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For an eye perfectly in focus, light from a distant star will be imaged on the retina.

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For an eye perfectly in focus, light from a distant star will be imaged on the retina. If the person is looking str - PowerPoint PPT Presentation


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For an eye perfectly in focus, light from a distant star will be imaged on the retina. If the person is ‘looking’ straight at the star, then it will be imaged on the fovea. Star. Light rays from a distant star are parallel. Retinal image of the star, on the fovea . Retinal Image of star 2.

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slide1

For an eye perfectly in focus, light from a distant star will be imaged on the retina.

If the person is ‘looking’ straight at the star, then it will be imaged on the fovea.

Star

Light rays from a distant star are parallel

Retinal image of the star, on the fovea

slide2

Retinal Image of star 2

The position of the retinal images can be determined by tracing the ray that goes through the centre of the pupil. (Compare this with a pinhole camera).

Star #1

Star #2

Visual Angle

Retinal Image of star 1

slide3

Retinal Image of star 2

For an eye perfectly in focus, light from two stars will be imaged at different places on the retina. As long as the visual angle is large enough (larger than one minute of arc for

the ‘normal’ eye) the stars will be seen

as distinct from each other - i.e. they

will be resolved.

Star #1

Star #2

Visual Angle

Retinal Image of star 1

slide4

For an Ametropic eye (in this case a myopic eye) the size of the blur circle on the retina depends upon the pupil size. The object is a distant ‘point’ object, e.g. a star and so the light rays enter the eye parallel. (The blur is a circular patch because the pupil is circular)

Light rays from a distant

star are parallel

Focal point

slide5

Given that the size of the blur circle on the retina depends upon the pupil size, as the pupil gets smaller the blur patch also gets smaller - even though the refractive error is unaltered.

slide6

With a very small pupil the blur circle approaches a pinpoint.

Although the light is not in focus on the retina, the retinal image of the star will still be fairly clear. This is the same principle as that of a pinhole camera.

Note that the centre of the blur

circle is in the same position as the

image would be if it were in focus.

slide7

Light from a second star, off

to the side of the first, will

also form an image (a blur circle)

on the retina.

Note, again, that the centre of the blur circle is in the same position as the

image would be if it were in focus.

slide8

With a large pupil the two blur circles will overlap,

and when this happens the two stars cannot be resolved.

Although this diagram looks complicated, it is simply a combination of the previous slide and the slide three beforehand. Each of these previous slides showed light from a single star, whereas this slide shows light from them both.

slide9

With a small pupil, however, the two blur circles will not overlap, and so the two stars can now be resolved, even though they are not in perfect focus.

Star #1

Star #2

slide10

An image of a test-chart letter

that is in focus on the retina

E

E

An image that is slightly out of focus on the retina. Here each point on the object forms a blur-circle on the retina

slide11

E

E

When the image is further out of focus, the blur-circles will overlap, and the letter can no longer be resolved.

slide12

An image of a test-chart letter that is OUT OF FOCUS BY THE SAME

AMOUNT AS IN THE PREVIOUS SLIDE, but the pupil is small

E

E

Although the image is out of focus on the retina, the blur circles are smaller, and so the letter can be resolved.

slide13

An image of a test-chart letter

that is in focus on the retina

E

E

An image that is slightly out of focus on the retina. Here each point on the object forms a blur-circle on the retina

slide14

An image of a test-chart letter

that is in focus on the retina

E

E

An image that is slightly out of focus on the retina. Here each point on the object forms a blur-circle on the retina

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