Spectacle and contact lens prescriptions are different in value but provide the same clarity of vision to the patient. It is important to assess the refraction of the patient before prescribing the lenses, especially if the contact lens is expensive to order.
When a lens power is at 4.00 D and above, you need to factor in the vertex distance between the back surface of the lens and the front surface of the cornea. Furthermore, bringing a -4.00 D lens closer to the cornea increases the dioptric power of the lens, thus, the power should be decreased to compensate for the decrease of the vertex distance.
The concept is the same for a +4.00 D lens, but the effect is the opposite. Bringing a +4.00 D lens closer to the cornea actually decreases the dioptric power of the lens, thus, the power should be increased for compensation.
To compute for the Effective power (ED) of the lens, we refer to the following formula:
EP = D/ [1 - (VD * D)]
D = power of the lens
VD = vertex distance (in m)
Example: Spectacle Refraction of RE: -4.50 DS Vertex Distance: 12 mm
EP = D/ [ 1 - (VD D)]
= -4.50/[ 1 - (0.012 -4.50)]
= -4.50/[ 1 - (-0.054)]
= -4.50/ (1 + 0.054)
= -4.50/ (1.054)
= -4.2694497 D
Since we only prescribe in 0.25 increments, we round it off to the nearest quarter of a diopter, so the EP is -4.25 D
For toric contact lenses, you may refer to the effective power chart conversion provided by Alcon here.