A diaphragm (or iris or iris diaphragm) is a mechanism in a camera that makes a variable aperture to control the intensity of light that passes through the lens. Along with shutter speed, this is what controls the exposure received by the film or image sensor.
A diaphragm may take many forms, from very simple devices for "point-and-shoot" film cameras consisting of just two notched pieces of metal, to more complex ones used in higher-quality cameras which have many blades arranged in a circle. This arrangement, also called an iris by comparison to the corresponding structure in the eye creates a nearly-circular aperture whose size can be varied as needed. There may be as few as 2 blades to as many as 24. In cameras with a small number of diaphragm blades, the shape of the aperture itself (e.g. a hexagon) can often be seen in defocused points of light, or in internal reflections. A lens may deliver a more or less pleasing bokeh with large aperture setting, depending on the aperture's position in the specific lens type's optical path and in most cases also depending on the aperture's roundness.
To transmit a particular light intensity, the physical diameter of the aperture must scale up with increasing lens focal length. Thus, apertures are expressed by the f-stop scale or "F-ratio" which is the focal length divided by the entrance pupil diameter provided by the diaphragm opening. Some professionals prefer to replace the f-stop scales of their equipment with "T-stop" scales which are saying more about the light transmission of the specific lens' glass whilst f-tops are just calculated from geometrical dimensions of the aperture.
Common diaphragm types
These are listed in historical order from oldest to newest.
- Waterhouse Stops - an aperture of a specific size is cut in a metal plate. To change the aperture size, the plate must be removed and replaced with a plate having a different sized aperture hole.
- Aperture dial (or Rotating Diaphragm, Aperture Disc) - a rotary brass dial, with several holes in different diameters. Each hole represents another F-stop. The dial's axis is parallel to the lens' optical axis. Turning the dial moves one of the holes into the lens' optical path.
- Aperture shifter - a strip of opaque material with a set of holes in different diameters. One of these holes can be shifted into the lens' optical path. Typical for simple box cameras.
- Iris diaphragm - a multi-bladed iris with an aperture that can be adjusted by some type of control on the outside of the lens or camera. The size adjustment may be continuous (so the user must rely on markings to position the control at a specific aperture) or provide detents at particular f/stop values.
- Preset - just for SLRs an improvement on the manual iris diaphragm, adding a second control ring with detents or preset locking points corresponding to the selected aperture. The photographer can focus and compose with the iris fully open for the brightest image; then (without taking their eye from the viewfinder) quickly close down to the chosen f/stop just before exposure.
- Automatic - an improvement on the preset diaphragm, which also allows focusing and composing at the maximum aperture; but stopping down to the selected shooting aperture happens automatically. Early lenses accomplished this with a pressure actuated plunger alongside the lens barrel, which aligned with the shutter release on the camera body. In the 1950s manufacturers began including an aperture linkage internal to the lens mount, for example in the Praktina or the Minolta SR-2. The clear advantages of this method have made it the standard for lenses ever since (except in situations, for example with bellows or tilt/shift lenses, where the required linkage would be difficult to engineer).
- Coupled - the aperture setting mechanism is coupled to the meter. An indicator shows the exposure settings' correspondence with actual enlightment situation.
- Automatically controlled - the coupled meter or exposure program electronics set the aperture size automatically.
|The Olympus Trip 35 (like many popular cameras of its era) has a roughly square
diaphragm, formed by two overlapping metal plates; part of a trapped-needle
image by the-hospice-of-st-francis (Image rights)
|Unnamed brass lens showing aperture dial
Front element removed to show internal mechanism
image by Dirk HR Spennemann (Image rights)
- The iris of the eye is itself named after the goddess Iris, who personifies the rainbow, because of the many colours that the eye's iris can be.