What Is Lens Speed?
Lens speed is basically the maximum amount of light that a specific lens can transmit in a specific length of time. Essentially, a lens that transmits a lot of light is called a “fast” lens, and a lens that transmits less light when fully open is called a “slow” lens.
However, the terms “fast” and “slow” have nothing to do with the ability of a lens to capture fast moving objects. The term “fast” when used to describe a lens, has to do with the amount of light it transmits in a specific time frame, whereas stopping motion depends upon using the proper shutter speed.
On your camera lens, somewhere either on the rim or side of the lens barrel, you will find some numbers that tell you what the maximum aperture of that specific lens is. This number will always begin with “1:”. “1:” means “the maximum aperture of this lens is…”. The next number or sequence of numbers, tells you the maximum aperture. So for example, 1:2.8 means that the maximum aperture of that lens is f/2.8.
One of the advantages of using a fast lens over a low lens, is that it enables you to take photos in lighting that is too dim for a slower lens without going to a slower shutter speed. So, a faster lens will give you a wider range of lighting conditions under which you can take photos without resorting to artificial lights or slower shutter speeds.
The main disadvantage of using a fast lens is the higher expense. A faster lens is also usually larger and heavier. Also, the vast majority of scenes can be shot just fine with a slow lens by changing the shutter speed, or raising the ISO setting.