BCST 1101 Light and Vision- The Eye

Vision

  • how we see

The eye is like a camera, in a way

  • compound lens

  • aperture

  • diaphragm

  • sensor

  • light absorbing interior

Eye- Camera comparison

  • both camera and eye form images using lenses

  • eye uses fluids to bend light for focusing

  • cameras use multiple pieces of glass

Camera

Eye

Sensor

Aperture

Lens

Black Coating

Retina

Iris

Lens

Choroid

The Human “Sensor” Retina

  • Key parts:

    • fovea

    • optic nerve

    • blind spot

    • photoreceptors rods and cones

Iris is the Aperture of the Eye.

  • The Iris is the opening in the eye that lets light to the Retina

  • The Aperture is the opening in the camera that lets lights to the Sensor

    Iris

    • Iris widens when the light dims and narrows when the light is brght

    • Iris movement is involuntary (meaning you can’t control your iris)

    • adjustments are automatic

    • The iris can help by raising or lowering the light level in the eye

    Main function of the iris

    • reduce aberrations-errors in focusing

    • sharpen image

    • increase depth of field


How do we see?

  • the human eye has a compound lens system

  • compound- more than one element

  • Element in this case is the cornea and the lens

Refraction

  • lenses are dense and transparent, so bend light

  • the curve in the lens helps aims the bent light

Cornea

  • the cornea is a curved window on a container of liquid

  • the liquid does most of the ray bending because of the change in density from air to cornea

What about the different materials?

  • The difference in densities between the liquids in the eye and the lens isn’t significant.

Remember Focal Length

  • The distance from a vertical line through the center or the lens to the “focal point” of the lens

Focus by Accomodation

  • The eye lens is fixed, unchanging distance form the retina at the back of the eyeball where the image is created

  • as the object distance changes, focus is achieved by the eye lans changing its focal lenght

  • this method of auto focusing is called accomodation

Camera by Contrast

  • Rigid Lenses

  • Manual or external adjustments

  • Some lenses require the sensor to move to meet their focal points

  • this is called back focus

Backfocus

  • adjusting the distance between the focal point and the sensor so that they are the same point in space.

Weird Eye tricks : Accommodation

  • The act of adjustment

  • Focusing you eye nearby requires shorter focal length (more bulgy) eye lens than focusing far away, since rays must be bent more for image to fall on retina.

  • is achieved by the eye lenses’ ability to change its focal length by changing its bulge.

  • Why Accommodate? Less bulge means less bending power and longer focal length, More bulge means more bending power and shorter focal length

  • Your eyes has a small depth of field

  • you can’t see something close and far with both objects in focus at the same time.

Retina : The Eye’s Sensor

  • know that the retina doesn't;t cover the entire back of the eye.

  • There is a hole where the rods and cones bundle together and head out to the brain.

  • the hole is your blind spot

  • can’t see blind spot? 2 reasons

    • It’s off to the side where your sight is less acute

    • the brain fills it in

  • Rods and Cones packed into retina

  • Sensitive to light like camera film

  • nerve fibers connect rods and cones to the brain

The retina is made of rods

  • Rods

    • the rods detects the brightness of the image formed on them

    • there are lots of rods

  • Cones

    • cones sense the colour

    • there are many fewer cones than rods

    • this makes us less sensitive to colour than light

Fovea

  • the fovea is the small region near the center of the retina

  • used for sharp, detailed viewing

  • has the most cones (precise colour vision)

  • has no rods (rods are used for low light, less precise viewing)

  • looking directly at someone means their image is on your fovea

  • if the image is not on your fovea, you see them “out of the corner of your eye”

Our eyes scan

  • your eyeball moves to see a sharp image

  • scans to make all parts of an image fall on the fovea

  • Why do we scan?

    • rods and cones

    • sharp and not so sharp

Darkness Adaptation

  • the retina is able to change its sensitivity to light as the light intensity varies by a factor up to 10 to the 13th power.

  • 10,000,000,000,000:1

  • The visual adaptation is happening constantly

  • sometimes you notice it but mostly you don’t

    When it suddenly gets dark

    • your rods and cones can detect lower and lower intensities as time goes on.

    It’s physiological Adjustment

    • at any given time in the dark, the minimum light intensity you can barely see gets lower and lower until you hit the lowest level perceptible

How does the lowering of your detection threshold work?

  • outside in the sun you are mainly seeing by using your cones

  • they enable you to see in full colour

  • this is called photooptic vision

Outside or bright vision

  • when you are outside, your detection threshold is high

Then it gets dark

  • your detection threshold gradually goes down

  • after 7 minutes you can no longer see with your cones

  • you switch to rods which are more sensitive to light

Dark and Monochrome

  • Scotopic Vision

    • your rods don’t allow you to see colours

    • rod sensitivity continues to improve in the dark for another 25 minutes.8