Many new photographers assume that the term automatic diaphragm refers to the automatic aperture adjustment offered by their dSLR.
In fact, the term auto diaphragm has nothing to do with automatic exposure. An automatic aperture is designed to remain at its maximum opening for focusing and composing, no matter which f/stop is selected. The lens only stops down to the desired aperture at the moment the shutter fires.
If you have ever used a camera with a manual diaphragm, you will understand why this is so important. 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 system of blades that create the adjustable aperture. Many 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 optics did not have adjustable apertures -- at least not the adjustable openings 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. A SLR is designed to show you exactly what the lens sees. If you reduce the lens opening in a manual diaphragm lens, 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.
Camera 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 fire the shutter, a mechanical or electrical devise rapidly stops the lens down to the chosen aperture, creating the proper exposure. You don't see this happening, because at the same time the camera is stopping down the lens, the viewing mirror is swinging up. As the mirror moves up to allow light to reach the sensor, 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 aperture 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.
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 aperture up 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 has 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.
Visit the basic photography area at Alphatracks.com to see photos and information detailing how the auto diaphragm operates.
Tom Bonner is the author of the Sony Alpha DSLR -A300/A350 Digital Field Guide, published by Wiley Publishing. A photographer for more than three decades, he offers photography and web design services to clients in the Charlotte, NC area.
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