optometric practice across human life span- lecture 4 LV calculations

What image would be formed for a positive convex lens if the object was between F and the lens?

- virtual

- upright

- bigger than the object

- on the same side

What would happen if you bring the object closer to a positive convex lens?

- image would be smaller than if the object was further away from the lens

- so in other words: moved object closer to lens = resulting image gets smaller

How would you achieve the highest magnification with a positive convex lens?

position the object as close as possible to the focal point on the object side

When will the field of view be greatest for a single lens magnifier?

when the lens is held closest to the eye

explain the Concept of Focal Point Alignment in Magnifying Lens

When the focal point of a magnifying lens lines up with a specific point inside your eye, it makes everything you look at appear bigger and clear.

You don't need to adjust how you look, and it works for things up close or far away. It's like having magical glasses that keep things in focus and bigger, no matter their distance.

where will the image be formed if an object is placed at the focal length of the magnifying glass?

- the image formed will be at infinity. the parallel rays from the image at infinity means that the image seen with the eye will be subtended at the same angle independent of the distance between the eye and the magnifying glass

Define magnification?

• Ratio between any linear dimension of the retinal image when a magnifying device is used

• Compared with the corresponding dimension when viewed without the magnifying device

What is angular magnification?

- used for telescopic devices (for distance vision)

- it is the ratio for the angle subtended by the image to that subtended by the object

What is the equation for angular magnification?

M = angle subtended by image at eye/ angle subtended by object at unaided eye

M= β/α

M= D objective / D ocular

What is nominal magnification?

• Assumes image size through the eye + magnifier

• Magnifier held at a working distance of one focal length

• Compared with viewing the object unaided at 250 mm from the eye

What are some assumptions made in nominal magnification?

- emmetropic or distance corrected eye

- refernce seeing distance 250mm

- distance between magnifier and eye is ewual to the foal length of the magnifier

What is the equation for nominal magnification?

M= F/4

Simple magnifier - object at f point

- image at infinity formed

-increase angle that is subtended at the retina

- assumes that d the distance between eye and magnifier is very small

- does not matter where eye is positioned

- eye distance from magnifier influences fov - further away , less fov

What is the maximum working distance for an emmetrope with single magnifier

- one focal length of the lens. minimum will depend on accommodation

Why does accommodation influence working distance - single magnifier

More accommodation, more magnification , resulting power increases , we changes

Increase in mag power and wd

- increase mag reduces wd of mag

Trade/maximum magnification - what is it

Mag depends on equivalent power of lens , not vertex power

- assumes 4D of accommodation is used , increases lens power + 4.00 D

What is the equation for trade magnification?

M = 1+F/4

Why use equivalent power?

• Used for calculating magnification in a thick lens system

• Object and image distances are measured from the principal planes

• Each principal plane is one equivalent focal length from the focal points

What is a symmetrical system?

principal planes are symmetrically placed in lens, equivalent focal length similar to back vertex focal length

What is an asymmetrical system?

principal planes are displaced relative to lens, equivalent focal length dissimilar to back focal length

What is the equivalent viewing distance?

• Ratio of the distance to the image produced by the optical system

• Compared with the distance an unaided near object must be held to subtend the same angle at the eye

Exit vergence of stand magnification - greatest mag

- when lens in contact with eye

Exit vergence of stand magnifier

- if object distance from lens is 1focal length

- magnification not impacted

- however fov influenced

Exit vergence of stand magnifier

Image formed below magnifier, user must be corrected and able to accommodate to see at the distance

How does telescope act as

-parallel light detector

-focus on distant object

- light rays focused from infinity assumed to be parallel

-add trial plus lens until distant object is clear

Exit vergence power vs telescope trial power

Opposite

E.g trial telescope power = +5.00

Exit vergence power =-5.00

How does a bright field magnifier work?

- the lens acts as a light collector

-

What does a galilean telescope consist of?

- highly powered -ve lens and low powered +ve lens

- negative closer to eye

-

What does an astronomical telesope consist of?

- two +ve powered lenses