Graphic Design/In typesetting, how is the appropriate leading determined?
Here's what I understand so far (from Robert Bringhurst and other sources):
It's dependent on the measure of the line (longer measure, more leading) so that the eye can easily find the next line as the reader scans the page. This generally means that within a fixed-width page, larger type needs less leading, since larger type results in a smaller measure.
Ascenders and descenders shouldn't collide (so this of course makes it dependent on the design of the typeface).
Unless you're setting with negative leading for graphic effect, the space between lines should be greater than the space between words.
It's that last point that's difficult for meľ see my image below. In this case, how is the space between lines determined? From the baseline (yellow) or descender (red) of the previous line? And to the cap height (green) or median (blue) of the next?
With heading or title text, and for graphic or stylistic effect, you are right in noting that the same rules for setting body text do not apply. You can see strong examples of this in magazine layout (check out Esquire Magazine or Real Simple Magazine for beautiful examples).
Type set "solid" has a leading that's equal to its point size (12 points of type = 12 points of lead). That is pretty standard for most typefaces and allows for enough spacing to avoid running your ascenders in to your descenders. Automatic leading in computerized type is often set at 1.2 times the point size of the type. Not particularly useful because it doesn't account for length of measure.
One way of calculating suitable point sizes for various measures is based on the relationship between point size and measure. Divide the measure (in picas) by the size of the type (in points). Round off to the nearest half point. Example: for 11 pt type set over a 24-pica measure, add 2 extra points of lead (24 divided by 11 = 2.18, rounded off to 2). This results in a setting of 11/13 (11 points of type/13 points of lead). increase the measure to 30 picas, results in a setting of 11/13.5. And I use this as a starting place, then use my eye to guide adjustments.
If you use a setting often, you can reset preferences in InDesign or Illustrator to use specifications defined by you.