Why DPI and Inches Don't Matter (Most of the Time)

Every now and then I get requests from people (including the photo club) to submit photographs that need to be a certain number of inches and/or DPI. I'm pretty sure that in most instances they do not intend to print my photographs directly but instead want to embed them into a slideshow for display on a computer screen somewhere or maybe to use on a website somewhere or even to make some kind of flyer. This gets me to thinking that there are a lot of folks out there who don't quite grasp the relationship between DPI (dots per inch), inches and pixels.

So let's be clear: DPI is not a property of an image. It is an arbitrary, separate and independent number that really just clutters up the header in the photo EXIF (exchangeable image file). It does not affect the pixels in the image file in any way. It is only an instruction for printing (or scanning) that is most often ignored by the person or service bureau doing the printing. In real life, it simply does not matter what this embedded DPI number is - it has no use, not until the time you actually print out the image on paper and even then it is not likely to be used. At the time of printing, it is up to the person setting up the print to decide how many inches the photo needs to be on a particular size of paper. The printer will set the DPI to match your size requirement, regardless of what is listed in the header.

Also, you often hear people say that their camera takes pictures of a certain size measured in inches but that is a meaningless statement. Your camera takes photos which can be printed at any length you care to specify. The quality of the print will vary, depending on your setting and the machine you are printing with. The camera itself has no clue what size you might choose to print them later, if you even decide to print at all.

Digital images are dimensioned in pixels - it is always about pixels

DPI and inches are unknown concepts in digital cameras.
The monitor screen is dimensioned in pixels and shows the image pixels directly, one-for-one (or shows a re-sampled temporary copy to fit on the screen).
The digital camera sensor is dimensioned in pixels.
The camera sensor size creates the image to be its same size, dimensioned in pixels.
None of the above involves inches or DPI in any way.
Paper prints do involve inches because paper is dimensioned in inches.
On paper, the concept of DPI is used to scale the image size (pixels) to the paper size (inches) and much of the time this scaling is set without regard to the DPI number that someone may have designated when they saved the photograph.

Computer Monitors

Computer screens are dimensioned in pixels and show the pixels directly - one-for-one without any concept of inches or DPI. If someone tells you that your image needs to be a certain DPI for the screen or the web they are simply wrong. It is the number of pixels they need to be concerned with, not DPI. If there are enough pixels on the screen to display all the pixels contained in the image (one-for-one) the photo will look its best. If there are more pixels in the image than there are on the screen, the computer will simply resample the image temporarily and throw out pixels it doesn’t need in order to display the photo. If there are not enough pixels in the image to fill one-for-one at a particular zoom level the picture will look pixelated. DPI has zero effect on images displayed on computer monitors.

Printing

DPI does play a role when you print images on paper but maybe not the way you think it does. If you print the image at home from image editor print menus, the computer will use the DPI values in the file header to compute the size of the image on paper if you do nothing to set print parameters. This can result in your grabbing the print out and realizing there's something that needs to be tweaked in the print settings.

If your photo is 2000 pixels wide and it says 180 DPI in the EXIF data, the printer will try to print 2000/180 = 11.1 inches but here's the deal: All print menus offer a way you can scale the size first to print on a specific piece of paper. If you scale this image to print 10 inches (to fit the paper), then it will scale to print at 2000/10 = 200 DPI. If you want 10 inches at 300 DPI, then you need to provide 10 inches x 300 DPI = 3000 pixels, scaled to print 300 DPI. The embedded DPI number is disregarded which is what happens most frequently. If you use a printing service, same thing.

To beat a dead horse

If you specify this image is to be printed at some size in inches, that original DPI number is not used. It is not a property of the image. It is just an isolated number, a separate number simply stored in the file, independent of the image and when it is used by the printer it is only to space the pixels.

When someone asks me or you to send them a photograph for a specific purpose, it is best to specify the size in pixels rather than a certain DPI or number of inches. They need to come up with those numbers on their own depending on the printing device.