1 - Camera lens.
2 - Extension tube.
3 - Film or sensor.
y - Object height.
y'- Image height.
l - Width of extension tube.
H - First principal plane.
H'- Second principal plane.
e - Distance between principal planes.
s - Object distance.
s'- Image distance.
ß - Magnification.
f'ob. - Camera lens focal lenght
Δ = Object - image distance.
Example:
f'ob. = 50 mm ; y = 33 mm ; y'= 24 mm ; ß = ? ; l = ?
ß = y' / y = 24 / 33 = 0.727
