I.  Introduction --  A camera is an interesting example of an optical instrument.  It is especially interesting for this course because it shares many important similarities with the eye.

In both the eye and camera, light coming from source, or reflected from some object,  is focused with a lens onto a detection plane.  In the eye, the detection plane is the retina, where cells which are sensitive to changes in light intensity encode the image and send it to the brain.  In the camera the detection plane contains either some type of film -- where chemicals that darken in response to light are found -- or some type of electronic detector, in which light intensity is converted to an electric signal.

Today we look at some very basic features of a camera.  Later we will see how this are also found in the eye.

Light "intensity" refers to how strong the light stimulus is.  More intense light generally appears brighter to us.  In physics terms light is more intense when MORE PHOTONS OF LIGHT REACH A CERTAIN LOCATION OVER A GIVEN AMOUNT OF TIME.

IN THIS DISCUSSION WE WILL CONSIDER THE FUNCTION OF A BLACK AND WHITE CAMERA.  WE WILL CONSIDER COLOR LATER IN THE COURSE.

A BLACK AND WHITE CAMERA CAPTURES THE SPATIAL DISTRIBUTION OF LIGHT INTENSITY THAT REFLECTS FROM AN OBJECT, WHICH ALLOWS US TO RECOGNIZE THE SHAPE AND TEXTURE OF THE ORIGINAL OBJECT.

II.  Basics of camera function

A camera has to achieve a few basic functions.  It has to focus the light from some object onto a plane.  It also has to control the amount of total light that strikes the detector, so that the detector can collect enough light when the intensity is low, or avoid overstimulation when the light intensity is very high.  Thus the first step is focusing the image onto some sort of detector.

In most cameras the principles of bringing the image into focus are straightforward.  A convex lens is used to create a real inverted image that falls on the detector plane.

If all the points or objects of interest are very far away then they will come to focus at the focal length of the lens.  However as objects move closer the image will actually move behind the focal distance.  Where then should the detector plane be located relative to the focal point of the lens.

In truth the detector plane should be behind the focal point.  The trick to bringing objects from different distances into focus, is simply to move the lens back and forth.

Moving the lens back and forth to bring the desired image into focus on the detector plane is called FOCUSING.

III. Controlling light intensity

What is an image?  It is a distribution over space of light intensity (or color, but we won't worry about that here) that matches the distribution of intensity at the object.

Ultimately the light is detected by some sort of film or chip.   In order to capture an image it is necessary to have some kind of detection device which responds to differences in light intensity.  In film there are small chemical grains which darken when struck by light.  The more photons they absorb the darker they become.  This leads to the creation of a NEGATIVE IMAGE -- because more light makes the film dark.

In video and digital cameras some sort of chip is involved in light detection.  Tiny detectors put out an electrical signal whose amplitude is proportional to the photons striking the surface.

Every sort of detector has what is known as a dynamic range  --- i.e. it can only record a certain level of light difference.  In order to recreate an image most of the variation in light  must fall within the range that the detector can handle.   If light levels are below its minimum response it cannot be detected.  At the upper intensity range the detector saturates -- i.e. it can tell the differences between different levels.

The difference in light intensity from a dark forest, or inside a room to brightly sunlight can be a factor of 1 or even 10,000x.  No detector can handle that range.  We have to have a way to control how much light gets to the detector in order to keep the light striking it within its operating range.
 

There are two main ways to do this:   either with a diaphragm -- which is a hole that sits in front of the camera lens and controls how much light (how many rays) gets it.   We can also control how long we let light strike the detector.  In still photography this is accomplished with a shutter which controls exposure time.  We open the shutter and let photons strike the film for a fixed amount of time.

The opening of the diaphragm is described by the f-stop.  The length of time the shutter stays open is the shutter speed.

IV.  Exposure time and video.

Light arrives in units called photons.  Each pixel, or dot on film sits there and collects photons as they arrive.  If the image being sampled is moving, it will blur.  In other words the image will slide over many pixels.   In still photography one tries to use short exposure times with moving objects so they won't blur on the film.

Video cameras capture a rapid series of still images.  In order to accurately encode motion the camera must sample the scene, then stop and start over again.

The camera actually takes a series of finite measurements or frames.  It sits there and each detector element "counts" photons for some short finite period.  It then stops, cleans the board and starts over again.  Motion is built up from these stills which are played quickly one after another.

If there is little light it is necessary for each exposure to be long enough to capture enough photons to build up an image.  However this can cause blurring.  Fine details of movement and spatial detail can be lost.   If there is a lot of light we can keep the detector elements within their active range by sampling for very short periods of time.  This improves resolution because the sample intervals are very short.

We will learn later that the eye makes the same trade-off.  In the dark we "sample" for longer time periods, and motion tends to get blurry.

V.   The detector plane.

Whether film or digital, the image has to be encoded somehow.  In this case of film there are chemicals that darken to varying degrees depending on their exposure to light.

Let's consider a video camera (or digital camera).

The detector is an array of photocells, a.k.a. pixels.  Each little square puts out a voltage or electrical signal that is proportional to how much light falls on it.   Let's call each tiny square that responds to light a "detector element."

Each detector element  only reports a light intensity.  To get a picture one needs to spread the image over many detector elements.  The picture is recreated from a mosaic of these  elements.  The bigger each detector element is, the more light each can capture and the more sensitive it is.  The smaller each element the finer the detail that can be captured.  This electrical information is then sent to some device which displays the relative intensity of each pixel and the image is thus redrawn.

The figures below show two different detector arrays capturing an image of an arrow.  The top picture in each shows the arrow imaged on the detector element array.  Each element gives an output (shown on the lower picture) proportional to the total amount of light striking that square.  Notice that the finer the detector array (the smaller the elements)  the better the quality of the image.

If the image is out of focus on the detector array,  the light from each point spreads across several pixels and the image gets fuzzy.  In real cameras the detector elements are very tiny relative to the size of the image so images can be recreated very effectively.

In order to see the image the output from each detector element has to be sent to some device which displays the intensity of each element.  The output from each detector element controls each PIXEL of a display device.
 
 

VI.  Lens size and image size.

The size of the image on the detector plane depends on the distance from the lens to the detector plane when the image is in focus.  This depends on the focal length of the lens.

A simple ray-tracing example shows why.    In the first case we have a short focal length lens.  It creates the image closeby, and it must therefore sit close to the detector plane.

In the second case we have a longer focal length lens which sits further from the detector plane.  The result is a larger image of the same object.  A longer focal length lens is known as a telephoto lens.

A zoom lens is a fancy device.  By using multiple lenses it is possible to create lenses whose focal length can change.  Thus we have the equivalent of all different focal length lenses.

VII.  Depth of field.

The detector array consists of tiny elements of finite size.  Each detector element measures all the light falling on it -- it cannot report anything about the spatial distribution of light striking it.

Imagine imaging a single point of light.  The rays emanating from it in all directions pass through the diaphragm.  What passes to the lens and gets focused represents a cone of rays.

If the image is perfectly focused on the detector plane this cone forms a dot.  If it is not perfectly focused it forms a circle.  So a point on the object becomes a small circle of light (called a blur circle).  If this blur circle is smaller than each detector element, the detector cannot show any difference -- it only records the total amount of light hitting it.   Only when the blur circle becomes larger than the detector element does the image begin to look out of focus, because the output from each point on the object gets spread over adjacent detector elements.

 

Thus, it turns out that there is not one position where the image looks in focus.  In fact the image can move back and forth over a small range and still appear to be in focus.  This is called the depth of focus.

Objects at different distances will form images at different locations.  If the difference in distance is such that the images fall within the depth of focus, both objects will appear in focus.  The distance apart that two objects are that are still in focus is called the depth of field.

The diagram below shows what happens when we make the diaphragm smaller.  Notice   Less light gets through, but there is another effect.  The cone of light reaching the detector plane is thinner.  As shown in this diagram, you can move such a cone a greater distance back and forth before the blur circle exceeds the size of the detector element.

If the blur circle is smaller this means the image can be moved back and forth a greater distance.  Thus a smaller diaphragm opening results in a greater depth of focus.   Since objects at different distances come to focus at different planes, the greater the depth of focus the greater the depth of field.

As illustrated below a smaller diaphragm opening increases depth of field at the same time it cuts down on available light.

 

 A smaller opening of the diaphragm results in a greater range of focus.  A greater range of focus translates directly into a greater depth of field.

For camera buffs.  If you want to maximize depth of field you use longest shutter speed you can get away with.  This allows you to stop down the diaphragm and increase the depth of field.

QUESTIONS

1.What is the detector plane?  Where does the image need to be brought to focus in a camera?

2. What happens to the image of an object as the object moves closer to the camera?

3.  What change needs to be made to get an image that is not in focus on the detector plane, back into focus?

4.  What do the terms "depth of focus" and "depth of field" mean?

5. What is a telephoto lens?  What does it accomplish and how?

6.  What is the purpose of the diaphragm of a camera?

7. How does making the diaphragm opening smaller influence the depth of field?

8.  What is the effect of changing the shutter speed on a camera?