Extended depth of vision (EDOF)
Light entering the eye is usually focussed very sharp at one point only. When we perform procedures to reduce a patients’ spectacle dependence, we imitate this sharp focus at the back of the eye. This used to be the rule with both the artificial lenses implanted during cataract procedures, as well as corneal laser procedures. This sharp focus however limits the distances that can be seen clearly by our eyes after the procedure. In a young individual their own natural lens can change the focus distance after a laser procedure, and therefore if they can see clearly at distance after the laser procedure, they can also see near, for reading. This is called accommodation. Our eyes start to lose this focusing ability due to the stiffening of our own lens in our forties, a condition called presbyopia. The laser procedure or artificial intra-ocular lens cannot change the focus of the eye from distance to near (it cannot accommodate), and therefore does not allow both sharp distance and near vision.
Extended depth of vision provides the solution for patients with presbyopia. This allows patients to see well at distance, but also improves their closer vision and therefore provides spectacle independence. By carefully and accurately changing the aspheric surface of the cornea or the intra-ocular lens, the focus is elongated to permit sharp vision over more distances.
During a LASIK or advanced surface ablation procedure the front of the cornea is reshaped and the focussing changed to a longer area. Patients that start to experience problems with reading in their forties benefit tremendously from the procedure. The procedure addresses both the spectacle error for distance vision, and also elongates the focussing distance to improve closer vision as well.
EDOF intra-ocular lenses implanted during cataract procedures are becoming the norm for a standard procedure, where a multifocal intra-ocular lens is not implanted. The shape of the lenses elongate the focus, thereby improving vision for both distance and closer work. The EDOF lenses does not permit the complete spectacle independence permitted by multifocal intra-ocular lenses, but by balancing the focus distance between the two eyes, patients are provided with spectacle independence for most situations.
With multifocal lenses, approximately 95% of patients are completely spectacle independent, but with EDOF lenses, approximately 70% of patients are completely spectacle independent. With multifocal lenses the vision is very similar in both eyes, but with EDOF lenses the aim is for some difference and overlap in the focus distance of the two eyes. This is called balanced vision.