2 - Thin Lenses, Pupils, Stops, Ports Flashcards
How to find power of thin lenses
Treat lens as 2 SSRIs -> calc power of each -> add them together
P = P1 + P2
(Thin lens power) = (front surface) + (back power)
Describe downstream/effective vergence
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
Downstream/effective vergence equation
Leff = L/(1-dL)
Effective verg) = (verg at r = 0)/(1-(dist)(verg at r =0)
__ of a lens changes depending on where the lens is located in front of the eye, hence the need to account for __ with spectacles
Effective power
Vertex distance
Vertex distance equation
Fc = (Fg)/(1-dFg)
Desired power at cornea) = (needed lens power)/(1-(dist)(needed lens power)
Vertex distance
- plus lenses
- minus lenses
(+) 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)
How to find power of thick lenses (not equations)
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
Describe aperture stop
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)
The pupils is the anatomical __ of the eye
Aperture stop
Describe field stop
Limits size of the object that can be imaged by the system
Works with aperture stop to limit field of view
Define pupil
Images of the aperture stop formed by lenses in front/behind the stop constitute pupils
Describe entrance pupil
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
How to find the entrance pupil
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
Describe exit pupil
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
If one knows the aperture stop of a system, the entrance + exit pupils can be easily found by
Imaging the AS thru lenses in front of/behind the AS
Focal ratio equation
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
Focal ratio of a camera is closely related to __, therefore is conventionally referred to as __
Exposure time
Speed of camera
Describe ports
Images of the field stop formed by lenses in front of/behind the stop constitute ports
Entrance port
Exit port
EnP: image of the field stop formed by lenses in front of it
ExP: image of the field stop formed by lenses behind it
Describe depth of focus
For the eye, the DOFocus is the interval surrounding the retina in which an eye sees an object as in focus
Describe depth of field
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
Super basic difference b/w DOFocus and DOField
DOFocus: image on retina
DOField: object in front of you
Why snellen acuity can improve with pinhole
Pinhole INCR DEPTH OF FOCUS
-note: won’t improve with media opacity/ct
General trends
- short focal length ->
- incr in aperture size ->
Short FL -> large DOField
Incr in AS -> decr in DOField, decr DOFocus
Describe field of view
Extent of an object plane that’s imaged
- measured as an angle or linear distance
- one common metric measurement is ANGLE OF HALF-ILLUMINATION
Field of view
- minus lenses
- plus lenses
(-) lenses = incr FOV
(+) lenses = decr FOV
Describe field of fixation
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
