# Blur Circles

```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
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).
Retinal Image of star 2
Star #1
Visual Angle
Retinal Image of star 1
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.
Retinal Image of star 2
Star #1
Visual Angle
Retinal Image of star 1
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
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.
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.
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.
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.
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
An image of a test-chart letter
that is in focus on the retina
An image that is slightly out of
focus on the retina. Here each
point on the object forms a blurcircle on the retina
When the image is further out of focus, the
blur-circles will overlap, and the letter can
no longer be resolved.
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
Although the image is out of focus on the retina, the blur
circles are smaller, and so the letter can be resolved.
An image of a test-chart letter
that is in focus on the retina
An image that is slightly out of
focus on the retina. Here each
point on the object forms a blurcircle on the retina
An image of a test-chart letter
that is in focus on the retina
An image that is slightly out of
focus on the retina. Here each
point on the object forms a blurcircle on the retina
```