Relative Aperture

In order to control luminous flux of lens, it is common to add a diaphragm (or called aperture).
Assuming d stands for the effective aperture of diaphragm, D stands for actual effective aperture affected by refraction phenomenon, A stands for the relative aperture of the ratio between D (actual effective aperture) and f (focal length).
A=D/f
Relative aperture size is decided by camera illumination. Illumination E is in direct proportion to the square of relative aperture. Commonly we think the reciprocal of relative aperture as diaphragm size. That is:
F=f/D

In this formula, F stands for F-number which is labeled on aris. The numbers include 1. 4、2、2. 8、4、5. 6、8、11、16 and 22 etc. For every two adjacent, the latter is increasing time number of the former. Due to image surface illumination is in direct proportion to the square of diaphragm, as soon as diaphragm changes image surface illumination becomes higher one more. The smaller F-number is, the larger diaphragm is, and the more luminous flux is.

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