Lec 11 pt 2

Question 1

• generated by any refracting surface,
  including a spherical one.
• induces significant blur only in the peripheral retina.

Answer 1

Off-axis (oblique) astigmatism

Question 2

• generated by a toric surface.

- can induce significant blur across entire retina, including the fovea.

Answer 2

On-axis astigmatism (cylinder)

Question 3

variation in meridional power for off- axis source that results in two line foci instead of a single image point

Answer 3

off axis (oblique) astigmatism

Question 4

the rule for off axis

Answer 4

if you displace the object along some meridian (vertical in this example), the lens acts like a plus cylinder with axis perpendicular to the meridian (horizontal axis in this figure).

Question 5

where is the tangential surface compared to sagittal surface?

Answer 5

in front of sagittal surface

Question 6

Oblique astigmatism causes the spatial contents of the object to be what?

Answer 6

to be separated onto two curved image surfaces.

Question 7

saucer is for

Answer 7

sagittal

Question 8

teacup is for

Answer 8

tangential

Question 9

the more astigmatism, what happens to focal points?

Answer 9

the further away focal points are (blurry image)

Question 10

if oblique astigmatism did not exist, the image of a plane would fall on a parabolic surface called the Petzval image surface.

Answer 10

curvature of field (petzval curvature)

Question 11

Target vergence for a flat object (decreases/increases) at periphery?

Answer 11

decreases off- axis, causing image to lie on a curved surface.

Question 12

The natural curved profile of the retina helps what?

Answer 12

minimize the impact of field curvature on refractive error of peripheral visual field.

Question 13

does the physical curvature of the retina overlap with the petzal surface?

Answer 13

NO. The eye also contains substantial oblique astigmatism that causes the sagittal and tangential image surfaces to lie anterior to the Petzval surface.
If the retina lies in the middle of Sturm’s interval, retinal blur is minimized and image quality at the retina is maximized. This happens for some eyes, but not all.

If so, the retinal image of flat objects would be free of blur from curvature of field.

Question 14

where is the retina located

Answer 14

in the middle of sturm’s interval (somewhere between the sagittal and tangential surface); by nature eye tries to minimize image blur