The size of the lens opening, also known as the aperture, a mechanism made up of a diaphragm, determines the lens’s speed. Depending on their optimum aperture openings, lenses are sometimes termed as fast or slow. The aperture determines how much light reaches the sensor of a digital camera.
The greater, or more open, the aperture is, the faster the shutter speed must be while creating exposure in any given light environment. To acquire proper exposure, you must instantly filter off a lot of light from passing through the lens.
An aperture’s diameter is measured in f-stops. Lowering the f-stop value widens the aperture, allowing more light to reach the sensor. Increased f-stop numbers constrict the lens opening, allowing less light to pass through. An f/2.8 lens has a larger aperture than an f/8 lens.
A large optimum aperture lens is termed a fast lens – the sharper the lens, the greater the aperture.
Fast lenses have a bigger aperture, allowing more light to penetrate in a shorter time. When there is lighter falling on a subject, faster shutter speeds are used.
A camera user can reduce the aperture of a lens using manual or semi-automatic controls if necessary. Stopping down refers to the procedure of decreasing the scale factor of an aperture. It’s important to remember that a lens’s clarity isn’t always at its best when it’s wide open or closed down too much, though.
In the very same light conditions, a closed or blocked aperture needs a slower shutter speed to achieve the very same exposure.
A lens with a low image sensor, such as F/4.5, f/5.6, or f/11 is a slow lens. Exposure duration will be longer with a slow lens because less light passes through to the sensor.
Zoom lenses with a longer focal length are typically slower. At the telephoto end of the zoom, they are slower, while at the wide end, they are faster.
The Depth of Field is increased while using a slow lens. The same can be said for a fast lens that has been slowed down. A photographer may desire deeper Depth of Field based on the graphics a photographer seeks to accomplish in a scene.
Fast lenses are more expensive than slow lenses.
The only difference between fast and slow lenses is the maximum amount of light they admit into the camera for recording. The minimum f-number or maximal aperture diameter –usually used to describe the lens speed of any photography lens. The faster a lens is considered, the more light it lets in. The necessity for a fast lens varies based on the type of photography you do and the environment you shoot in.
Certain camera features are only fully functional with a lens faster than f2.8 because it affects focusing since specific focus points behave differently. More lenses with this or wider (faster) apertures are available, with the maximum aperture conspicuously labeled on most lenses when marketing.