Fundamentals of geometrical optics

Examples

1 :

We take into account a 6 lenses shooting objective, the object is at the infinity. An iris put in the middle of the objective is used as an opening diaphragm for the system. The entrance and exit pupils are virtual.

The full light field points are lighted up by the D.O. Figure 52 shows the edge of the full light field image, for a point beyond, the beam of light will be limited, in part, by the frames of lenses.

Figure 52
Figure 52 [zoom...]
2 :

Refracting telescope: the object is at the infinity; the objective of the refracting telescope makes the beams limitation, it is D.O and entrance pupil, its image by the ocular is the exit pupil which is the place where the eye pupil of the observer has to set itself.

The entrance and exit pupils are real. A diaphragm situated in the focal plan of the ocular executes the field limitation. The image full light field is infinite; it is confounded with the infinite image of the ocular diaphragm of the ocular field.

Figure 53
Figure 53 [zoom...]
Second diaphragm : Combined object or image of the field diaphragm

In an optical system with two dominant diaphragms (we do suppose that other diaphragmations do not play a part), we calculate the full light field in the following way :

  • Diaphragms are brought back to the same space, for instance image space.

  • The pupil is the object conjugate or D.O. image seen under the smallest angle from the centre of the image fiel (point A).

  • The secondary stop is the object or image conjugate of the field diaphragm.

  • The side of the full light field (point B) can be obtained by seeking the intersection with the image field - the closest to the centre of the field - of the ray which joins the side of the pupil and the side of the secondary stop (in the same space : object, image or intermediate).

For a point outside the full light field (point C for instance), rays issued out of the pupil converging in C are partly obstructed by the secondary stop, diaphragmation is said to be "cat"s eyed".

Figure 54
Figure 54 [zoom...]
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