d_o is always positive
d_i is positive if real
d_i is negative if virtual
f is positive (convex)

do is always positive
d_iis always negative (virtual)
f is negative (concave)

image properties vary with object's position
sample ray diagrams

do ∞	di = f real	M 0
do > 2f	2f < di < f real	M < 1
do = 2f	di = 2f real	M = 1
2f < do < f	di > 2f real	M > 1
do = f	no image is formed	- - -
do < f	0 < di < -∞ virtual	M > 1

 

virtual, upright, M < 1
sample ray diagrams

do ∞	di = -f virtual	M 0
any do < ∞	0 < di < -f virtual	M < 1

#2 top of the object, passes through the center of the lens unbent

#3 top of the object, aims for F', strikes the lens, refracts parallel to the axis

#2 top of the object, passes through the center of the lens unbent

#3 top of the object, aims for F', strikes the lens, refracts parallel to the axis (dot back refracted ray)