It's not possible create a QFont for the 'base' font of a family which doesn't have a regular weight such as Myriad Pro Light on Windows

On Windows, the common Adobe font Myriad Pro is split up in to several families, each of which have a 'base' font and bold/italic variants that allow you to access the whole range of fonts, as explained here: http://www.adobe.com/type/browser/html/readmes/MyriadProReadMe.html?PID=4172912.

Qt apps don't allow you to access Myriad Pro Light, however, as this family has no regular weight, just a light and a semibold. Thus QFontDatabase attempts to find the nearest match for the regular weight (50) and matches semibold (63) instead of light (25) because it's numerically closer. Try it in any of the Qt demo apps to see what I mean.

This isn't good, as it means you can't access that font. It would be preferable for QFontDatabase to fall back to the light variant in the case where bold has not been specified, as the bolder weight can be accessed by adding the bold flag.