How does optical margin alignment really work?

In my experience the best way to set the optical margin alignment is to set it at the size of the body text. It allows characters to extend over the margins, but the amout is depending on the shape of the character. If you zoom in on text you will see on normal text that round characters like o u and sharp characters like v w normally extend below the baseline. A similar thing happens at the Cap height if you set a guide on the flat of Hor N. These extensions are so that the baseline is optically correct.

In a text like a novel with indents and much dialog you will notice that without optical alignment the indents that start with a quotation mark or an EM-dash appear larger than normal indents. Optical alignment helps to make such indents appear more regular/balanced.

The right margins are also affected but this is mostly seen if you have justified text (avoid full justified text). 

There are ofcourse situations that automatic optical margins cannot resolve and you may want to kern manually.

Hi @apa68177991 ,

Thanks for reaching out. As per this article, the point size determines how far into the margin characters will extend, but it doesn’t necessarily have to correspond to the point size of your text. 

I am leaving the discussion open for experts' suggestions.

Regards

Rishabh