The job of the lens is to create an image of the outside by projecting it on the surface of film or digital image sensor. To accomplish, light from objects traverse the transparent lens. The shape (figure) of the lens and the density of the lens material cause these image forming rays to alter their direction of travel. We can draw a trace of these emerging rays. When we do, a trace discloses that this revised path takes on the shape of a cone of light. This action is called refraction --meaning to bend inward.
The distance from the lens to the apex of this cone of light can be measured. You will find that light rays from distant objects come to an apex nearer the lens than light rays originating from objects closer to the lens. In other words, the length of the cone of image forming rays is a variable based on the object’s distance from the lens. The focal length of a lens is a measurement taken lens-to-apex, when the lens is imaging a far distant object (objects at infinity symbol ∞). A measurement of the distance lens-to-apex made when the lens is observing nearby objects is called the “back focus”.
The key to answering your question is the fact that a lens has limited ability to refract (bend inward). When we focus the camera, we are adjusting the distance lens to sensitized surface in an attempt to cause the apex of the cone of image forming rays to just kiss its surface. If the distance is right, the object will be in focus. If the focus adjustment causes this apex to fall too short or too long, the resulting image will be fuzzy.
Again, as we focus we are changing the distance lens-to-sensor. The focus mechanism of most cameras limits the amount of movement lens-to-sensor. This is because most camera lenses are optimized to work distant subjects. A macro lens design is optimized to work in close and slightly compromised when tasked to work distant subjects.
Standard cameras lenses usually stay the forward movement at a distance of about ½ meter (20 inches). If our desire is to image super close with a standard lens, we must install tubes/rings or bellows or resort to using close-up supplemental lenses.
We commonly use extension tubes when working in close. The tube merely increases the lens to sensor distance. In other words they elongate the distance lens to apex, and this accommodates the now stretched out back focus distance. With a tube in place we get revised lens to subject distance values. Because the lens has limited ability to refract, the now revised span of focus is limited to close-up work; we can only work narrow span of close by object distances.
