2 - Thin Lenses, Pupils, Stops, Ports

Question 1

How to find power of thin lenses

Answer 1

Treat lens as 2 SSRIs -> calc power of each -> add them together

P = P1 + P2
(Thin lens power) = (front surface) + (back power)

Question 2

Describe downstream/effective vergence

Answer 2

For a lens at some distance (x) in front of a screen, the vergence of light striking the screen is different that the vergence of light leaving the lens
-i.e. light LOSES VERGENCE AS IT TRAVELS further from object

Question 3

Downstream/effective vergence equation

Answer 3

Leff = L/(1-dL)

Effective verg) = (verg at r = 0)/(1-(dist)(verg at r =0)

Question 4

__ of a lens changes depending on where the lens is located in front of the eye, hence the need to account for __ with spectacles

Answer 4

Effective power

Vertex distance

Question 5

Vertex distance equation

Answer 5

Fc = (Fg)/(1-dFg)

Desired power at cornea) = (needed lens power)/(1-(dist)(needed lens power)

Question 6

Vertex distance

• plus lenses
• minus lenses

Answer 6

(+) become weaker as they closer to the cornea (more minus)
-hyperopes require higher plus in cls (more plus)

(-) become stronger as they closer to the cornea (more minus)
-myopes require less minus in cls (more plus)

Question 7

How to find power of thick lenses (not equations)

Answer 7

Dr. Fenska says: “thick lenses: TAKE THE LOSS”

Treat lens as 2 thin lenses that are separated by some material*
Use Gauss system and cardinal points

*called successive imaging

Question 8

Describe aperture stop

Answer 8

Physical entitity
Limits amount of light passing into an optical system
Can be actual aperture (camera), the edge of the lens, etc.
Limits field of view (along with field stop)

Question 9

The pupils is the anatomical __ of the eye

Answer 9

Aperture stop

Question 10

Describe field stop

Answer 10

Limits size of the object that can be imaged by the system

Works with aperture stop to limit field of view

Question 11

Define pupil

Answer 11

Images of the aperture stop formed by lenses in front/behind the stop constitute pupils

Question 12

Describe entrance pupil

Answer 12

Image of the aperture stop formed by all of the lenses in front of it
-if there are no lenses in front, the entrance pupil itself is the AS

Determines the size of the cone of light that actually enters the system

Question 13

How to find the entrance pupil

Answer 13

1) go to axial object point
2) look towards the front of the system (to the right)
3) EP = element/image of an element that subtends the SMALLEST ANGLE
4) therefore, it looks smallest when seen from the object point

Question 14

Describe exit pupil

Answer 14

Image of the aperture stop formed by all of the lenses behind it
-if there are no lenses in front, the exit pupil itself is the AS

Determines the size of the cone of light that actually exits the system

Question 15

If one knows the aperture stop of a system, the entrance + exit pupils can be easily found by

Answer 15

Imaging the AS thru lenses in front of/behind the AS

Question 16

Focal ratio equation

Answer 16

f over __ = f/D
(Focal length over__) = (focal length)/(diameter of entrance pupil)

Example: a camera with entrance pupil diameter of 30mm and a 60mm focal length will have an f/2 focal length

Question 17

Focal ratio of a camera is closely related to __, therefore is conventionally referred to as __

Answer 17

Exposure time

Speed of camera

Question 18

Describe ports

Answer 18

Images of the field stop formed by lenses in front of/behind the stop constitute ports

Question 19

Entrance port

Exit port

Answer 19

EnP: image of the field stop formed by lenses in front of it
ExP: image of the field stop formed by lenses behind it

Question 20

Describe depth of focus

Answer 20

For the eye, the DOFocus is the interval surrounding the retina in which an eye sees an object as in focus

Question 21

Describe depth of field

Answer 21

The interval surrounding the fixation plane in which an object can reside and still be in focus (with NO CHANGE IN ACCOMMODATION)
-i.e. if the object is located within this region, there will be no perceivable blur on the retina/screen

Question 22

Super basic difference b/w DOFocus and DOField

Answer 22

DOFocus: image on retina

DOField: object in front of you

Question 23

Why snellen acuity can improve with pinhole

Answer 23

Pinhole INCR DEPTH OF FOCUS

-note: won’t improve with media opacity/ct

Question 24

General trends

• short focal length ->
• incr in aperture size ->

Answer 24

Short FL -> large DOField

Incr in AS -> decr in DOField, decr DOFocus

Question 25

Describe field of view

Answer 25

Extent of an object plane that’s imaged

• measured as an angle or linear distance
• one common metric measurement is ANGLE OF HALF-ILLUMINATION

Question 26

Field of view

• minus lenses
• plus lenses

Answer 26

(-) lenses = incr FOV

(+) lenses = decr FOV

Question 27

Describe field of fixation

Answer 27

Angle made from optical axis by entrance port as measure at the CENTER OF ROTATION OF THE EYE
-in the eye, the center of rotation is typically 14mm FROM THE CORNEA