# The sign of a focal length

We know that for converging lens, $f>0$ , for diverging lens, $f<0$. But for many materials I have read so far, it says that: "the focal length of a concave lens is 8 cm."

I thought that the focal length in this case must be negative. Or when it comes to focal length, we need to find the absolute value of $f$?

• More on sign conventions in optics. Jan 9 '16 at 7:36
• Thank for your reference. I know that for concave lens f<0 . But how about the focal length. Should we take the focal length as the magnitude of f so that the focal length is positive for all lens? Jan 9 '16 at 9:17
• I think the magnitude has been quoted. The negative sign is implied. Also, there might be other sign conventions, so leaving off the sign avoids conflicts related to different sign conventions. Jan 9 '16 at 15:07