DPI: a concept you should forget.

The question of “what DPI do my photos need to have?” comes up often in forums about online photo sales. It can set off a firestorm, with some replies containing total misinformation. Arguments can ensue.

A newspaper photo up close.

I’ll offer a simple way to think about this subject, and the reasons why you shouldn’t. (If you already know where I’m going with this, skip the rest of this post and just watch this funny video instead.)

The acronyms DPI and PPI use the words Dots, Pixels and Inches. And here is the key:

Dots means dots of ink.
Pixels means points of light.
Inches means inches – a physical dimension.

A paper print has dots, and inches. So it can have a DPI value.

A display device (monitor or phone) has pixels, and inches. So it can have a PPI value.

A photo from a camera has just pixels. That’s all – no physical size and therefore, no inches. It has pixel numbers, like 6000×4000, depending on its sensor size, but the image has no physical size. So it can’t have a DPI or PPI value. It just can’t.

This part is tough for many people to accept: DPI and PPI are meaningless for a photo. DPI only applies to prints, and PPI only applies to displays. You probably don’t know the PPI of your monitor, and never change it. If you’re selling prints through an online POD, you don’t know how big they’ll be or how they’ll be printed, so you have no control over DPI. Just relax and forget about these things. Never think about DPI again and everything will be fine.

“But” – you may be saying – “My camera has a DPI setting! My JPG images have a DPI value! How can they not mean anything?” And I am telling you that in today’s world, for photography, they do not. Cameras should not even be showing us these values, much less allowing us to twiddle with them, because people think they’re somehow changing the “resolution” of their camera, or the size of the output image. In reality all that happens is that a different number gets tucked into the JPG (or raw) file. It means nothing.

“But” – your next objection – “300 DPI is the standard for publication images. Surely my photos should be 300 DPI”. And again, no. This is just an old canard (love that word) that keeps making the rounds. 300 DPI was, indeed, a standard for magazines and printed publications because they used printing presses, which involved things like “offset printing”, “screens” and “halftones”. And these print media were viewed at a reading distance, not hung on a wall.

Photos that we sell online will be printed by inkjet printers, which could not care less about 300 DPI. Check their specs and you’ll see claims of DPI in the hundreds, or even thousands – but they’re not even really “dots”. Inkjets lay down microscopic drops of ink in proprietary patterns, possibly overlapping, possibly not. We don’t know. And the software for those printers resamples the image as necessary without regard to whatever DPI number it contains.

As a final line of defense, some claim that we need 300 camera sensor pixels per inch of the final print, skipping right over any consideration of printer dots, viewing distance, or how the camera pixels might be interpolated or combined by a printer driver. It’s like the thing about drinking 8 glasses of water per day – people believe it because they’ve heard it since forever, even though absolutely no one can point to any theory or research behind it.

Sharpness is subjective, always – it’s not a numbers game or a formula. There is no requirement for 300 of anything, anywhere. The final, viewed image just needs to be sharp enough that viewers like it.

So forget D PI and PPI. Never resize or resample your photos, you’ll just create problems for yourself. Don’t worry about “resolution”. Don’t adjust a DPI or PPI “setting”. Just use the pixels that your camera creates. Let your online print supplier’s software perform whatever magic it needs to, to get the best print at whatever physical size your customer ordered. It will be fine.

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