焦比

Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one.

標誌

ƒ-number公式中，ƒ/#，又標作為${\displaystyle N}$，公式是這樣的：

${\displaystyle f/\#=N={\frac {f}{D}}\ }$

級數、光圈級數慣例及曝光

相對級數

標準全級光圈值尺度

 ƒ/# 0.5 0.7 1 1.4 2 2.8 4 5.6 8 11 16 22 32 45 64 90 128

${\displaystyle 2\times 1.4^{2}\approx 2^{2}}$
${\displaystyle 2\times 5.6^{2}\approx 8^{2}}$
${\displaystyle 2\times 8^{2}\approx 11^{2}}$

典型半級光圈值尺度

 ƒ/# 1 1.2 1.4 1.7 2 2.4 2.8 3.3 4 4.8 5.6 6.7 8 9.5 11 13 16 19 22

典型三分之一級光圈值尺度

 ƒ/# 1 1.1 1.2 1.4 1.6 1.8 2 2.2 2.5 2.8 3.2 3.5 4 4.5 5 5.6 6.3 7.1 8 9 10 11 13 14 16 18 20 22