What are we compensating for in single vision freeform lenses? Let’s start by answering why back vertex distance (BVD) is compensated in ophthalmic lenses. We learned in basic optics that moving a lens closer or further from the eye changes the effective power perceived by the wearer. When the BVD is closer, minus power increases, and plus power decreases. Although the change in effective power is nominal for powers less than +/-7.00 D in modern personalized lenses, even nominal deviations in power from the prescription can be compensated, so why not provide the patient with the most precise reproduction of the vision they experienced during refraction?
Ideally, ophthalmic lenses reproduce the power as prescribed in the patient’s finished eyewear. However, to reproduce the prescribed power in the finished lens, the back of the lens surface must sit the same distance from the eye (corneal apex) as the distance from the phoropter lens to the corneal apex during refraction. The dispenser typically does not know the distance of the phoropter lens from the patient’s eye, but to get the full advantage from freeform personalized lens technology, the BVD is provided for the measured fit of the frame. The freeform software calculates the lens surface requirements to produce the prescribed power in the lens based on the actual as-worn BVD along with many other parameters (Rx, refractive index, base curve).
Furthermore, to reproduce the prescribed power, the as-worn lenses should form the same vertical and horizontal angles between the lens optical axis and the patient’s visual axis as the phoropter lenses. But we know that the phoropter lenses sit flat forming a 90-degree angle between the optical axis of the lens and the visual axis. However, the frames as worn do not sit flat. Instead, frames sitting on the face have pantoscopic tilt and faceform. Personalized freeform lenses can compensate, but the software needs the actual position of wear to produce the same effective power in a tilted lens as the prescribed power derived during refraction with flat lenses. Ideally when the eye is in the primary position, looking straight ahead through the lens’ optical center, a clear image forms on the far point sphere, the imaginary spherical curve representing the ideal focal points corresponding to the curvature of the retina, for all gaze directions. However, in the real world, lenses are tilted in front of the eyes, and we look through points on the lens away from the optical axis. Both scenarios form angles between the lens optical axis and the patient’s visual axis leading to lens aberrations. When angles are formed between the lens optical axis and the patient’s visual axis, lens aberrations degrade vision. Lens aberration is a failure of a spectacle lens to sharply focus an image on the retina. The prescribed power of the lens is intended to focus images at the far-point sphere of the eye. For a deep dive into the topic, check out the free CE from HOYA “360° of Single Vision Comfort – MySV™” at 2020mag.com/ce.
• Deborah Kotob
Pro to Pro Director