Macro
Simulator
The ruler lies away from the lens at an angle, so the sharp band on it is the depth of field, read off in millimetres.
Magnification changes everything
Getting closer than a lens was built for is easy. What it costs is light, working distance and almost all of the depth of field, and the three of them get worse together.
Magnification, not distance
Macro is measured as the size of the subject on the sensor against its size in life. At 1:1 a ten millimetre insect covers ten millimetres of sensor. Everything else about the shot follows from that ratio rather than from how close you are.
Depth of field collapses
Beyond about 1:10 the depth falls away with the square of the magnification. At life size, f/8 on a Super 35 sensor leaves well under a millimetre, which is less than the thickness of most of the things worth shooting that close.
Working distance is the real constraint
The same magnification from a longer lens puts the front element further away, which is the difference between being able to light the shot and casting your own shadow on it. That is what you are buying with a 100 mm macro over a 50.
Extension costs light, diopters cost sharpness
Moving the lens further from the sensor spreads the same light over a bigger circle: two stops gone at life size, before you have lit anything. A close-up diopter on the front costs nothing in light and some corner sharpness instead.
Focus by moving, not by turning
At these magnifications the focus ring changes the magnification as much as the focus. Set the ratio you want, then move the whole camera back and forth on a rail until the right plane is sharp.
Everything moves
A millimetre of depth makes breathing, floor vibration and a draught through the room into focus problems. Macro is the one place where a heavy tripod and a still subject are worth more than any lens.