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

1. 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

2. 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

3. 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

4. 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

5. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. 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

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

12. 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.

13. 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

14. 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