Wednesday, March 4, 2015

Printing your travel photos: PPI vs. DPI

HP Z3200 Designjet Printer - Image courtesy of Hewlett PackardWhile these days, most travelers display their photos online instead of in the photo albums of the past, many travelers still print more than a few images to keep and show off their travel memories. Some even hang prints of their travels on the walls of their home.

Unfortunately, many travelers, like other photographers, are often confused about sizing their images for printing because countless photographers incorrectly use the terms, PPI (pixels per inch) and DPI (dots per inch) interchangeably.

PPI and DPI are not at all the same, and they are independent of each other.

PPI has to do with the image itself, while DPI has to do with the printer and how densely it's applying its ink to paper.

Let's start with DPI. This refers to how many dots of ink printers lay down on paper, per inch, to print each image. DPI is independent of PPI. Many photo printers today will print at 1200 to 1440 DPI. Printers which utilize ink dots in that density range typically give prints well filled images, with sharp edges, as they cram 1200 to 1440 ink dots into every inch of each print.

Using that many dots means a printer will almost always be utilizing multiple ink dots to print a single pixel, but that's quite typical and a plus for image quality.

Now let's look at PPI, pixels per inch. Photographic images produced by digital cameras are made up of pixels. In fact, the pixel is the fundamental unit of digital images. The word “pixel” was created by combining the shorthand expression for pictures, “pix” and “el” for element. Pixels are picture elements.

In the camera, we can reference pixels to the photosites on the camera's sensor. Once the camera “captures” the image and stores it in a file, for practical purposes, I think we can think of the pixel as the smallest single component of a digital image. When we put all the pixels together we have our image. The more pixels we have to work with, the larger the image, and the larger we can natively print that image.

To figure out print size we start with the number of pixels in our image. Then we use the image's resolution to determine the maximum size the image can print at that resolution.

In a 16.2MP camera I've used, it produces an image size of 4,928 x 3,280 pixels. Those are the pixels you have to work with natively to make a print from that camera. At a resolution of 300 PPI, a pixel density which can produce a high quality, sharp print, the print size will be 4928÷300 by 3280÷300 or 16.43” by 10.93”. If we reduce the resolution to 240 PPI, which should still produce a great looking print for many photos, the size you can print the image jumps to 20.53” by 13.67”.

So, if you send a full 16.2MP image to your printer with its resolution at 300 PPI, you can natively get a 16.43” by 10.93” print, regardless of what DPI your printer is set to use, as they are totally independent of each other.

If you had a 36.3MP camera, your full size images would be about 7,360 x 4,912 pixels, so at a resolution of 300 PPI those images could be natively printed to a size of 24.53” by 16.37”, again regardless of the printer's DPI setting, much larger than the print from the 16.2MP camera.

Of course, you can get larger prints by reducing your images' resolution. Sometimes that makes a lot of sense.

Most people view prints most comfortably at a distance to the print of 1.5 times a print's diagonal length. We typically view 4” x 6” prints from about a foot, but 16” x 20” prints, generally from more than three feet away. That means we can reduce the resolution needed for larger prints, as our ability to see detail at distance diminishes. While you'd want a resolution of about 300 PPI for a 4” x 6” print, an 8” x 10” print could have a resolution of around 200 PPI and a 16” x 20” print a resolution of about 100 PPI. By the way, billboard images are generally printed at a resolution of just 10 PPI.

If you haven't ever done the calculation, you might be surprised that at a resolution of 100 PPI, a 16.2MP image can be natively printed at 49” by 33”.

Sometimes because it's necessary to crop an original image, you don't have the necessary pixels to make a print at the desired size. There are programs and plug-ins which can interpolate and fill in more pixels based on the ones around them, to produce enlarged prints. OnOne's “Perfect Resize” is one of the best programs which plug into Adobe's Photoshop, to make such enlargements.

When you are trying to determine print size, remember, it comes from the number of total pixels in your image and the resolution you use when you print, not the dots per inch you have set your printer to use.


Sasha (Chicago) said...

I thought they were the same thing. What little I knew. Thanks for the great article.

Will (Dallas) said...

Great article. I didn't realize I could reduce the resolution for large prints and still get good looking prints. That's very helpful.

Tony (Miami) said...

Like Sasha, I thought PPI and DPI were the same, and really didn't understand how they worked or what they meant. Thanks for the great article.

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