L6 Flashcards
(12 cards)
What is the difference between a spherical and a cylindrical lens?
A spherical lens refracts light equally in all meridians to a single focal point, while a cylindrical lens refracts light to a focal line because it has two principal meridians with different powers.
What is the axis meridian of a cylindrical lens?
The meridian with no refractive power; orientation is specified in degrees.
What is the power meridian of a cylindrical lens?
The meridian with the maximum refractive power; always perpendicular to the axis meridian.
How is the orientation of a cylindrical lens specified?
By the axis meridian, written in degrees (as if facing the patient, contra-ocular view), with 0° written as 180°.
How is a cylindrical lens labeled in standard UK notation?
By sphere power, cylinder power (difference between meridians), and axis (orientation of the most negative power meridian), e.g., +4.00/-2.00 x 90.
Why must the orientation of a cylindrical lens be specified, but not a spherical lens?
Because rotating a spherical lens has no optical effect, but rotating a cylindrical lens changes its effect due to axis specificity.
What does ‘along’ mean in power cross notation?
It refers to the power meridian, not the axis meridian.
What is the process for correcting astigmatism during retinoscopy?
Identify principal axes, correct one meridian with a sphere, assess the perpendicular meridian, and correct with a cylinder as needed.
Why is working distance important in retinoscopy?
Because the examiner is not at optical infinity; working distance must be accounted for to determine the correct spectacle prescription.
What is the circle of least confusion (COLC) in astigmatism?
The blur circle midway between the two focal lines in an astigmatic eye.
When recording power crosses, which meridian should be noted?
The power meridian, not the axis meridian.
Why does the cylinder value remain the same when correcting for working distance in both meridians?
Because the working distance vergence is added equally to both meridians; the cylinder is the difference between the two.
