Speed of Lens

You might have heard from some friends about slow and fast lens. These terms result from the property of the aperture of the lens. The lenses with larger apertures are faster than lenses with smaller apertures. So that would mean that a 'f/4.6' lens is 'faster' than a 'f/8.0' lens. Let us try to understand why aperture is measured in terms of its speed.

You would have understood that the larger the maximum aperture of the lens, the more the light that enters the camera. This should be obvious by now. Also from the previous post, you would remember that the slower the shutter speed, the more the light that enters into the camera. When we take the photo of an object then the image is formed as a result of the light entering the camera. This amount of light has to be regulated and should be as accurate as possible for a good photograph. Too much or too less light and the image will not look as good as it is meant to be. Of course there are creative techniques which make use of these less-than-perfect images and we will have a look at it later.

So, amount of light is directly proportional to aperture and indirectly proportional to shutter speed. Since both apertures and shutter speeds are the property of the camera, the image that is formed depends on both the aperture setting and the shutter speed setting. Let us fix two camera/lens at the maximum aperture value and compare them. One lens is F/2.8 and the other is F/8.0. In the case of the F/2.8 lens, a lot of light is entering the camera because the aperture is wider. Now we will set the shutter speed to a desired value so that we get a good image, say 1/500s. At F/2.8 and 1/500s we get the image that we desire.

Now let us use the other lens - the F/8.0 one. Since this has a smaller aperture value than the F/2.8 lens, it would be transmitting lesser amount of light into the camera. Now since the amount of light has reduced, the image will look darker. As this is the widest aperture of the camera, we cannot increase the aperture any further to allow more light into the camera. So we are left with only one other choice(as of now) - to decrease the shutter speed. Since the shutter speed will now be open for a longer time, it will allow more light into the camera. If we decrease this setting enough then we will have the same image quality for the F/8.0 aperture as we had for the F/2.8 + 1/500s combo. This would be approximately at a shutter speed of 1/2s. So, the same image results at a slower shutter speed.

So, when taking photographs, you are limited by the aperture and hence, while you can shoot fast on a lens with a wider aperture, you will have to shoot slow using a slower shutter speed on a lens with a smaller aperture. It must be understood that the F2.8 lens can be stopped down to F8.0 since it is only a matter of making the aperture narrower. On the other hand the slower lenses are limited by the smaller maximum aperture and hence you cannot increase it beyond the maximum possible value to make it faster.

Do drop in your comments and do let me know if the reason for labeling a lens slow or fast is clear.