Principal focal length
The principal focal length is the distance between the principal focal point and the centre of the lens.
Principal focal length 1 = Principal focal length 2
![](/images/user/base/Maths/Logarithms/Reference%20Tables/reference-log-table-1.4df6ee6.jpg)
![](/images/user/base/Maths/Logarithms/Reference%20Tables/reference-log-table-2.d516bb3.jpg)
![](/images/user/base/Maths/Logarithms/Reference%20Tables/reference-anti-log-table-1.b0b0513.jpg)
![](/images/user/base/Maths/Logarithms/Reference%20Tables/reference-anti-log-table-2.f89189d.jpg)
The principal focal length is the distance between the principal focal point and the centre of the lens.
Principal focal length 1 = Principal focal length 2