W8 Vision Sphere

What is an objective procedure?

Retinoscopy

When do you carry out subjective refraction?

When no objective refraction (ret) is carried out

How is subjective refraction carried out?

Find best vision sphere, check cyl with fan and block or jcc, recheck sphere

What is the definition of best vision sphere (BVS)?

The most plus or least minus spherical lens that provides the best visual acuity

What is the circle of least confusion?

Dioptric midpoint between the anterior and posterior focal lines

What does BVS do?

You move the circle of least confusion onto the retina

What are the 2 BVS methods?

Plus/minus technique and maximum plus to maximum visual acuity (MPMVA)

How do you do the plus minus technique?

…

Which lenses influence accommodation?

Positive lens relax accommodation, negative lens stimualte accommodation

How do you do the MVMPA technique?

…

What is fan and block generally used for?

Determine the axis and amount of astigmatism- often used when Jackson Cross Cylinder cannot be done

What are the fan lines in fan and block?

Fixed at 10 degree intervals and marked to indicate the axis setting of the minus cylinder used in the test

What is Maddox V?

Determines the meridian of the posterior focal in the astigmatic eye- Maddox V and blocks underneath can be rotated to any setting

What happens once BVS is in place?

Circle of confusion will be close to/on the retina

What does halving the cyl to estimate sphere do in fan and block?

Brings the posterior focal line onto/near the retina, helping us to find principle meridian of astigmatism

How do you know astigmatism has been corrected in fan and block?

Each cyl increase pushes anterior focal line towards the retina so when the parallel and perpendicular blocks appear equally clear astigmatism has been corrected

How do you refine the sphere after fan and block?

Plus/minus method or duochrome

How do you alter sphere and cyl when refining sphere?

+1.00DS sphere prevents overcorrection in sphere with negative lens

+0.50DS cyl prevents overcorrection of astigmatism

How is Maddox V used to determine principle meridians in astigmatism?

Rotate the arrow towards the darkest fan line so both sides of the arrow head should be equally clear