Maybe it would be a good time to learn how to inspect the aperture diaphragm inside your camera lens.
Anyone who has ever been shopping for a previously owned lens for their SLR or DSLR camera has probably heard or read the phrase, “The aperture blades are clean, oil-free and snappy”. Do you know what this ubiquitous photography phrase means? Is this information that you should know regarding your own camera equipment?
…read on and decide.
In photography, a diaphragm is an adjustable device with a variable opening that is called the “aperture” or “iris” of the lens. It’s the diaphragm’s job to control the amount of light that will “expose” the image. The diaphragm is located within the lens. It obstructs the stream of light as it passes through the optics of the lens. The adjustable diameter of the aperture regulates the amount of light that will pass through the lens to the sensor or film in the camera.
The aperture opening is controlled by adjustable “blades” that can be set to the desired f-stop. A very simple lens can have as few as two blades, but most SLR lenses usually have five or six aperture blades. A high quality professional lens’s diaphragm will have as many as nine blades. With more aperture “blades” the iris opening becomes more perfectly rounded, which can significantly effect certain aspects of image quality. In most cases, the more “blades” in the diaphragm the better. However, a nine-blade diaphragm equipped lens will tend to be the most expensive model.
The smallest f-stop number on a lens (usually between f/1.2 to f/4) indicates an aperture that is “wide open”. It is like a faucet that is fully opened, allowing the maximum flow of water to blast through the spigot. A larger number, like f/16, indicates a “stopped down” aperture, limiting the amount of light, like a faucet allowing a controlled stream of water. The aperture should not be confused with the “shutter”, which is a part of the camera’s body. The aperture determines the amount of light that will pass through the lens, while the shutter controls the length of time that the light will be allowed to “expose” the film or be “captured” by the digital image sensor. To put it more simply, for proper exposure, the aperture controls QUANTITY of light and the shutter controls the TIME allowed for exposure.
The reasons for these functions are an integral part of photographic science. There are many excellent sources for learning all of these specific photographic principals. If you want to learn more about the science of photography, you will have no trouble finding the information in hundreds of excellent books and even on the Internet for free. For the purposes of this article, just be aware that the aperture is an adjustable “light valve” that is used to control the amount of light that will be permitted to pass through the lens to the camera. A properly working diaphragm will “snap” to the selected f-stop instantly. The iris blades must be perfectly clean to be able to move fast and stop at the right position accurately. If the aperture is not working properly you will not be able to take properly exposed images!
To fully understand the meaning of the common phrase “clean, oil-free and snappy”, you need to know just a little about the aperture diaphragm and how it functions. The simple explanation above should suffice here, at least for the purposes of this tutorial.
I have never seen a simple aperture diaphragm inspection procedure printed anywhere for amateur photographers to see anywhere. This is the procedure that is taught to camera service technicians, but anybody can do it, quite easily …and I will show you how.
All photographers need to know if their lens’s aperture is working properly or if it isn’t. I will describe a simple procedure that can be used to check lenses with an electronically activated aperture diaphragm. Most older SLR lenses have an aperture (f-stop) ring or small “lever” in an arched slot in the rear of the lens to allow an actuator in the camera body to set the aperture opening. Most modern lenses perform this function entirely within the lens itself, activated electronically. There is no mechanical “ring” or lever on the lens baffle to permit you to simply test the aperture’s function without a camera body attached. Electronically controlled aperture diaphragms are “sealed units” that are much less susceptible to failure, but serious problems can still arise.
There is another important reason for performing this simple inspection, that has nothing to do with checking for a malfunction. Lens manufacturers, on several occasions, have changed the aperture diaphragms on certain models, sometimes to upgrade to more blades (to be more competitive with a rival company) …but sometimes, they just want to save manufacturing cost by substituting a less expensive diaphragm in a popular model. They do not always announce these change to the public. The only absolutely certain way to know the exact number of “blades” that make up the diaphragm inside your lens is to actually count them yourself. I’ll show you how to do that.
The inspection procedure is really very easy and it takes less than a minute to complete. First, it helps if you have an SLR camera (digital or film) with "Depth-Of-Field Preview” function. This button is usually located very near the lens mount. It allows you to preview your depth of field directly through the viewfinder, prior to snapping the picture. If your camera has this feature, you are ready to inspect the lens using this method.
Mount the lens that you are examining on the camera and remove the front cap and any screw-on filters. Turn the camera “On” and set it to “Aperture Priority” mode. This is often labeled as “AV” mode. Adjust the aperture setting to f/8 or so. Look carefully through the front of the lens and press the “Preview” button. You should see the aperture “snap” quickly into the proper position (about half closed). When you release the “preview” button, the aperture should instantly “snap” back to the wide-open position. The photo below is of a normal aperture, in preview mode, stopped-down to f/8. Please excuse the annoying light reflections, it’s not easy to take a picture of the inside of a lens (try it sometime).
5-Blade Lens @ f/8
While you are looking at the “stopped-down” aperture, count the number of blades (the segments that make up the diaphragm). Sometimes it is easiest to look at the geometric form that they resemble. If it looks live a pentagon, it is a five-blade design. If it looks like a stop sign (octagon) you have an eight-blade aperture, etc. Generally, the more blades that are used, the smoother the circular opening will be. Smoother is better (and more expensive). Look-up (or Google) the word “bokeh” sometime for a detailed explanation about why this is so important.
A 6-Blade Aperture @ f/11
Next, adjust the aperture to the largest numeric setting available, usually either f/22 or f/32. Now press the “preview button and hold it down. Using a small flashlight, or with strong overhead lighting, inspect the “blades” of the diaphragm. Your diaphragm could have anything from five to nine aperture blades. These overlap to form a very tiny opening when you preview at the maximum f-stop (minimum opening).
A Simple 5-Blade Aperture Diaphragm @ f/22
• Inspect the blades for any signs of dirt, damage, debris or any “oily” looking stains. If the blades look clean and metallic gray (or nearly black) with no visible foreign matter, you have “clean”, “oil-free” aperture blades. Oily blades sometimes have a “pattern” of contrasting surfaces which are easy to spot. The blades should all be consistent in color and finish. The finish should be non-reflective and completely uniform. Any variations should be cause for suspicion.
Now, release the preview button and press it again, several times. If all is working properly, the blades will “snap” into place and back almost instantly. Each time the aperture closes down, it must be at the exact same size opening. If all is well, this is described as a “snappy” aperture. Obviously, this simple test cannot verify that the aperture is stopping down to the precise positions or that the duration is absolutely accurate, that requires some very sophisticated lab instruments. However, it will at least let you know that everything “appears” to be normal and working as it should. (If your lens passes this check, you can be 99% sure that everything is OK.) That’s it; you are all finished, you have just completed your first aperture diaphragm inspection.
If your camera doesn’t have a “DOF Preview” button, try this procedure instead:
*1. Mount the lens on the SLR camera with “MF” (Manual Focus) selected on the lens. It may be helpful to remove any filters that are mounted on the lens.
2. Activate the camera and set it to “M” (Manual) mode, with a 3 second (or longer) shutter speed. Alternatively, you could use “B” (Bulb) mode, if you need the shutter to remain open for as long as you hold down the shutter button. Next, adjust the aperture setting to the highest number (f/22 or f/32, if available). Also, disable the built-in flash function, if your camera has that feature.
3. Now, look through the front of the lens, with the camera pointed directly at you, and press the shutter button all the way down while you are looking inside the lens.
4. The shutter will stay open with the aperture “stopped down” for the shutter duration you selected or until you release the shutter button in bulb mode.
5. This should give you enough of time to get a look at the aperture diaphragm “blades” (or segments), to confirm how many there are, and to inspect their condition.*
HERE IS WHAT YOU ARE TRYING TO AVOID!
THIS IS AN OILY, CONTAMINATED, MALFUNCTIONING CANON EF LENS APERTURE.
Notice the shiny “pinwheel” pattern on the aperture blades. It is most visible when the aperture is “stopped-down” to f/22. This aperture does not work properly. It “closes” down to different size openings when activated multiple times at the same f-stop. Sometimes it sets to the correct opening size, but often it “sticks” slightly too far open and it allows excessive light to pass through, causing over-exposed images.
THIS IS A “HEALTHY” SIX-BLADE APERTURE!
This is the exact same model of genuine Canon EF series lens (A Canon EF 24mm f/2.8 prime lens, to be exact) as the faulty one shown above it. These two identical lenses clearly show the difference between a clean, perfectly functional aperture diaphragm and one that must be avoided. Aperture replacement is expensive. This $300.00 lens could probably not justify the high cost of repair.
HOPEFULLY, YOU WILL NEVER SEE ANOTHER APERTURE THAT LOOKS LIKE THE BAD ONE PICTURED ABOVE, BUT IF YOU DO, YOU WILL NOW RECOGNIZE THIS PROBLEM IMMEDIATELY!
See, that wasn’t so tuff. Now you know more about aperture inspections than most people ever will. From now on, you will be able to determine the actual number of aperture blades in your lens for yourself, as well as whether the aperture blades are truly “clean, oil-free and snappy”!
Any reports of your personal lens aperture inspection findings or any questions that arise may be left as comments to this journal. All comments are welcomed.
Please note that this tutorial is fully copyright protected work by Gene Walls (AKA ProfAudio)
© 2010 Gene Walls
All copyright and reproduction rights are retained by the author. None of this text and / or any of the photos may not be copied, reproduced or altered by any way without the express written permission of the author. This includes my images AND THE 100% ORIGINAL TEXT. A copyright watermark is embedded within each of the images to guarantee successful prosecution, in the event of any violation.