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Circle of Confusion
The circle of confusion is simply a spot where light rays come together on the digital sensor of your camera. Most people probably won't consider learning about it a worth while use of time. But, learning about the circle of confusion will help you to understand some very useful and interesting aspects of digital photography. Things like depth-of-field, lens resolution, sharpness and bokeh all are related to the circle of confusion.

Light is a straight line What is focus?
First, what is focus? I mean, what actually happens to the light as it bounces off an object and enters your camera focused or out-of-focus?

To understand focus you need to understand how light rays find and enter your camera's lens. It is important to understand that light travels in a straight line. You can only see light that is directed straight at you. If you shine a flashlight in the dark you cannot see the beam of light unless it is pointed right into your eyes. You can only see what the beam of light hits. Once a ray of light hits the surface of an object, it bounces off in an infinite number of directions, dispersing the light so everybody around can see it. This is due to the imperfections of the surface material the object is made of (see figure 1).

Light is a straight line This is where your camera's lens goes to work. Its job is to collect as much of that light as possible and focus it onto the camera's light sensitive chip (CCD). Every ray of light that hits the subject bounces in a different direction. There are trillions of light-rays shooting off the object you are photographing. Only a small portion of that light actually makes it directly into your camera but even that small percentage of light rays is a very large number. This light enters through the lens as cone shaped points of light which take the size and shape of the aperture and focus down to a tiny point on the CCD. The smaller the point, the sharper the image (see figure 2).

The point where these lines hit the CCD is known as the circle of confusion. The size of this area depends on how much the point is in focus. The sharper the focus, the finer the point. An out-of-focus point will be much larger.

Spherical aberrations Spherical Aberrations
The finer the focal point, the sharper the image. But how fine can this point be? That depends on the quality of the lens. The problem is that the rate light bends as it passes through a lens at different angles is not a constant. The greater the angle between the light ray and the lens surface at the entry point, the greater the change of angle as it passes through the glass. This means that the rays of light that pass through the lens at its outer edge bends more than the light that enters in through the center. Yes, we expect this to happen but look at the diagram to see what I mean. The light rays towards the outside edges are bending too much so that they are not all meeting at one fine point. This makes the sharp tip of the cone of light dull. It's like drawing with a dull pencil. No matter how hard you try, you can't draw sharp lines. This is know as spherical aberration. No matter how much you try, you cannot focus down to the smallest point at the tip of the cone.

There are ways lens manufacturers can design lenses that will minimize spherical aberrations but this brings the cost of the lens up. If your lens is not creating sharp images, it could mean that it is suffering from severe spherical aberrations. Sharpness is not the only thing to be concerned about. There are also resolution concerns.

Lens Resolution Lens Resolution
Let's assume we have a camera with a CCD resolution of 100 pixels. Yes, that is very low but it makes it easy for us to see what is going on. Now let's assume we have a basic stock lens with some spherical aberrations. Take a look at the diagram here. The smallest point we can focus down to actually fills in one pixel on the CCD. This means that our lens is matched in resolution quality with our camera.

Lens Resolution vs camera resolution Lens Resolution vs Camera Resolution
Now we buy a new, higher resolution camera but we keep the same old lens. Our camera resolution is now 400 pixels. Look at the diagram. Our old lens cannot focus to a small enough point to cover one pixel. It takes four or more pixels to cover the smallest point of focus. Even though we improved our camera resolution, our photographs will not be any sharper because our lens cannot focus down to a fine enough point that will take advantage of the extra pixels our camera can capture.

So at times it may be wiser to buy a better quality lens than to buy a better quality camera. What does this say about those new cell phone cameras that go up to 40 megapixels? I have seen reviews that showed these cameras are no sharper than a 12Mega pixel digital SLR camera.

The same thing applies to upgrading your lens when your camera cannot produce the resolution. A high quality lens capable of producing a very sharp image will be wasted on a camera with a low resolution sensor. The CCD will not be able to capture the sharpness the lens is capable of.

Depth of field
Coming soon...