Aspects of Camera Lenses: Principal Focus
Camera lenses are the complex tools that make it possible for an image to be recorded on a photographic medium like an image sensor or film. One aspect of a camera lens is focus. It is the lens' ability to focus, which keeps the images looking sharp and accurate instead of like big globs of light. The areas that appear sharp is where light converges. Some areas appear blurry because the light does not converge very well at that distance from the lens. An important concept in focus is the principal focus, also know as the focal point.
Two Focal Points
Because it is possible for light to travel through the lens glass in two directions, there are two focal points. One is the distance of the subject to the lens, which can be adjusted with the focus ring. The other focal point is the distance of the lens to the photographic medium. We refer to this as the principal focus because it is at this exact spot where the image appears sharp.
If the distance is slightly off between the photographic medium and principal focus, then the image will either be soft or blurred. The same principal can be applied for the subject as well. If they are not at the focal point then they will be blurred.