0

Assuming a font size of 9 pt (pretty widespread minimum font size), what PPI / scaling is recommended for reading it from a typical computer display? Is the 96 PPI standard close to optimal, or is it just a legacy of old days when higher display PPI were not possible?

Windows dialogs certainly suggest it's the optimum: enter image description here

1 Answer 1

1

This is a tricky one to answer!

Firstly, DPI and PPI aren't the same thing conceptually. One is a measure of the dots per inch on a printed output and one the pixels per inch of a digital display.

I believe as a result you're kinda talking cross purposes. The windows DPI setting of 96 is the default https://support.corel.com/hc/en-us/articles/115001485408-Display-Options-How-to-change-DPI-Scaling-Level-for-displays-in-Windows-10-#:~:text=DPI%20setting%20controls%20the%20size,has%20setting%20of%2096%20DPI.

And by changing this what you're effectively doing is scaling a ratio (as I understand it) the number of "dots" (which doesn't make much sense in digital displays) which would make up an inch (I assume, given the windows dialog) of screen. What this results in, (by reducing DPI) is more pixels representing the same inch that 96 would have, thereby given a fixed hardware resolution display making the screen elements appear smaller.

As a side point, Windows 10 has done away with this dialog and shows interface scaling as a percentage.

So, context aside, your original question asks what the optimum PPI / scaling is recommended for reading a typical computer display. What you're asking is at least two questions.

The reason it's at least questions is that the output "size" of text on screen is a function of both PPI and scaling. For example, a 24" 4k monitor with a high DPI and scaling of 200% will look identical (in size) to a 24" 1080p monitor with a scaling of 100%. The 4k monitor will look sharper at this setting because there are 2x the pixels per inch and techniques like pixel rounding and smoothing are less pronounced. This is especially true of mobile devices which may have a very high hardware PPI, but internal scaling (on android its just "smallest to largest" which they control. I'm not aware of the technical scaling factors used here, I cite it to illustrate that different devices handling scaling differently in their terminology.

Finally, we come to "optimum" there are many ways to look at this:

  • Minimum text size to meet WCAG accessibility guidelines, but beyond this, recommended best practice for "base" font sizes. 9pt/12px has been given as the literal minimum before you starting failing A11y checks and should certainly not be used for body text for example where something close to 16px as a minimum would be recommended.
  • Optimum font spacing, line lengths and justifications, for readability (https://www.w3.org/TR/WCAG21/#visual-presentation)
  • Optimum for your content, userbase and use cases -For example if you're expecting users to read in specific environmental conditions, you should consider this

Those three points alone could warrant and answer each, all longer that this answer! DPI as a number in that context is largely meaningless, because it's a measure from a single OS 20 years ago.

In summary, there is no optimum, scaling is a user controlled preference and should be treated as such, if a user chooses to change it, you should accommodate it (https://www.w3.org/TR/WCAG21/#resize-text).

5
  • Tow questions remain: (1) does this minimum of 9pt/12px apply to a standard 96 DPI display, the 160 DPI "baseline" display for mobile device, or something else? and (2) what happens below this limit: eye damage or inability to read the text? Commented Jul 14, 2021 at 7:43
  • 1. The 12px minimum will be scaled on devices with high DPI, by the software or OS to meet viewing standards required. Do a bit of research on retina displays for example to understand. 2. The standards are the very minimum for the very minimum of readability. Try reading any long form text as that side and you will experience eye strain at a minimum. For anyone with less than perfect eyesight, even the minimum may by unreadable. Even for someone with perfect eyesight, in the wrong conditions, high motion, high ambient light and low colour contrast, it may still be unreadable. Commented Jul 21, 2021 at 18:58
  • (1), so e.g. for a Retina display with 226 PPI, which scaling factor should be applied by the OS? (2) So I suppose those standards have nothing to do with health & safety requirements, which really was the core of the question (it was migrated from med SE). Commented Jul 22, 2021 at 9:07
  • 1. That's a user preference as I mentioned in the answer, if people find the size of elements on screen too small, they will increase the scaling to say 125% (of whatever the OS default is) which in turn will make the same element take up more pixels (as the number of pixels per cm of physical screen space is a constant) there is a decent table on Wikipedia en.wikipedia.org/wiki/Retina_display Commented Jul 22, 2021 at 11:10
  • 2. Yeah those minimum requirements are around readability as a facet of accessibility. Health and safety would be a whole area of its own, you could, for example, think holistically, if the screen can't be read, does that put the user in danger? or in a medical context, potentially a patient? Another way to think about it is longer term user eye strain and therefore damage. Larger is nearly always better until you get to the point where readability is effected negatively due to short line length and broken words, affecting the amount of Saccades en.wikipedia.org/wiki/Saccade Commented Jul 22, 2021 at 11:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.