Many new photographers assume that the term automatic aperture refers to the automatic aperture adjustment offered by the Sony Alpha (and other dSLRs.)
In fact, an automatic diaphragm has nothing to do with auto exposure. A lens with an automatic diaphragm will remain at its maximum aperture for focusing and composing, only stopping down to the set aperture at the moment of exposure.
If you have ever used a camera with a manual diaphragm, you will understand why an automatic diaphragm is such a huge advantage. A fully automatic diaphragm is a delight to use, particularly in dim light or when the lens is stopped down to maximize depth of field.
The term diaphragm is used to describe the lens aperture system — the system of blades that create the adjustable aperture. Some photographers use the terms aperture and diaphragm interchangeably. They are closely related, but the aperture is actual lens opening, while diaphragm is the collection of components that comprise the aperture assembly.
Turning the clock back to the early days of photography, you will find that early lenses did not have adjustable apertures — at least not the adjustable apertures you and I are used to. When the first adjustable diaphragms appeared, they allowed the photographer to change the lens opening to allow more or less light through the lens. The diaphragm contained a number of preset openings, which became known as f/stops or f/numbers. Smaller f/numbers actually indicate larger lens openings, while the smallest lens openings are represented with higher numerical f/stops. This system continues more or less today.
The thing that sets modern lenses apart from the older adjustable aperture lenses is that the older units use a completely manual diaphragm. As you change the lens opening from say f/4 to f/11, the amount of light passing through the lens is reduced accordingly. That regulates the exposure properly, but it causes problems for focusing and composing. Since a SLR shows you exactly what the lens sees, if you stop a manual diaphragm lens down, the image in the view finder becomes dimmer. In low light conditions it may be impossible to see anything through the viewfinder. Even in bright sunlight, a lens stopped down to f/22 or smaller is difficult to see through.
Lens designers solved this problem by creating an automatic diaphragm. This design has been more or less standard on SLR cameras for the last fifty years. With an auto diaphragm, the aperture remains wide open, no matter which lens opening is selected. You may select an f/stop of f/16, but the lens will remain opened to it’s maximum aperture at all times. This provides the brightest possible image in the viewfinder.
When you actually take an image, a mechanical or electrical devise rapidly stops the lens down to the chosen aperture, to create the proper exposure. You don’t see this happening, because at the same time the camera is stopping down the lens, it is also swinging the viewing mirror up to expose the sensor (or film in film SLR). When the mirror swings up, the viewfinder turns black for an instant. It is during that time that the auto diaphragm goes to work and stops the lens down. As soon as the exposure is made, the diaphragm springs wide open again, presenting you with a bright viewfinder when the mirror returns.
Of course if the selected f/stop corresponds to the lens maximum aperture, nothing happens; the lens cannot be stopped down, so it simply remains wide open.
Is this just some interesting photography trivia? Not at all. Even though all of your current lenses probably contain an automatic diaphragm, there are several reasons you should familiarize yourself with how a manual diaphragm works.
1: Many teleconverters and lens mount adapters do not offer an automatic diaphragm. Thus when you use these adapters, your modern new dSLR reverts to a manual diaphragm camera.
2: There are numerous manual diaphragm lenses still on the market. Some of these are new special purpose lenses, while others are older (but still usable) optics. You need to understand just what using a manual diaphragm entails before investing in one of these lenses.
3: Many new dSLR’s lack a proper depth of field preview option. When you stop a lens down, DOF grows wider, while opening the lens aperture causes DOF to appear shallower. Since an automatic diaphragm lens is always wide open, you cannot see DOF in the viewfinder. Even if you have a lens that will stop down to f/32 you won’t see the effect in the viewfinder unless your camera allows you to switch to a manual diaphragm preview mode.
4: If you understand about how an automatic diaphragm works, you will understand why a DOF preview is worth having. This could influence your future camera buying decisions.
The automatic diaphragm, although it had been around for decades, is just as important to modern SLR cameras as auto focus, auto exposure and image stabilization. Understanding how it works and why it is needed is crucial to getting the most out of your dLSR.
Other articles in the Basic Digital Photography series: