Im not entirely sure, but i remember a formula that could be used to find the distance of the image, aswell as the height of the image for lenses, using their focal legth and other stated variables.
1/F = 1/Do+1/Di M=-Di/D0=Hi/Ho
F=Focal Length
Do=Distance Object
Di=Distance Image
H=height
If you are given variables, maybe you could rearrange to find F.
