Module 12 F-Stops and Creative Focus Techniques C ats and owls can see in dim light better than we can, in part because the lenses of their eyes allow more light to enter.
Lens speed is defined as the maximum amount of light that the lens can transmit.
Like the pupil of an eye that automatically adjusts to varying light levels, camera lenses have an iris that controls the amount of light that can go through the lens. In the bright sunlight the pupil contracts in an effort to avoid overloading the light-sensitive rods and cones in the back of our eyes. A lthough an iris can be smoothly adjusted from a very small opening to the point of being wide open, certain specific points within this range are marked according to degrees of light transmission. These numerical points are called f-stops . Contrary to what you might first assume, the smaller the f-stop number, the more light the lens transmits.