How to determine the focal length:
The position of the lens is fixed and the camera (or the screen ) is moved depending on the object position, that you get a foczussed image (in the image center). Different object positions result in different camera- or screen distances
How to determine the focal length of an objective (= (= lens system)):
The Position of a lens (and the lens singlets in it) are fixed and an arbitrary Point O on the optical axis is marked as reference point, for example the center of the lens or the center of the first lens element).
Now we measure the distance x from the reference point to the object, the distance x’ to the image and the image size B.
You get a list of Magnifications
and equations from refererence Point to object
and reference point to image:
Where h und h’ are the distances from object side resp. image side principal planw to the reference point.
The value Vd is given by
which defines the Abbe number with respect to the yellow Fraunhofer-Line d (or D3) helium line at 587.5618 nm wavelength.
It can also be defined using the green mercury E-line at 546.073 nm:
where F’ and C’ are the blue and red cadmium lines at 480.0 nm and 643.8 nm, respectively.
see: circle of confusion
Angle that the lens can see in the direction of a given sensor measure.
Actual aperture angles are influenced by the length of used extension rings,and even focus distances (because they are mostly achieved by simulating extension rings) max possible aperture angles aren’t
Changing the sensor size changes the actual aperture angle, max possible aperture angles aren’t
The smaller the number, the more light a lens can collect, the brighter the image, the smaller the depth of field.
The larger the number, the darker the image, but in generally the greater depth of field. At the same time, we generally lose resolution, see Rayleigh Criterion.
The inverse of the square of the f-number is a measure for the image brightness of a lens.
The value is only valid in paraxial optics, ie for objects close to the optical axis.
Further off the optical axis, the focal distance of distant objects is affected by the spherical aberration.
(back focal distance = Back Focal Length = BFL)
The difference between the usual color and monochrome – ( ” black and white ” – ) cameras is an additional layer of small color filters , mostly arranged in the so-called ” Bayer pattern ” (patented by Bryce E. Bayer, 1976, employee of Eastman Kodak ) .