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 focused 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.
In paraxial optics each single refracting surface satisfies the Abbe’s Invariant Q in the paraxial Area, that relates the front focal distance s of an axial object point with the back focal distance s’ of it’s conjugated point behind the surface
are used in paraxial optical design.
The angles are measured in radians!
A beam is described by a distance r from the optical axis and a offset angle from the optical axis
An ABCD Matrix that describes the optical element is formed
The ABCD Matrix is multiplied by the input vector
The result is an output vector that describes the output beam with a new distance from the optical axis and a new angle off the optical axis
is a short for for the equation system
Examples of ABCD matrices for simple optical elements :
Where d = reduced distance= thickness / refraction index
Where d_i = reduced distance_i= thickness_i / refraction index_i
R = radius of curvature, for a convex surface (centre of curvature after interface)
= initial refractive index
= final refractive index
effective radius of curvature in tangential plane (horizontal direction)
effective radius of curvature in the sagittal plane (vertical direction)
With for convex mirrors (centre of curvature after interface)
f = focal length of the lens, where for convex/positive/converging lenses. Valid if if and only if the focal length is much bigger than the thickness of the lens
deviation from perfection of an optical system.
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