Why doesn't the same Windows font look the same when migrating to emacs 26.1?

Question

I'm trying to migrate from emacs 24.5 to emacs 26.1. I installed 26.1 and running both with the same config files. I took several tries and fixing small things in the config, for emacs 26.1 to start without errors.

However, the font setting has problems. It is set by a frame parameter in initial-frame-alist:

(font . "-outline-Consolas-normal-normal-normal-mono-18-*-*-*-c-*-iso10646-1")

It works fine in 24.5. but in 26.1 the exact same font setting results in smaller font (emacs 26.1 below):

Why is that?

According to describe font the same font and size is used, only the font version is different for some reason:

emacs 24:

name (opened by): -outline-Consolas-normal-normal-normal-mono-18-*-*-*-c-*-iso10646-1
       full name: Consolas-13.5
            size: 18
          height: 22
 baseline-offset:  0
relative-compose:  0

emacs 26.1:

name (opened by): -outline-Consolas-normal-normal-normal-mono-18-*-*-*-c-*-iso10646-1
       full name: Consolas-11.0
            size: 18
          height: 22
 baseline-offset:  0
relative-compose:  0
  default-ascent: 17
          ascent: 17
         descent:  5
   average-width: 10
     space-width: 10
       max-width: 20

Why emacs 26 finds a different version of the same font in the first place?

Answer 1

Emacs has changed on its way from 24 to 26 its mind on what the forced size "18" means in your initial-frame-alist.

In your examination the size is 18 in both cases, and the 26 screenshot looks a bit more like it would be 18 pixels.

I use (add-to-list 'default-frame-alist '(font . "Anonymous Pro-12")) to setup my emacsframe and I guess if you use "Consolas-13.5" in the font value, your display will look the same in both versions - minus the possibility that the fractional part might be interpreted different.

Window FAQ on fonts from the Emacs manual