samco Printers is a full service commercial printer providing exceptional offset, digital and large format printing in Vancouver, British Columbia, Canada using the latest pre-press, printing and finishing technologies. We also have a fully equipped art department to handle your graphic and web design needs.
Need help? Let us help you get started or put the finishing touches on your printed materials. Our Art Director will be happy to spend some time with you. Give us a call at 604.683.6991 or send an email to techsupport@samcoprinters.com
Don’t need help with design? Talk to us anyway to ensure that the files you’ll be supplying meet our specifications.
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