Object distance and image distance – Concave lens
The object distance is the distance from the object to the centre of the lens. This is denoted by the symbol `do`.
The image distance (sometimes confused with the focal length) is the distance from the image to the centre line of the lens. This is denoted by the symbol `di`.
With the principal focal length added, this would look like:
![](/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)