Lens ratio describes the relationship between the diameter of the lens and the focal length and simply an indication of the amount of light that a lens can allow in. Sometimes called focal ratio, f-ratio, f-stop, or relative aperture, the lens ratio is the focal length divided by the diameter of the lens. A lens with a focal length of 50mm and a diameter of 20mm therefore has a lens ration of 2.5 (50/20=2.5), which gives as an f-ratio of 1/2.5 or f2.5.
This ratio in SLR lens is normally indicate somewhere in the lens, often in the barrel of the lens as shown below. In the figure below, the lens ratio is f/1.8.
For a photographer to get the precise exposure, a combination of f/stops and shutter speeds are used to ensure the right amount of light hit the sensor (or film). Shutter speed of a camera represents the length of time a camera shutter (remember the shutter) is open to expose light into the camera sensor (if you like, the amount of time the camera shutter stays open when you hit the fire button).
The shutter speed thus regulates how long the sensor is exposed to light coming through the lens by either freezing action or blurring motion. On its part the f/stop regulate the amount of light that is allowed through the lens by changing the size of the hole the light comes through. From the many different combinations of f/stops and shutter speeds, there is one particular amount of light that correctly exposes the image.
The amount of light that hits the image sensor is determined by the aperture, the diameter of a lens. A lens with shorter lens ratio (closer to 1) produces brighter images than those whose ratio is higher ratio number. A lens with low f-stop implies wide maximum aperture, better quality lens and thus allow the photographer to do more with it. If for instance, you want to take photos in low ambient light conditions, such a lens with low f-stop will allow you to achieve quality exposure (and thus brighter photos) with a very shallow depth of field compared to a higher f-number.
While high quality zoom lenses (variable focal length) deliver a constant lens ratio in the focal range, the ratio on lower quality lens varies upwards as the focal range changes. The focal length and the lens ratio are key determinants of the price tag of lenses. It is worthwhile to mention that a lens with lens ratio <=f/2.8 can be considered to be in the professional