lunedì 4 aprile 2011
Diaphram STOPs & Area of a Circle
Do you know why photo diaphragms stops have these strange numbers like
f/1.0 f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8 f/11 f/16 f/22 ?
It is very easy. This comes from the Area of a Circle formula! Lens are circles. The amount of light captured by a lens is proportional to the area of the aperture. The area of a Circle is : r^2 * pi ;
To double the AREA of the circle, we have simply to multiply for the square root of 2 the radius!
Example1: radius = 1.00 cm, Area = 3.14cm^2
Example2: radius = 1.41cm, Area = 6.28cm^2
So, in a photographic lens
where f is the focal length and N is the aperture Number. That explain why each diaphragm stops corresponds to the half of the light captured by the lens, and that-s, why, to have the proper exposure, you can simply double the exposure time.