Most modern cameras automatically focus a scene before a photograph is acquired. This is way easier than the manual focus that occurred in the ancient world of analog cameras. When part of a scene is blurry, then we consider this to be out-of-focus. This can be achieved in one of two ways. The first way is by means of using the Aperture Priority setting on a camera. Blur occurs when there is a shallow depth of field. Opening up the aperture to f/2.8 allows in more light, and the camera will compensate with the appropriate shutter speed. It also means that objects not in the focus plane will be blurred. The second way is through manually focusing a lens.
Either way, the result is optical blur. But optical blur is by no means shapeless, and has a lot to do with a concept known as the circle of confusion (CoC). The CoC occurs when the light rays passing through the lens are not perfectly focused. It is sometimes known as the disk of confusion, circle of indistinctness, blur circle, or blur spot. CoC is also associated with the concept of Bokeh, which I will discuss in a later post. Although honestly – circle of confusion may not be the best term. In German the term used is “Unschärfekreis”, which translates to “circle of non-sharpness“, which inherently makes more sense.
A photograph is basically an accumulation of many points – which represent the exact points in the real scene. Light striking an object reflects off many points on the object, which are then redirected onto the sensor by the lens. Each of these points is reproduced by the lens as a circle. When in focus, the circle appears as a sharp point, otherwise the out-of-focus region appears as circle to the eye. Naturally the “circle” normally takes the shape of the aperture, because the light passes through it. The following diagram illustrates the circle of confusion. A photograph is exactly sharp only on the focal plane, with more or less blur around it. The amount of blur depends on an objects distance from the focal plane. The further away, the more distinct the blur. The blue lines signify an object in focus. Both the red and purple lines show objects not in the focal plane, creating large circles of confusion (i.e. non-sharpness = blur).
Here is a small example. The photograph below is taken in Bergen, Norway. The merlon on the battlement is in focus with the remainder of the photograph beyond that blurry. Circles of confusion are easiest to spot as small bright objects on darker backgrounds. Here a small white sign becomes a blurred circle-of-confusion.
Here is a second example, of a forest canopy, taken through focusing manually. The CoC are very prevalent.
As we de-focus the image further, the CoC’s become larger, as shown in the example below.
Note that due to the disparity in blurriness in a photograph, it may be challenging to apply a “sharpening” filter to an image.