Hello,

I think your quesztion is wrongly typed,do you mean the focal length of convex/concave lens?as there is no focus lens called conveyance allowance.Please comment below to mention your doubt clearly.

But Focal length is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror till the point where parallel rays of light meet, or converge.

Hope this helps.