7: depth of focus, depth of field and the pupil

what happens when the pupil size varies

get a variation in retinal illuminance- less light reaches the retina when the pupil size falls

get a variation in the diameter of the blur circle in eyes with refractive error

get a change in the depth of field and depth of focus

implications of changing pupil size exist for what

accomodation testing

testing vision using a pinhole disc

what is the pupil

aperture within the iris, through which light penetrates into the eye

located slightly nasally to the centre of the iris

function is to regulate the amount of light admitted into the eye, to optimise the depth of focs and to mitigate ocular aberrations

retinal illuminance: dependence on pupil diameter

Ret lumin: amount of light falling on the retina

luminance flux T on the retina : T = LS

where L is the luminance of the stimulus and S is the area of the pupil is mm²

T is given in trolands

wy does retinal illuminance decrease with age

due to the absorption of the lens and to a reduction in pupil size

may represent a reduction of at least 10 fold in an 80 year old eye as compared to a 20 year old eye

so amount of light entering the older adult eye may be 10x less compared to yound adults eye

because the illuminance is calculated from the area of the pupil, when the pupil diameter is halved, retinal illuminance is reduced by a factor of 4

blur circle and their dependence on pupil size

myopic eye as rays of light being brought to a point focus in front of the retina

gives rise to a blur circle on the retina

red ra at the top and red ray at bottom. gap between the two corresponds to the blur circle

if the pupil is smaller or made smaller using a pinhole disc , blur circle reduces

myopia remains the same/ uncorrected but its impact is reduces when pupil is smaller as peripheral rays are cut out

depth of field

for a given setting of an optical system, it is the distance over which an object may be moved without causing a sharpness reduction beyong a certain tolerable amount

dof increases increases when diaphragm or pupil diameter reduces in size, such as when the pupil gets smaller

dof is bigger in older eye

dof examples

viewing at infnity, dof is from infinity up to 3.6m from the eye for a pupil diameter of 4mm. at 3.6mm target will still look in focus unless it is closer than 3.6. no change in accom is required between infin to 3.6

so without any change accomodation, objects up to 3.6m appear in focus

but depth of field runs from infinity up to 2.3m from the eyes for a pupil diameter of 2mm

what is depth of field rrange for viewing at 1m

will range from 1.4 to 0.8m for a 4mm diameter pupil

from 1.4-0.8 objects remain clear without accomodating change

what is dof for viewing at 1m when pupil diameter is 2mm

dof ranges from 1.8m to 0.7m - bigger dof as pupil is smaller

dof is much bigger on the right as the aperture is smaller

more detail is seen

eye in focus on B

accomodating to point B and forms image on retina at B’

object moves forward to B1, so eye is under accomodating giving rise to blur on the retina

B2: if target is brought further away it will still be clear

as close as B1: will still be clear

what is depth of focus

for a given setting of an optical system, it is the distance in front and behind the focal point over which the image may be focused without causing a sharpness reduction beyond a certain tolerable amonnt

depth of focus is inversley proportional to the diameter of the diaphragm/ pupil

depht of focus is larger in older eyes as pupil is smaller

units given in D

what is the difference between depth of field and depth of focus

depth of field is the range of object distances in front of the eye that appear clear without changing accom

depth of focus is the range in which the image can shift on the retina whilst still being clear - object doesnt move

depth of focus example

if the depth of focs is 0.50D for an eye looking at an object at a particular distance, the light can be brought to a focus by up to 1/4D in front of the retina or by up to 1/4D behind the retina and the image will still be percieved as in focus

depth of field isin front of eye

dep of focus ar retinal level

at point O, without change in accom eye is accom here

without any change in accom the object will be seen clearly as long as it doesnt fall outside of d-of-field

provided the image of the object falls somehwre along the arow of the d-of-focus , object appears clear

how can we make use of the increased depth of field and depth of focus which accompany a smaller pupil?

increased dof/dofoc make the eye less susceptible to blur

it doesnt remove the cause of the blur but makes us less aware of it

accom testing also impacted by pupil size

pupil size and unaided vision

when the pupil size is reduced, less light nters the eye but the impact of aberrations and uncorrected refractive error is reduced

reduction in blur circle means the same uncorrected refractive errro has a much reduced effect on vision

impact of a given amount of myopia on unaided vision will be greater at night than during the day as pupil enlarges

purpose of pinhole disc

reduces the pupil size artificially so the pupil diameter is 1mm

if the diameter is any smaller than 1mm optical problems can occure from diffraction

pinhole disc

used when practitioner having difficulty improving VA in an eye

if VA has imporved to 90 during subjective refraction but no further improvement, either there is an optical problem or more uncorrected refactive error

or the neural system is not capable of better VA as there could be ie retinal disease

pinhole disc allows us to distinguish between these options

if pinhole in place and va jump from 90 to 110 then the problem is optical

if with pinhole it doesnt improve likely there is a neural problem

amp of accom fall sto 0 at age 60

even tho amp falls to 0 by age 60, graph shows around 1D

this is due to depth of focus

small pupil means that objects from infinity up to 1m away appear clear

inside 1m , blurred

this gives the false appearance that amp of accom is 1D

reduction in pupil size with age helps to maintain clear vision up to 1m despite loss of accom