Matt Spinelli Photography - Understanding Resolution

By Matt Spinelli, 4/19/2006 (revised 4/20/2006)

"Help, no matter what I do, every picture I take with my camera comes out to be 72 DPI when I load it into my photo editor. I've heard that 300 DPI is what I should have. Is there a problem with my camera? Does this mean I only have low resolution images? What about my printer? It says it is 1200 DPI. I'm so confused!" I often hear people making such statements or asking for help. They are worried that their camera is not working properly or that they are doing something wrong. Others are perplexed about the difference between the terms DPI and PPI. In this article we will eliminate any misconceptions about what these terms represent and how they are different from one another.

What is PPI?

We all know that our digital images are made up of pixels. The actual image itself is a matrix of so many pixels wide by so many pixels high. A 6 megapixel image, for example, roughly has a width of 3000 and a height of 2000 pixels.

PPI stands for pixels per inch. PPI is used to describe how densely the pixels that make up an image are packed within a span of an inch. We are not referring to square inches here. A 6 megapixel camera at 300 PPI would mean that it would be 10 inches wide (3000 pixels / 300 PPI) by 6.67 inches tall (2000 pixels / 300 PPI). Thus, PPI determines how large an image will print. By itself, the term has little meaning unless the image size dimensions or pixel count are known as well.

To calculate PPI and the various print sizes it produces for any digital camera's image, I have created this helpful PPI calculator. Please note, however, that when printing an image, either we or the printer interpolate the image (add pixels) to reduce jaggies and/or graininess when the PPI values become too small. For professional quality photographs, we need to have an original PPI value of 300 before any post processing interpolation is applied to the image. Very Good results can be achieved with PPI of 200-300. Rarely, should we want to have an image with lower than 150 PPI before interpolating.

Keep in mind that some images demand more PPI if they are very detailed, such as a landscape. Others, like a portrait, we can get away with less initial PPI. Also noteworthy, is the fact that not all cameras (their lens, sensors, and processing) produce equal quality images. Some cameras produce images that resolve far more detail than others, even though they may be the same megapixel count and therefore the same PPI at a given image size. So just remember when using my PPI calculator, or my megapixel calculator/analyzer, that in the real world all cameras are not equal. These calculators, while providing mathematically accurate results, do not necessarily mean that just because the original PPI of an image from your camera is 200 at the size you plan to print, that it will be just as sharp as an image from another camera at the same 200 PPI.

What is DPI?

DPI stands for dots per inch. It often is used interchangeably with PPI to describe the resolution of digital images and scanners (which produce digital images). This is why there is some confusion, because DPI also refers to the resolution of an output device such as a printer. Printing technologies differ from each other, some use chemicals, other spray dots, and yet others use dye sublimation. Printer DPI varies from each technology and model, but in this article we will discuss one of the most popular printing devices, since almost everyone owns one.

With the common inkjet printer, DPI represents how many dots of ink are placed within the span of a linear inch on a printed image. These printers use a limited number of colored inks to produce the millions of colors possible in a photograph. Some printers have 3 colors (magenta, cyan, yellow), others have 4, and some have 6 or more. To compensate for the limited number of colors, each pixel of the digital image is represented by mixing many tiny dots of ink together.

For example, a 2400 DPI printer has 2400 dots of ink packed together within an inch. If an image were 300 PPI, the printer would use about 48 dots to make up one pixel from the digital image. A 1200 DPI printer would use about 16 dots per pixel. It can get a little more complicated than this, but suffice it to say this generally is how it works.

One thing to keep in mind with printers, though, is that they have a native resolution at which they print best. Native resolution is the PPI of the digital image. When a digital image is sent to the printer, and if it doesn't have enough PPI to print at the printer's native resolution, the printer will automatically upscale (interpolate) the image to the correct PPI amount. If the digital image has too high a PPI value, it will downscale the image before printing. Note that interpolating to achieve a higher PPI value does not add detail to the image.

Comparing the difference between PPI and DPI

Below I have an example to make PPI and DPI easier to understand. I printed this photograph at home on my 1200 DPI inkjet printer. It has a native resolution of 600 PPI. Instead of letting the printer upscale my image to 600 PPI, I did this myself in Photoshop. I then printed the image as an 8.5x11 inch photograph. Next, it was scanned in at 2400 PPI in order to resolve the ink dots. In this example, each pixel of the digital image is made up by roughly 4 ink dots.

As the above image shows, the higher DPI value inkjet printed image doesn't have any more resolution (in terms of sharpness and detail) than the lower PPI value digital image. Both, PPI and DPI, when it comes to resolution, are relative. For the printed image, resolution is relative to the quality of the digital image and the ink dots being sprayed. For the digital image, resolution is relative to whether or not the image has been interpolated, the quality of the lens, sensor, and camera processing. So the bottom line is that neither PPI nor DPI is a truly accurate term in describing how sharp and detailed an image is. All they do is describe resolution in the sense of pixels or dots per inch. It's that simple.

Final Comments

PPI and DPI are terms often used to describe the resolution of digital images, printed images or printers, scanners, monitors, and other imaging tools. Even though dots and pixels can be different, as in the example above with regards to printer ink dots and digital image pixels, the term DPI can be used with both examples. Some prefer to use PPI instead of DPI, but nonetheless, if DPI is used to describe a digital image, we all know it is being used to describe pixels per inch. If the term is used to describe a printer, then we realize, by context, that it refers to ink dots. If anything else, just remember that DPI is the common term used for resolution in general. PPI is a term that refers to just a digital image's resolution.