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# Diaphragms and Relative Aperture Effect on Perspective and Intensity 71

## lens, diaphragm, effective, diameter, called and stop

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DIAPHRAGMS AND RELATIVE APERTURE : EFFECT ON PERSPECTIVE AND INTENSITY 71. Relative Aperture of a Diaphragm. The diameter of the beam of rays incident parallel to the axis which, after refraction through the lens components in front of the diaphragm, completely fills the latter is called the effective diameter of the diaphragm. Thus, D, D' and D" (Fig. 53), although of different diameters, all have the same effective diameter d.

If, without altering the position of the stop, the real diameter is altered, its effective aperture varies proportionally. The constant ratio be tween the effective and the real aperture is sometimes called the coefficient of the effective aperture, and is equal to I only if the beam of light reaches the stop before meeting the lens (the case with single lenses). In the general case, in which the stop has in front of it one or more lenses forming a convergent system, the coefficient is greater than 1. As the value depends on the construction of a given lens, obviously no rule can be given, but it may be stated that with symmetrical anastigmats it generally lies between xi and 1-15, whilst with anastigmats consisting of three separated lenses it often amounts to 1-3.

If the diameter of the effective aperture is i/nth the focal length F, the aperture is said to be F In, which is also called the relative aperture of the diaphragm considered. If, for example, the real diameter is o-8 in. and the effective aperture is o-92 in. of a lens of 4-6 in. focal length, the relative aperture is F/5.

The relative aperture of the largest stop a lens can use is called the maximum relative aperture, or, more simply, the maximum aper ture of the lens. We shall see later (§ 90) that the maximum relative aperture of a lens is the principal factor governing its speed.' 72. Different Types of Diaphragms. In order to be able to get all possible apertures with a lens, modern objectives are usually fitted with an iris diaphragm (Fig. 54) having an aperture which can be varied by means of a rotating ring or external lever on the The thin blades of the iris are of ordinary steel or ebonite.

Though ebonite has the advantage of not rust ing like steel, in damp climates, care must be taken not to subject it to great heat. Hence an ebonite iris should not be used in the enlarging or projecting lantern using a condenser, or there will be danger of the blades melting or burning.

With lenses of which the component glasses are too closely spaced to accommodate an iris, a rotating diaphragm is employed (Fig. 55). Here an eccentric disc has a number of different apertures, which, by rotation of the disc, are brought into position concentric with the axis of the lens. The size of the aperture in position is indicated by a number engraved on the part of the projecting disc opposite the aperture.

In many old lenses and in modern lenses for process Waterhouse stops (Fig. 56) are inserted through a slot in the side of the lens tube.

73. Pupils of an Optical System. The beams of light passing through an optical system are limited by the aperture of the diaphragm. Now the components of the system in front of the stop (lens Fig. 57) form a virtual image (called the entrance pupil, of the stop D. The entrance pupil is such that the prolongation of rays through which afterwards are just bounded by the diaphragm D, reach the outline of the entrance pupil. The diameter of the entrance pupil is the effective aperture of the diaphragm just mentioned.

In like manner the components behind the diaphragm in Fig. 57) form a virtual image of its aperture, called the exit pupil the outline of which is reached by the prolongations of those rays which (before passing through just reached the outline of the diaphragm D (E. Abbe, 1890). The two pupils" are thus con jugate with respect to the complete lens.

If we suppose the diaphragm aperture gradu ally reduced to a small opening 0 on the axis, admitting but a single ray of light, this single to the position and dimensions of this image, from the positions of the nodal points and the foci.

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