Concave lens: Principal focal length – be careful
To compare one concave lens with another you need to know the focal length of each lens.
Some books define the focal length of a concave lens as the distance from the centre of the lens to where the image will appear to a viewer the other side of the lens. But this is not correct, this is the "image distance" not the principal focal length.
The focal length of a concave lens is only the distance between the centre of the concave lens and the principal focal point and should always be called the principal focal length. See below:
![](/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)