Learn the easy way to inspect the aperture diaphragm in your Canon EF Series SLR camera lens.
Anyone who has ever been shopping for a used lens for their SLR camera has heard or read the phrase, "The aperture blades are clean, oil-free and snappy".
This is very important information to know about a lens that you are thinking about purchasing. Unfortunately, many novice or "first time" photo gear sellers who use these words actually have no idea what they really mean. This very common phrase just "sounds good" to them and it seems to be pretty important, so they parrot those words in the description of the lens that they have up for sale. I recommend that you make certain that the seller actually understands the meaning of this phrase and can confirm that it accurately describes the actual lens that they are offering for sale.
To understand the meaning of the phrase "clean, oil-free and snappy", you should know a little bit about the aperture diaphragm and how it functions. I am not going to teach a photography course here, so a very simple explanation will suffice, at least for the purposes of this short eBay Guide.
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 and eventually past the shutter, onto the film (or the digital image sensor in a Digital SLR).
The smallest f-stop number on a lens (usually between f/1.2 to f/4 or so) 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 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 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 all about photography, you will have no trouble finding the information in hundreds of excellent books and even on the Internet for free.
However, even with all of the information that is available, I have never seen this simple inspection procedure explained anywhere for amateur photographers to see. This is the exact procedure that is taught to every certified Canon service technician. All Canon factory authorized service technicians are instructed that every lens must pass this inspection.
Anyone who is selling or buying a used camera lens needs to know weather the aperture is working properly or if it isn’t. Yes, you can tell if the aperture is merely functional by checking a a lot of photos that were taken with the lens. However, Canon recommends a much faster, more thorough way to do it.
This procedure applies particularly to Canon EF series Auto-Focus lenses, which have an "electromagnetic", electronically activated aperture diaphragm. Most lenses have a 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. Canon lenses perform this function entirely within the lens itself, activated electronically. There is no mechanical lever on the lens baffle to permit you to simply test the aperture without a camera body attached. The Canon EMF aperture diaphragms are "sealed units" that are much less susceptible to failure, but problems can still arise.
There is another very important reason for performing this inspection, that has nothing to do with checking for a malfunction. Canon has, on several occasions, upgraded certain model lenses with improved aperture diaphragms. They do not always announce this change to the public. The only absolutely certain way to know the exact number of "blades" that make up the diaphragm is to actually count them yourself.
The inspection method that I describe here WILL WORK WITH ANY SLR LENS, and it will also test the aperture coupling between the lens and the camera, whether it is "mechanical" or "electromagnetic". Most lenses for other camera systems allow you to do a much more simplified inspection of the aperture itself, but since Canon EF lenses do not, I thought that this information might be very helpful, especially for Canon EOS camera owners.
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. Turn the camera “On” and set it to "Aperture Priority" mode. This is often labeled as "AV" mode. Adjust the aperture setting to around 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).
A Simple 5-Blade Aperture Diaphragm @ 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 "Preview" button, try this procedure instead:
1. Mount the lens on the EOS camera with "MF" (Manual Focus) selected on the lens.
2. Turn the camera "on" by setting the Command Dial to "TV" mode and then rotate the "Main Dial" (located just behind the shutter button) until the highest number (like "22" or "32") is displayed in the LCD window.
3. Look through the front of the lens with the camera pointed directly at you and press the shutter button all the way down.
4. The shutter will stay open and the aperture "stopped down" for several seconds (depending upon how much light is available, the less light the longer).
5. Now you should have plenty of time to get a good 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!
I have heard it falsely stated that Canon EF series lens aperture diaphragms "NEVER fail". It has also been said that you can determine if there is an aperture problem by simply taking a few photos and checking the pictures for exposure problems. Unfortunately, following that advice would have been a real disaster in this case. The first few pictures that I shot with the "bad" lens turned out beautifully. If I had based my evaluation on those test photos alone, I might have bought this useless lens, erroneously convinced that the aperture was working perfectly. Canon's very simple 30 second inspection saved me from getting "stuck" with a completely worthless lens!
I inspect a lot of Canon EF series lenses. This is the second EF lens that I have encountered with this problem so far this year (2007). I would be thrilled if this condition was as rare as some uninformed pundits proclaim it to be. Maybe I am just the most unlucky Canon lens collector around, or possibly, it is because I actually do carefully inspect each and every lens. No matter how rare this problem is, if you are the unfortunate buyer who finds yourself the "proud owner" of one of these, you would surely wish that your lens had been checked before you bought it. It's half a minute very well invested!
UPDATE: Today is November 21, 2007 and I am presently holding the third Canon EF lens with the same aperture problem that I have encountered so far this year.
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 tough. Now you know more about aperture inspections than most people who are selling their used lenses on eBay. From now on, you will be able to determine the actual number of aperture blades for yourself, as well as whether the aperture blades are truly “clean, oil-free and snappy”!
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