A covert lens with an 11-mm FL and a 6-ram aperture has a theoretical f-number of 1.
The most important characteristics of a pinhole lens are its optical speed - the lower the f-number the greater the speed - and its ease of installation and use.
This automatically doubles the f-number of each lens (only one-fourth of the light transmitted).
Generally, the larger the f-number a lens can be set to, the better the picture.
R equals the amount of light that reaches the camera from a known source divided by the amount of light that would have reached the camera through a perfectly transparent lens of the same f-number.
60, shows the relationship between the f-number and the R factor.