Friday, January 5, 2018

Dots Per Inch and Other Assorted Fairy Tales

Red Line by Mark Becwar on

Back in the day, when I used to shoot film, I knew exactly what size my photos were. I shot 35 mm film, so my exposures were all that size. There was no doubt about the image size. With digital we're left with a question.

What size is a digital photo?

First let's consider the detector in our digital camera. This detector is a certain number of pixels wide by a certain number of pixels tall. In my camera that is 5760 pixels wide by 3840 pixels tall. Cool. That's the answer, right?

Not so quick.

How big is a pixel?

Enter dots per inch (DPI) -- a measure of how many pixels we're displaying/printing per inch*. Like with everything else in photography, there's a tradeoff here. Higher DPIs make for crisper, better looking images, but that comes at the price of final image size. High quality prints that you might show in a gallery, or sell as art are typically around 300 DPI.

Doing the math for my camera, that means that the largest image I can produce at 300 DPI is 19.2 x 12.8 inches. (Divide the number of pixels by the DPI to get the size.) If I accept a lower output quality of 150 DPI, the result is 38.4 x 25.6 inches.

I won't give any hard-and-fast rules here. It's a matter of personal preference, and something that you need to decide on your own. Most galleries and competitions want image submissions at 300 DPI.

A Quick Rant to Finish

A lot of competitions and galleries specify in their submission requirements that the digital file should be 300 DPI. This is nonsensical. While some file formats do have the ability to store a DPI for calculation of the 'native' image size, it is generally optional. What matters is the actual resolution (in pixels) of the image. When you open the file on a computer monitor, it isn't being displayed at the 300 DPI suggested by the file, it's being shown at the DPI of the monitor, which may be as low as 72 DPI. When you print the image, you select what size you want. If you want images that you can print at 300 DPI and have a minimum size, then do the conversion, and post the requirement in pixels. (If you want an image to be a minimum of 10 x 10 inches at 300 DPI, then just make the requirement that the image must be at least 3000 x 3000 pixels.

About the image:
The photo is of a Los Angeles Red Line subway car, shortly after delivery to the north side of Milwaukee, WI, for rebuilding by Talgo. Taken by yours truly at a press event on 14 July 2017.

* Technically PPI and DPI are different. This is an intro, so I'm simplifying things a bit to make it easier to understand. The two are, for our purposes today, identical.

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