I normally don't consider myself that stupid but I'm having trouble understanding what I should be measuring in relation to this comment "To use, simply measure or estimate the perpendicular distance from the lens
to the plane of focus" from this article http://www.davidsummerhayes.com/Focusing%20the%20tilt-shift%20lens.pdf
So, perpendicular to the lens axis I assume? To the plane
of focus? A plane, by definition, take up linear space so where on that plane do I measure or estimate to?
I assume this measurement is done with the lens' tilt mechanism zeroed.
Can someone help a blind man see?