The iris diaphragm

The intensity of the light transmitted by a photographic lens is controlled by the iris diaphragm or stop which is usually an approximately circular aperture. In some simple cameras the aperture is fixed in size; in others the diaphragm consists of a rotatable disc bearing several circular apertures so that any of them may be brought in line with the lens. Such fixed apertures are known as Water-house stops and are used, for example, in some fish-eye lenses. This arrangement is limited in scope. Most lenses use an iris diaphragm, the leaves of which move to form an approximately circular aperture of continuously variable diameter. When the camera shutter is of the between-lens type the diaphragm is part of the shutter assembly. It is operated by a rotating ring, usually with click settings at half-stop intervals, and calibrated in the standard series of f-numbers. The interval between marked values will be constant if the diaphragm blades are designed to give such a scale; in older lenses with multi-bladed diaphragms the scale may be non-linear, and the smaller apertures cramped together (Figure 9.8).

The maximum aperture of a lens may not be in the conventional f-number series, but can be an intermediate value, e.g. f/3.5. The minimum aperture of lenses for small-format cameras is seldom less than f16 or f/22, but for lenses on large-format cameras, minimum values off/32 to f/64 are typical. The SLR camera requires automatic stopping down to the chosen aperture immediately before exposure, in

Figure 9.8 The iris diaphragm. (a) Principle of the iris diaphragm. Ring A contains a number of slots. An iris leaf D has a fixed hinge C at one end and a pin B at the other which moves in a slot in the rotating ring A. As ring A is rotated, pin B moves so that the leaf moves in an arc. The action of several overlapping leaves is to give larger or a smaller central aperture. (b) Multi-bladed iris using a simple radiused shape for each leaf. This gives a circular aperture but an unevenly spaced aperture scale. (c) Another multi-blade design, where the more complex blade shape gives an aperture scale with equidistant spacings suitable for automation or servo control

Figure 9.8 The iris diaphragm. (a) Principle of the iris diaphragm. Ring A contains a number of slots. An iris leaf D has a fixed hinge C at one end and a pin B at the other which moves in a slot in the rotating ring A. As ring A is rotated, pin B moves so that the leaf moves in an arc. The action of several overlapping leaves is to give larger or a smaller central aperture. (b) Multi-bladed iris using a simple radiused shape for each leaf. This gives a circular aperture but an unevenly spaced aperture scale. (c) Another multi-blade design, where the more complex blade shape gives an aperture scale with equidistant spacings suitable for automation or servo control order to permit viewing and focusing at full aperture up to the moment the shutter is released.

In early SLR cameras, the lens was stopped down manually by reference to the aperture scale. The use of click-stop settings assisted this process. Next, a pre-setting device was introduced which, by means of a twist on the aperture ring, stopped the lens down to the preset value and no further. This arrangement is still occasionally used, for example, on perspective control (PC) lenses. The next step was to introduce a spring mechanism into this type of diaphragm, triggered by the shutter release. The sequence was thus speeded up, but the spring needed resetting after each exposure and the diaphragm then re-opened to its maximum value. This was known as the semi-automatic diaphragm. Finally, with the advent of the instant-return mirror came the fully automatic diaphragm (FAD). In this system an actuating lever or similar device in the camera, operated by the shutter release, closes the diaphragm down during the shutter operation. On completion of the exposure the diaphragm re-opens and the mirror returns to permit full-aperture viewing again (Figure 9.9). A manual override may be fitted to allow depth-of-field estimation with the lens stopped down. Some cameras with early forms of through-the-lens (TTL) metering required the lens to be stopped down to the preset aperture for exposure measurement. Cameras with a shutter-priority mode have an additional setting on the lens aperture scale, usually marked 'A', where the necessary f-number is determined by the camera and set just before exposure, the user having selected an appropriate shutter speed. The aperture set may be in smaller increments than the usual half-stop increments, possibly to 1/10 stop if motorized and under digital control.

In most modes the TTL metering system in a camera normally requires the maximum aperture of the lens in use to be set into the metering system to allow full-aperture metering with increased sensitivity. Various linkages between lens and body are used for data transfer.

As focal length increases, the problems of fitting a mechanically actuated automatic diaphragm increase, so electrical systems are preferred. Some ultralong-focus lenses are not connected electrically and require manual setting of the aperture. Extension tubes usually transmit the actuation for diaphragm operation via pushrods or electrical connections, but extension bellows may not do so, and manual operation, or use of a double cable release, may be necessary.

Lenses fitted to large-format technical cameras are usually manually operated, although some presetting devices are available, as is electronic control from film plane metering systems. Some lenses have fixed apertures such as mirror lenses and those used in simple cameras.

Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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