This is a question we are asked constantly by our clients. So, we thought we’d have our Production Director, Ken Goldner, break down what the heck a “high resolution” image actually is. One thing to note before we get started: the explanation below concerns raster images (images made up of pixels) as opposed to vector images, which are resolution-independent.
terms to know:
raster = pixel-based art
vector = resolution-independent art
ppi = measurement unit that means “pixels per inch”
RGB = color space for digital designs
CMYK = color space for printed designs
300 ppi = print resolution
72 ppi = digital resolution
what’s the size?
It’s commonly believed that, for printing purposes, 300 pixels per inch (ppi) is HiRes. In most cases that’s true, but that also depends on the physical size at which the image will be used. For example, if the source image is 4 inches wide and 300 ppi, but will be used at 8 inches wide, that’s a 200% enlargement, so the reproduction resolution becomes 150 ppi, which is no longer HiRes.
do not enlarge
As a rule, raster images should never be enlarged beyond the optimum usage resolution*. It is okay to reduce them, as long as they start and remain at optimum or higher resolution.
billboards have their own rules
As mentioned, for most standard printing purposes, 300 ppi at usage size will suffice as HiRes. But what if the reproduction will be a large billboard? In this case, optimum resolution is usually about 100 ppi. Resolution requirements should always be confirmed with the vendor.
we need context
We see that knowing usage size is necessary to determine HiRes status. Also necessary is knowing the reproduction method. What if the image is not for printing, but for use on a website? In this case, the optimum resolution would be about 72 ppi at usage size. Also, knowing that the reproduction process will be digital on a screen tells us that the image should be in RGB format, instead of CMYK format, which is used for printing.
for future requests
The next time you request a HiRes image, be sure to include the usage size and the reproduction method. Even better, include the usage resolution if you know it. For example: “eight inches wide at 300 pixels per inch.”
*Since raster images contain a finite amount of pixel information, they cannot be enlarged without lowering resolution. They can be up-sampled to a higher resolution, but the application doing that will create new pixels by using existing pixels to try to figure out what the new pixels should look like. This usually results in blurring. Up-sampling an image, along with careful retouching as necessary, should be a last resort, used only if there’s no other way to achieve the desired results.
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