Registered design-variation axis tag: 'opsz'
Name: Optical size
Description: Used to vary design to suit different text sizes.
Valid numeric range: Values must be strictly greater than zero.
Scale interpretation: Values can be interpreted as text size, in points.
Recommended or required “Regular” value: A value in the range 9 to 13 is recommended.
Suggested programmatic interactions: Applications may choose to select an optical-size variant automatically based on the text size.
The Optical size axis can be used as a variation axis within a variable font. It can also be used within a STAT table in non-variable fonts within a family that has optical-size variants to provide a complete characterization of a font in relation to its family within the STAT table. In the STAT table of a non-variable font, a format 2 axis value table is recommended to characterize the range of text sizes for which the optical-size variant is intended.
The scale for the Optical size axis is text size in points. For these purposes, the text size is as determined by the document or application for its intended use; the actual physical size on a display may be different due to document or application zoom settings or intended viewing distance.
In applications that automatically select an Optical size variant, this should normally be done based on the text size with a default or “100%” zoom level, not on a combination of text size and zoom level.