Image file resolutions what you need to know

image file resolutions cover picture of twilight dusk landscape western NSW Australia. By Australian Photo Adventures and Excitations.

Image file resolutions all those numbers.

There is a lot of confusion around image file resolutions. We are regularly asked to supply a file at 300dpi. When we ask what size they need the image, we get a puzzled look or are told: “I just told you”. But they haven’t told us. All they have told us is the print resolution, not the file size.
So for the record, file resolution does not equal file size.

I do not want to write a ten-page documentary. Here is a much-simplified version.
DPI, short for Dots Per Inch is a printing term referring to the number of ink dots laid on paper per linear inch. Three hundred DPI has become the accepted standard print resolution. Despite a lot of printing being carried out at other resolutions.
For example, newspapers printed on a very absorbent paper-stock. The ink spreads out. Bleeds is the term used, too many dots, too close together would result in a patch of solid ink with nop tonality or definition.

Think PPI instead of DPI

What we need to know as photographers are how many pixels across the image. Mostly we ask for the number of pixels across the longest edge. As soon as we have this number, we know how large our files need to be. The old, I need 300dpi file is meaningless.
Take, for example, someone asks for an image at 300dpi. I could supply an image that was 300 pixels across, and it would likely be useless to them. Unless of course, they only wanted to print the photo to a size of one-inch width. If they were enlarging the image to four inches across, they would have a shortfall of 900 pixels. Upsizing the file, in this case, would result in a blurry print job.

Different resolutions for different outputs.

Do I need different image file resolutions for various purposes? In the real world in 2019, it is no longer a big deal. Or it shouldn’t be. For example, modern internet browsers are now way more sophisticated than just a few years ago. Most will automatically resize images to fit the space allotted although I would still advise uploading a file sized as recommended by the website you are supplying.

For example, if their preferred size is 815 pixels across, that is what I would upload if you upload a picture with fewer pixels than needed up-sampling takes place. Put another way the software expands the picture size, notices gaps in the information available and fills in the gaps. Not a place you want to be if you’re a photographer who cares about how your photos look. Conversely, loading an image bigger than recommended can result in auto down-sampling. Again this occurs in results ranging from excellent to bad.

Likewise, supplying photos for printing, you need to provide sufficient pixels to cover the area to be printed. Just make sure you know what print resolution they work on, and supply files with an adequate pixel count to do the job. Again upsizing has its limits. The good news, however, most competent printers are very capable of down-sampling without loss of quality.