Working Distance
The clear space between the front of the lens and the subject at closest focus.
Formula / rule
Working distance = MFD − lens length − flange depth
Measured in
mm
In depth
Working distance, not magnification, is what makes close-up photography practical: too little and the lens shades its own subject, frightens an insect, or leaves no room for a light. It is the main reason macro lenses are made in 60, 100 and 180 mm versions — all reach 1:1, but the longer focal lengths do it from much further away.
Examples in practice
A 180 mm macro gives about 25 cm of working distance at life size, where a 60 mm gives under 10 cm.