Subjective and objective refraction. Prescription of glasses.
Refractive error = optical defect that does not allow light to be brought into sharp focus on the retina, resulting in blurred/ distorted vision
Subjective & objective refraction = parts of a comprehensive eye examination during which the doctor determines a patient’s refractive correction, which is done best by a combination of the two types of examinations
Objective refraction
= refractory exam using different instruments, obtained without receiving any feedback from the patient
Examination is done by using Retinoscopy:
- The instrument projects a streak of light, called the “intercept”, that passes through the pupil to eventually shine on the retina & produce a “retinoscopic reflex”
- Parallel alignment of the intercept & retinoscopic reflex indicates the presence of only a spherical error
- If the intercept also coincides with one of the principal meridians, a cylindrical error is also present
- The intercept is then swept across the patient’s pupil & the effect on the retinoscopic reflex is noted
- → If it moves in the same direction “with the movement”, plus lenses are applied
- → If it moves in the opposite direction “against the movement”, minus lenses are applied
- → This procedure is performed until the pupillary reflex fills the whole pupillary aperture & no movement is detected = point of neutralization
Subjective refraction
= subjective refraction requires responses from the patient
- It is performed after the objective retinoscopy for more accurately refining the measured refractive error
- It relies on the patient’s response to alterations in lens power and orientation, using the objective refraction or the patient’s current refractive correction as the starting point
- The spherical correction is checked by small changes, initially increasing the plus power to overcome accommodative efforts → until the clearest image is obtained!
- It is a determination of the combination of sphere & cylindrical lenses