Re: measuring exact focal length of camera lens

Assuming that the lens in question is a thin Bi-Convex eg like this (),
the lens maker formula could be used
1/f = (n-1) * (1/R1 - 1/R2) + (n-1)^2 /n * t/R1*R2
Where f=focal length
      n=refractive index of lens material
      R1=radius of curvature of 1st surface
      R2=radius of curvature of 2nd surface
      t= lens thickness
nb 

^2 means squared
either R1 or R2 is taken to be negative usually R2
t can be measured using vernier calipers or micrometer
A spherometer is needed to find R1  R2

If the above is of no use a simple high school physics experiment can be
used to find the focal length (but not as accurate as the above method)
below is a diagram of the experimental setup see any high school physics
book for a better diagram

EQUIPMENT
	  lens,lens holder*,cross hairs cut into a piece of metal/card etc
           screen (sheet of white paper),and meter-stick


  lamp¦       ()                  |screen
       
      '   u    '      v           '

where ¦ is the piece of metal etc containing the cross hairs
      () is the lens 
       u is the distance between the cross-hairs and the lens 
       v is the distance between the lens and the focussed image
nb 
  all measurements are made from the middle of the lens 
  experiment is best preformedwithout any background lights
  * lens holder can be simply a piece of wood witha slot in it ti stop lens
   falling over

by putting the above results in the following formula the focal length can
be obtained   1/f= 1/u + 1/v

the magnification m =v/u.