Optics review

What is a way to determine L' of a stand magnifier?

- L' is the vergence of light emerging from a stand mag - can use trial lenses or a ret bar to find the highest power convex lens that allows clear vision of the virtual image for an eye corrected for refractive error positioned close to the stand mag lens

A person with a 5mm pupil looks through a 4X12 telescope, how much light is lost?

- a 4x12 TS has a 12/4 or 3mm exit pupil - 3^2/5^2=9/25=0.36 so 36% of the light is lost, plus any light lost at optical surfaces - the person will see things more dimly than without the telescope

For a prism half eye, what is the assumed PD at near and what is the effect of anything other than this PD?

- assumed PD of 64 - if patient PD is >64,there is a BI effect from the lens so BI increases - if patient PD is <64, there is a lower BI effect

When looking at a person's eye, their pupil is an image from the ___________________ which is ___________ and _____________. Where do we find the entrance pupil of an eye?

- cornea - larger and closer - in front of the actual pupil

For what VA does Kestenbaum's rule work for?

Distance or Near VA ex. 0.2/1.6M 1.6/0.2=8D to read 1M

Formula for enlargement ratio (ER) of a stand magnifier.

ER=(L'-F)/L' L'=vergence of light emerging from the stand mag (always -'ve, D) F=equiv. power of the lens (always +'ve, D)

CCTV is what type of magnification?

Electronic

What kind of image is formed for a convex lens if the object is placed between the focal point and the lens?

Erect and virtual outside of the focal plane of the lens.

Formula for magnification of a telescope with a cap (telemicroscope).

M(tms)=M(cap) X M(ts) M(tms)=rF X M(ts)

Formula for magnification of a hand magnifier if you know the distance held.

M= 1 + hF M=mag h=dist from lens to eye in m F=diopter strength of the lens

Telescope magnification power by lens diameter

M= entrance pupil diameter / exit pupil diameter

A patient reads 4M print at a distance of 40cm through a +2.50 add over the best correction. What size print could she read with a +5.00D hand-held lens held 20cm from the print?

M=rF M=(0.4)(5) M=2X so she should be able to now read 2M print with the print held 20cm from the lens

What is the counting method of predicting near add?

count up and down the logMAR chart to get required add or size of print

What is CPS?

critical print size

What is epidemiology?

the branch of medicine that deals with the incidence, distribution, and possible control of diseases and other factors relating to health.

What happens when an object is held beyond the focal point of a convex lens?

the image is inverted

Telescope brightness formula

TS reduction in brightness = (exit pupil diam.)^2 / (eye pupil diam.)^2

How would one determine the equivalent power of a stand magnifier alone?

- hold it up to an object at a fixed distance and size - use a CL reticule to measure the image size - with similar triangles, use the ratio of the distance/object size=f1/image size to find f1 which is the focal length of the lens - 1/f will give the D power of the lens

What is the goal of a telemicroscope when used for non-reading, mid-range tasks such as music?

- maximize RDM to preserve VF - for music, determine the closest distance at patient can accomplish the task

What is the estimated near pd using distance pd and add power? What pd requires an "adjustment?"

- near pd is 1.5X the distance pd, subtracted from the distance pd - if the pd is >65, do the above but add 1 more, if <=65, do the above

A stand magnifier enlarges 3X and has an image location 30cm behind the lens. The patient is totally presbyopic and has a reference acuity of 0.25/8M through a +4.00 add. She prefers to hold the magnifier 14.5 cm from the spectacle plane. What size print should she read with this stand magnifier, and what add would be needed?

- the image will be 44.5 cm from the eye (30+14.5=44.5), so +225 add - M(total)=RDM X ER - M(total)=(25/44.5) X 3 - M(total)=0.56 X 3 - M(total)=1.68X Magnification=Referemce Size/Goal Size - 1/68X=8M/Goal Size - Goal size=8M/1.68X - Goal size=4.76M

How does one construct a custom prism? What is the amount of prism used and what PD is used?

- use 2 pd BI more than the near ADD power in each eye - calculate the near PD

Magnification of a telescope (lens power)

-(Fe/Fo)

The Bailey-Lovey chart has a progression of ____________________ between each line.

0.1 log unit

How do you record a patient reading 6M print at 40cm?

0.4/6M

What is the MAR of a 20/20 letter?

1' which is the "resolving capacity"

As with hand magnifiers, we can't trust the X power of the label, but why if you are given the D of stand mags, you don't know the mag? (2 things)

1. 2 different stand mags of the same optical power may have different heights, resulting in different ER and i (dist to image) 2. 2 different people can have different adds which = diff. mag

How do you predict the near magnification based on continuous text reading performance?

1. hold chart at a specified distance based on add, etc. 2. figure out the CPS (critical print size = smallest print that can be read at the maximum reading speed) 3. CPS replaces the near acuity to determine the predicted add power

What are 4 ways of predicting near magnification?

1. visual acuity equivalency 2. base it on letter visual acuity - Kestenbaum's rule 3. counting method 4. base it on continuous text reading performance

What is the logMAR of 10'?

1.0

What is the size of a 1M letter?

1.45mm tall (tan 5'=x/1000mm)

if the CPS is 0.4/1.6M what is the predicted near add for 1M?

1.6/0.4=+4D Add

One minute of arc is ________ of a degree.

1/60

If we need 20% reserve on the MN Read for a predicted add, what should we use for Kestenbaum's rule? If the resultant effect is different with the patient, what can we assume?

100% or 2X with Kestenbaum if the predicted add of MN Read is much more then there are problematic scotomas when reading vs letters

How much prism is in each lens of a prismatic half eye.

2 pd more than the dioptric power per lens

What is the MAR of 20/50 and the logMAR?

2.5' MAR and 0.4 logMAR

For predicting near magnification, how is "visual acuity equivalency" used? Show by using a VA example, let's say somebody who wants to read 1M print with a VA of 20/200.

20/200 = x/1M x=0.1m=10cm (the EVD), therefore 10D is the equivalent power or Fe

Definition of legal blindness

20/200 in the better eye or less than or equal to 20 degrees in the widest diameter

A lower contrast chart example would be what?

25% contrast chart has higher contrast than a 9% contrast chart.

0.4 M is what in N notation

3N

Five minutes of arc is __________ of a degree.

5/60

approx size of a 20 foot (6M) letter at 20 feet

8.75 mm (figure out by tan 5' = x/6)

1M is what in N notation

8N

What is an optotype?

A standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols

Vergence amplification of a telescope

A(TS)=A X M^2 (A(TS)= Acc demand of TS, A= Acc demand of target without the telescope)

Formula for angular magnification

AM=magnified size/unmagnified size

What kind of system is a telescope?

Afocal, has a dioptric power of 0

Why is it not always correct that the magnification of a lens is always determined by dividing its dioptric power by 4?

Because not everybody has a reference distance of 25cm (4D).

For formula M=rF, why does the distance from the eye to the lens not matter?

Because we are measuring r, the "reference distance" which is the distance of the object without the lens in front.

If a patient has better contrast sensitivity binocularly, what type of visual aid should be tried?

Binocular

What is the "eye relief"

Eye relief is the distance (in millimeters) between your eye and the binocular/ spotting scope eyepiece that allows the full field of view to be comfortably observed. It measures the spacing from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. Your eye should be positioned here to see the full field of view of the eyepiece

Formula for equivalent power of a telemicroscope.

Fe= Fcap X M - Fcap is the power of the cap - M= mag of the TS

What is the equivalent power of a stand magnifier?

Fe=ER X Add Power

A simple magnifier used with an add formula.

Fe=F1+F2-(t)(F1)(F2)

Formula for the equivalent power of two lens systems.

Fe=F1+F2-(t)(F1)(F2) t is in meters

What is the equivalent power of an electronic magnifier?

Fe=M X Add - a function of angular mag ("enlargement ratio)=M and working distance

What telescope has a (-) ocular?

Galilean

2 types of tesescopes

Galilean and Keplerian

If someone needs 10X magnification to read 1M, what power lens will they need if they read 10M at 40cm? What would a manufactured call a 10X in terms of diopters? Why?

M=rF 10=(0.4)(F) F=25D manufacturer will use a 25cm reference point (4D) so they will call this a: 10=(0.25)F F=40D or, the 25D lens will be called a 6.25X

Formula for magnification provided by a hand magnifier or a microscope, when the object is in the focal plane of the lens.

M=rF where r is the reference distance (m) and F is the dioptric strength of the lens

What are some examples of a continuous text reading card?

MN Read, SK Read, Bailey-Lovie Word Reading Card

The area of a magnified image of a telescope is _________ of the unmagnified image.

M^2

Magnification of a telescope (by lens power and exit pupil)

Mang = - F2/ F1 where F2 = power of eyepiece and F1 = power of objective lens Mang = 1 / (1-dF1) where d = the tube length of the telescope F1 = power of objective lens Mang = objective entrance pupil diameter / eyepiece exit pupil diameter

How can you figure out the size of a letter in M notation if it is not labelled?

Measure the size of the letter and divide by 1.45m

Telescopes have what lenses?

Objective and Ocular

What is the best way to figure out the final Rx of lenses in a trial frame?

Place the trial frame in a lensometer to measure back vertex power of the combined lenses rather than adding the combined lenses.

The total magnification provided by a stand magnifier is a combination of two events. What are they?

RDM (change in distance so get relative distance magnification or minification) and as well as enlargement by the lens itself.

Formula for RDS (relative distance magnification)

RDM=r/d r=reference (original dist.), d=new distance

Formula for RSM (relative size magnification)

RSM=New Size/Reference Size

What is the definition of a 20 foot letter?

Subtends 5' or arc at 20 feet

What is the definition of an 1M print?

Subtends an angle of 5' arc when located 1 M from the eye.

Total Magnification of system

TM=RSM X RDM X AM (maybe an example would be a CCTV at a certain distance with a telemicroscope)

How does one calculate the MAR? What is the MAR of a 20/200 letter?

Take the reciprocal of the VA quotient. Ex. if VA is 20/200, the quotient is 0.1 (which is the decimal acuity), reciprocal is 10, so the MAR is 10' (the MAR of a 20/20 letter is 1')

Detailed definition of the exit pupil.

The exit pupil is the width of the beam of light leaving the eyepiece, usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of our Sport Optics should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5 mm and 9 mm (the maximum amount for the human eye). Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam falls on your iris and doesn't go into your eye. To calculate the exit pupil, simply divide the size of the objective lens by the magnification of the binocular or spotting scope. For example, the exit pupil of 7x42 binoculars is 6 mm.

Why is the term enlargement used instead of magnification with a stand magnifier.

To avoid confusion with total magnification, which is a result of enlargement by the lens and RDM

For a lens with an add, once the Fe is determined, how would you figure out the magnification?

Use M=rF, assuming the object is located at the anterior focal point of the system and F is actually Fe for the system

For a convex lens, the max angular magnification is achieved where? Where is the image?

When the object is placed at the focal point of the lens At optical infinity.

What is adaptive technology?

adaptive technology refers to special versions of already existing technologies or tools that provide enhancements or different ways of interacting with the technology. The adaptation helps individuals with a disability or impairment accomplish a specific task.

What is the telescope rule of 57

at 57 cm, 1 cm = 1 degree (can change the units) or at 114 cm 2 cm = 1 degree

Why do people hold a magnifier within the focal point of the lens?

because at the focal point less of the object can be seen than within the focal point

Why do we add only a 20% reserve on the predicted add on the MN Read?

because this is not letter acuity we are using

why is near snellen a problem?

because you may have a card that is calibrated at 16" but measured at 10", so saying 20/100 at 10" is meaningless

How is RDM determined in a stand magnifier?

by comparing the reference distance to the distance of the image (so you need to know where the image is located)

It is best to prescribe lenses based on what? What should we not use?

dioptric strength, not a designated "X" value given by a manufacturer, since the reference distance used by the manufacturer may differ from that used by the clinician

When a hand magnifier is held with the object not at the focal point but within, it is acting like a ____________________.

stand magnifier

When a dioptric power is specified by a manufacturer, it is not necessarily the _____________________

equivalent power

How to use the formula M=rFe to figure out the "visual acuity equivalency" to predict near magnification.

ex. M=r*Fe if want to go from 6M to 1M mag is 6X, r=ref. distance ex 0.4, F=power needed over the DV Rx for 1M in this case its 15D

If I put my eye at the exit pupil of the same size, what is the benefit of light in relation to magnification?

get a benefit of light also by M^2

How do you determine the image distance (i) of a stand magnifier?

i=-100/L' the answer is in cm

What does M=(Fe/4) + 1 account for

it accounts for the 4D of accommodation or add needed for 25cm

What does M=Fe/4 mean?

it compares the magnified image to the unmagnified image held 25 cm away

what does a higher Add do to the magnification of a stand magnifier?

it decreases the working distance which provides RDM

How can you avoid failure with stand magnifier or to prepare?

label each magnifier in the diagnostic set with its enlargement factor and the position of the image (to ensure that the patient has the correct bifocal in place or holds the magnifier at the correct location to use the bifocal that he or she has)

Formula for linear FOV of a magnifier.

linear field=(df)/h d=lens diameter f=magnifier focal length h=distance from the lens to the eye *** all of these are the same unit

Because the B-L chart has progression of letter sizes and spacing based on the logarithm of the _______________________ , the chart is referred to as a ________________________

minimum angle of resolution logMAR chart

for a myope using a prismatic half eye, what happens to the BI effect?

myopes will get less than the desired BI effect as they have to move print closer

What is the exact formula for near pd from the distance pd and add?

near PD=dist PD X (W/W+h) where W=WD in mm based on add, h=dist from the spec lens to the eye's center of rotation in mm (approx. 27) so formula could be: near PD=dist PD X (W/W+27)

A patient is totally presbyopic and has a best corrected near acuity of 0.4/4M through a +250 add. She tries a stand magnifier that has an enlargement of 2.5X and an image location of 20 cm behind the lens. How can she use this stand magnifier to read 1M print?

needs 4X to read 1M, has only 2.5X, so needs 1.6X based on RDM - M(total)= ER X RDM - 4=2.5XRDM - 1.6=RDM - so 0.4/1.6=0.25 so need image at 25cm but the stand image is 20cm so the eye has to be 5cm from the lens and will need a +4 add

Is any light lost if the exit pupil is larger than the eye's entrance pupil?

no, the only light lost is at the optical surfaces (the eye pupil is "filled" with light)

For a hand magnifier, if the object is located in the focal plane of the lens, the image is formed at _________________. Parallel light leaves the lens so the image will be in focus when viewed through __________________.

optical infinity the best distance correction

where is the exit pupil for a Keplerian telescope? Galilean?

outside of the telescope; within the telescope

Where is the ideal location to put your eye to get a "light funnel" with a telescope?

put your entrance pupil at the exit pupil of the telescope

When reading with a continuous text reading card, what do you instruct the patient to do?

read as fast and as accurately as you can

For predicting the near add, what is Kestenbaum's rule?

reciprocal of the VA fraction gives the ADD necessary to read 1M print

A lens provides the ________ magnification regardless of how far it is held from the eye as long as the object is maintained at the __________________. Why?

same focal point of the lens Reason: because as you bring it closer you get RDM increasing but MAM (max ang mag) decreasing so the total mag remains constant.

A good LV VA chart should have optotypes of equal __________________, have tasks of ________________ difficulty on each line, and have a ____________________ relative change in optotype ___________________ between all lines.

spacing equivalent consistent size

The exit pupil is an image of what?

the objective

How is magnification determined after using the formula for equivalent power of two lenses.

use the formula M=rF

A telescope focused for near acts as a ________________.

vergence amplifier

The formula M=rF applies when the object is held at the focal length of the lens but when does it apply when the object is held within the focal length of the lens?

when the distance from the lens to the eye is the focal length of the lens

In what situation will a lens with an add be equal to the lens alone?

when the lens is held at the focal length of the lens Fe=F1+F2-(t)(F1)(F2) since t*1/t=1 the F2's cross out so Fe=F1

When is Fe<F1 in a lens with add system (system has less power than the lens alone)?

when the lens to eye distance is more than one focal length of the lens (do not use the add in this case)

When is Fe>F1 in a lens with add system?

when the lens to eye is less than the focal length of the lens Fe=F1+F2-(t)(F1)(F2) in this case (t)(F1) is <1

What will a binocular telemicroscope require?

will require TS's or BI prism caps

What happens when you hold a magnifier at the distance of the focal length of the magnifer + the focal length of the add?

you get an inverted and minified image, like a reverse Keplerian