Resolution - ppi vs. dpi

Digital image resolution and print resolution have nothing in common!

There, I said it – whew. I feel better now, thanks.

It seems like this has to be one of the most misunderstood topics for large format printing. I just had a phone call from a new customer that asked what resolution I use when printing. Since I already knew what machine and material I’d use for his vehicle wraps, the answer was 720 dots per inch (dpi). But I had to say, “why do you ask”….because I thought I already knew the answer and hoped to save him some grief from having to struggle with too large of a file size. He said “I need to know if my customer’s file has enough resolution for your printer”. Just as I suspected.

Many designers are taught that an image’s resolution in pixels per inch (ppi) is a function of the printing resolution. This is driven by the offset printing world where the rule of thumb has been to multiply the printing resolution by 2 to get the optimal resolution for digital images. It used to be that common offset print resolution was 150 lines per inch. So 150 x 2 = 300 ppi in the digital image. Because of differences in the printing technology between inkjet and offset, together with the differences in how the respective images are viewed, this rule does not apply in digital inkjet printing.

text made from pixels

First let’s look at the 2 totally separate types of resolution we’re talking about here. One is pixels per inch. Odds are you’ve seen an image that didn’t have enough pixels, it looks “jaggy” as the pixels become large enough to be noticeable. We certainly do need enough pixels to avoid this jaggy look and have nice, sharp images. But pixels are not everything, as we’ll see in a moment. For now, isolate that we are talking about the number of pixels in a digital image….NOT how it will print.

Print resolution is usually expressed in dots per inch (dpi). Obviously, the more dots per inch, the better. But that’s not the only thing that matters. There are other very influential aspects to inkjet printing that effect the perception of resolution beyond how many locations a printer can put dots within an inch. One of the biggest is the addition of light cyan and light magenta to the conventional cyan, magenta, yellow, and black inks. These light inks do not add to the color gamut as some sales people will have you believe. What they really do is hide the dots from your eye in lighter colors. Since the inks are a lighter color they don’t contrast the white background as much as the dark cyan and magenta inks. This way you perceive the print to be a higher resolution that it really is. All of our printers have at least 6 color inksets that include light cyan and light magenta. The fine art printer adds several shades of black (or more precisely: greys) as well as red and blue inks.

resolution in dots

All of this techno speak is trying to get to one fundamental truth. For inkjet printing, digital image resolution and print resolution have no relationship whatsoever. You could have 1000 ppi image and have it print at 300 dpi and it would look grainy because of the low print resolution. You could have a 10 ppi digital image and print it at 1000000 dpi, the jaggy pixels would be printed very cleanly, but it would still be low resolution.

So let’s get to some practical guidelines. Generally speaking you need 100-200 PIXELS per inch in your digital images to get a nice clean print from our systems. As the dimensions of the print get larger and are viewed from further away, the needs for pixel resolution drop off a cliff. Common image resolution for vehicle wraps is 70-100 ppi. Image resolution for  a large banner may be as low as 20 or 30 ppi if it will be hung on the side of a building and viewed from 10+ feet away. The answer to my customer’s question is 70-100 ppi for the vehicle wrap he was calling about. This would save him hundreds of megabytes or possible gigabytes in file size and still get a clean print.

Notice how you really don’t (usually) need a huge file size to get great results. Notice how there is no reference to the print resolution. That’s because we’ll always print as high a resolution as is needed and we just need to make sure your images have the appropriate pixel resolution.

Of course there are exceptions to these guidelines. When in doubt, call us. We’re always eager to make sure you get the best possible results for your job.