Resolution of a file is specifically speaking about raster (variable data) images which are photographs or illustrations created using raster image programs such as photoshop or captured photographs. A raster image uses pixels to create every inch of graphic, and resolution is how many pixels are used to create that inch, that is what PPI or DPI is, Pixels Per Inch or Dots Per Inch, i.e., 100dpi has 100 lines by 100 lines of pixels of information in every inch of your graphic, the more pixels the better quality of image. Pixels on your computer are converted to Dots of ink at the printer which is why the terms PPI and DPI are interchangeable.
When images are scaled up or down in a layout your resolution (dpi) drops or rises in proportion to the percentage of scale, i.e., 300dpi at the original size becomes 100dpi when enlarged 300%. This is why it’s important to know just how large you can scale your image for best printing results. Manipulating the image size in Photoshop will not add quality to your image. It will cause a poor image to pass a preflight and possibly get printed, so don’t do it. We look for those numbers and can help images lacking resolution with specialized software.
For large and grand format printing an effective resolution of 72-100dpi at final size is typically acceptable and considered high resolution. The resolution can be higher, however resolutions lower than 72 dpi should be called out to your printer, they may have recommendations on how to improve the image quality. Acceptability will vary depending on subject matter and end use. The decision will need to be made by the customer. Sending a file to be printed can be implied as having accepted the image quality as is by some vendors.
In large format printing, viewing distance is measured in feet rather than inches, the eye won’t see blurry and jagged edges the further away the viewer is from the product. Here’s a chart to help you determine what resolution is optimal for your viewing audience.
Note: If you built your design to half scale the image resolution in your layout should be double the ideal dpi and 4 times larger if at quarter scale.
Tip: Review your image quality at the printed size in your graphic program (i.e., view at 100% or if it’s half scale 200%). Is the quality you’re seeing on screen acceptable? If not, it may be best to use a different image or reduce the size of your image for better results. You can also ask your vendor if they can help you improve it with sharpening or running it through a fractals generation program for better enlargement results. We do not recommend that you force a higher resolution on an image in Photoshop, the results may not be acceptable.
Tip: Watch for bad clipping paths, your high quality image may print well, however if you haven’t clipped the object out of the background properly, the edges could be very jagged or you could be missing parts of your image.
Tip: Avoid saving your high resolution files too many times as a JPG, this file format may compress your file with each save and has a tendency to leave JPG artifacts around the objects in the image that appear as a dirty halo or strange groups of bright green or pink pixels. It’s better to save your files as a PSD until you’re done with all image manipulation then you can save a JPG if you need a much smaller file size.
How to Check the Size and Resolution of your Raster Image
If you have a file you want to print in a large format you can check the image size and resolution to determine if it will scale to the size you need with good printing results. Here’s how you can do that in a few popular programs:
- Windows Paint: Under the File menu click “Image Properties”
- Macintosh Preview: Under the Tools menu click “Adjust Size…” we found this was the best tool to give you size in inches and the dpi.
- Adobe Photoshop: Under the Image menu click “Image Size…”
- Corel Photo-Paint: Under the Image menu click “Resample”
- GIMP: Under the Image menu click “Print Size…”
- Artweaver: Under the Image menu click “Image Size…”
How to Calculate Enlargement Proportions
Example: You have an 8.5″w x 11″h file at 300dpi and want a 24″x36″ poster
If the 24″ width is the critical dimension:
Width: 24 divided by 8.5 = 2.824 – you are enlarging your file 2.824 times
Height: 11 multiplied by 2.824 = 31.06
Resolution: 300 divided by 2.824 = 106.23
If the 36″ height is the critical dimension:
Height: 36 divided by 11 = 3.273
Width: 8.5 multiplied by 3.273 = 27.82
Resolution: 300 divided by 3.273 = 91.66
Proportionally you could produce a 24″x31″ or 27.82″x36″ poster with ideal resolution values.