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I'm a novice type designer attacking my first face with FontLab 5. I feel like my naivety and idealism have lead to a couple of conflicts, the most significant being with UPM and the actual cap-height when set. I imagine you guys can help clarify some things for me.
The Way I Understand UPM
As I understand it, UPM is an essentially arbitrary number that determines the number of Font Units per Em—usually defined as 1000 or 2048. This value is used to scale the font when set. If1000 UPM is used, then when set at 10pts those 1000 units will be scaled to 10 pts high.
While 1000 UPM is described in FontLab's Manuals as a standard, which as a multiple of 10 makes sense, it appears no one uses 1000 units as the cap-height. Usually a number closer to 800 units is used (Am I correct in this belief?), making the actual cap-height when set significantly smalller than 10 pts. To make matters worse there seems to be little reason or standardization in how many units tall the cap-height is. So when set at 10 pts, two fonts can have different cap-heights.
My Beef With the Situation
If someone asks me to draw 2 inch letters, I'm going to draw them so that they have a 2 inch cap height. If they asked me to draw 10 pt letters, I'd draw letters with a 10 pt cap height. I imagine you would do the same. Who wouldn't? It only makes sense to draw the letters so that the tallest letters are at the specified height. Ignoring for the moment the growing tendency for ascenders to rise slightly above the cap height that would make the cap height the tallest point in the font. So, why would you build a font that when set at 10 pts is actually some arbitrary size, like 8 pts?!
Am I Crazy?
Or just naive? What am I missing? What is the rationale for this annoying and confusing discrepancy? If I decide to build my font with a 1000 unit cap-height/ascender height will this severely impact my vertical metrics. Will my font merely be noticeably quirky—requiring it to be set at 7 or 8 pts to read like everyone else's 10 pt—or will it be completely wrong, bad, amateurish, or whatever degrading modifier you wish to use? Additionally, if I use something other than 1000 units for my cap-height, how do I determine what number of units to use for my cap-height?