I'm looking for a way to find - as exactly as possible - the real focal
length of my lenses (as used on a D-SLR) to give me an indication about
how
reliable the EXIF-given focal length really is for zoom lenses.
Assume:
1. that I know the distance between the CCD and an object that I make a
picture of
2. the quotient between the distance between two points on the object and
the distance between corresponding points on the CCD (as computed from the
pixeldistance on the resulting image)
D := Distance between CCD and object
Q := Quotient (> 1 if image is smaller than the real life object)
F := Focal length - searched for
After some deductions from the simple lens formula I get:
F = (D * Q) / ((Q+1) * (Q+1))
Then, given the 'fact' the the simple lens formula is only an
approximation
when applied to non-simple lenses, what relative error should I expect
from
such a computation?
What other methods could I use to get the true focal length?
TIA /BJ
