There is no rule for doing the optical compensations, they are always done by eye. Just thicken them. Check this out:

http://briem.ismennt.is/2/2.3.2a/2.3.2.05.verticals.htm

http://briem.ismennt.is/2/2.3.2a/2.3.2.06.illusion.htm

Interesting.

Thanks for the link!

Briem is my hero. I will never make another diagram. (not)
These optical illusions are one of the critical areas of type design, and a tough aspect to teach. You need good eyes for it.

It's tempting to say:
Aha! Helvetica is 7.34% more blah blah. My font will be 7.34%.
Here are two beginning excercises. You may find printing is needed because your monitor will lie to you:

illusions.pdf (164.8 k)

Could anyone please share any advice with diagonal lines? They seem to thin out compared to the other lines, despite using the same width as the vertical lines.
When you slant them, yes they get thinner. When you rotate them, they actually get thicker. The above pdf shows the difference. The top excercise shows slanting, the bottom is rotational thickening.

Have fun!
Randy

> When you rotate them, they actually get thicker.

If you rotate them towards the vertical - otherwise the opposite.

hhp

Just to be clear, using no pronouns to confuse :-)
When trying to make two lines look the same thickness...

If you take a vertical | and rotate it to