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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.

Advantage and Disadvantages of Third Party Camera Lenses

So let us start first what is a third party camera lens, a third party camera lens is one that has been developed by a different company in which you are using of a different brand of camera. For example you are using a Nikon or Canon Camera, and then you are using a Sigma or Tamron as a camera lens for your camera.<br /><b r />

Mainly the reason that most photographers or other camera enthusiasts purchase these lenses is because it is more cost effective for them. Now let us discus what may be the advantages or disadvantages of having this type of camera lens. <br /><b r />

What are the Advantages? <br /><b r />

First off is that third party camera lens are a lot cheaper than the branded camera lenses. Second is providing excellent image quality, where can you find a camera lens for example Tamron which provides excellent image quality as well as a lot cheaper than Nikon and Canon. Lastly some brands like Nikon or Canon models which do not sell some specific lenses are mostly can be found in generic brand lenses such as Tamron or Sigma. <br /><b r />

What are the Disadvantages? <br /><b r />

First is quality is still the issue, no matter how cheap the generic brand is what’s important is the quality it gives you, meaning the durability of the lens. If you do intend to resell or upgrade your lens in the future. Generic brands do not hold their value not like Canon or Nikon. There is no guarantee that the generic lens you buy will be compatible with any Nikon or Canon EOS camera. It is always best to really look into this fully before purchasing a third party lens, and do not simply presume because the salesperson which is by the way paid by a commission always says it is compatible and they will add that it will fully work with your particular digital camera, and that there will be any problems encountered. Lastly, there are third party lenses do not have an option of turning on fully manual modes. They also do not offer extra features such as Canon USM for fast focusing. <br /><b r />

What you should do? <br /><b r />

In any way, you cannot interchange that Nikon Camera Lens if for Nikon Camera and Canon Camera Lens is for Canon Camera, but if you will consider the money involved you might want to think otherwise. Eventually what you think is the best fit for you is the best thing that matters. <br /><b r />

About the Author

John Christian Smith is a writer and a freelance photographer. John Christian Smith also writes for the http:// sigma-macro-lens.com and http:// nikon-macro-lens.com

lens converter for manual lenses

so i have a canon rebel eos, and i got a gift well a camera from my poppa its an older cannon a-10 or something like that i dont reallt care about the camera itself but it comes with really nice lenses now i need a converter for it to use the leses...so what di i get?
it is a canon AE-1 with a rebel eos but the eos is not digital its manual

Because of the differing flange focal distances between the old Canon FD-mount and the newer EOS system's EF-mount, FD lenses will not work completely with your EOS Rebel.

There are lens adapters available on eBay and from retailers but you won't be able to focus to infinity. Some have optical correction lenses that'll allow you to focus to infinity but you'll have to deal with a massive loss of quality.

http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=fd+to+ef&category0=

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