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How to Test the Aperture (Diaphragm) of a Lens

 

Introduction

One of the most important lessons I ever learned about camera equipment is that even though a lens may be clean, scratch-free, and smooth in the operation of its controls, it nevertheless may not be functioning correctly.  So what else could be wrong?  The answer lies in the aperture blades.

For a discussion of what the aperture does, click here.  For a brief explanation of how the aperture blades work, read on.  Undoubtedly many persons reading this article already understand the function of the aperture blades, and if so you can scroll on down to the testing sections.  However, many others may have ended up here because they are curious about what all can go wrong with their camera or lenses and end up ruining their photos.  In the case of malfunctioning aperture blades the result is, generally speaking, overexposure.  But as we shall see, overexposure is merely the most obvious manifestation of a broader set of problems.

 

Background

Have you ever noticed that little window above the LCD screen on the back of your DSLR?  It's called a viewfinder, and in the days before digital cameras that was how you framed your shot.  It's still the best way in many if not most situations, particularly with higher end cameras that have good viewfinders.  If you have ever used the viewfinder, you will have noticed that the viewfinder goes black during every exposure.  This is because there is a mirror in front of the film or sensor that reflects light coming in through the lens up into a prism in the top of the camera.  The prism reflects the light back through the viewfinder.  This mirror flips up and out of the way so the film or sensor can see through the lens.  When the mirror flips up, the viewfinder goes black.  When the mirror flips back down, you can see through the viewfinder again.  Ordinarily, all of this happens in a fraction of a second.  You have probably seen this mirror many times when removing or changing lenses.

What all is happening while the viewfinder is black?  If you said that the shutter opens and closes you are correct.  But more has to happen.  In fact, after the mirror flips up but before the shutter opens the aperture blades must close down to an f-stop value selected either by the photographer or by the camera.  Then the shutter opens and closes.  Then, the aperture blades open all the way back up.  Then the mirror flips back down.  During the routine picture-taking process, the photographer never sees that the aperture blades inside the lens have actuated.  Hence, if they fail to actuate, the photographer is equally unaware of this unfortunate fact.

In order to ensure that the consequence of faulty aperture blades is clear, let us consider a specific example.  Suppose we are using a 50mm f/1.8 with aperture blades that are frozen fully open.  Suppose further that the camera is in Program mode and has selected f/8 @ 1/250 for a scene you are about to photograph.  Since the aperture blades are frozen fully open, they will not stop down to f/8 before the shutter fires.  The resulting photo will be shot at f/1.8 even though the camera's metering system decided that f/8 was needed for proper exposure.  The photo will be slightly more than 4 stops overexposed.  If the aperture blades are not frozen but rather are sluggish due to congealed oil, the blades may manage to close partially before the shutter fires.  Let's say the blades make it down to f/4.  The photo will be overexposed by 2 stops.

Please note that attempting to compensate for the overexposure in the above example is not a solution to the problem unless the lens is always to be used at its widest aperture.  The bottom line is that the lens will not function reliably at any aperture other than f/1.8.  Thus, the photographer has no control over depth of field.  Furthermore, lenses tend to be less sharp wide open and therefore cannot achieve the optical performance for which they were designed.

If you have a lens with aperture blades that will not stop down reliably and wish to shoot wide open with the lens, switch to Aperture Priority mode and manually select the lowest f-stop available.  Outdoors, you will need some pretty fast shutter speeds and/or very low ISO values usually.  Indoor flash with TTL should work fine, but outdoor fill-flash is likely to be hit and miss at best.

 

Testing Method 1 - DOF Preview

If your camera has a depth-of-field preview button, you can perform a quick and dirty test of a lens' aperture blades.  Turn on the camera, take the lens cap off, and switch to Manual Mode or Aperture Priority Mode.  Select a fairly small aperture value, say f/11.  While looking through the viewfinder, press and release the DOF Preview button.  The viewfinder should quickly darken and then brighten as the button is pressed and released.  If the darkening does not take place, the first thing I would recommend is to consult your camera manual and make completely sure you are using the DOF button correctly.  If you are using A mode, switch to M mode.  If you cannot get the darkening to happen, try a smaller aperture - f/16 or f/22.  Then try a different lens.  If one lens works and another lens doesn't, I'm afraid you've got a bad lens.

To get the most out of this method, it is essential to compare how dark the viewfinder becomes when you press the DOF Preview button at different f-stops.  If the lens has a maximum aperture of f/4, start at f/5.6 and see if you can tell a difference.  Then go to f/8 and see if it's a little darker than f/5.6.  It should be a little darker with each increase in f-stop.  Make certain that you can tell a difference between f/16 and f/22.  Also try to make certain that the viewfinder is darkening rapidly when the button is pressed.

While this method is certainly informative, I do not consider it to be definitive.  I would probably pronounce a lens bad if it fails this test, but I would not pronounce a lens good if it passes.  This is because I do not believe we can sufficiently judge the 'snappyness' of the blades in this manner.

 

Testing Method 2 - Exposure Comparison

This method is best for digital cameras or for film cameras if you don't mind wasting a few frames.  In short, we will take a series of photos with different f-stops while keeping the shutter speed and ISO constant.  We will then compare the resulting photos to ensure that the exposure decreases for every increase in the f-stop.

First select an environment.  I would recommend doing this indoors away from any window lighting because the light coming in through the window may change in between photos due to clouds moving over the sun, etc.  I would include a lamp in the photo, but no TV, computer, person or pet that may change from one photo to the next.  Put your camera on a tripod, and keep it as stationary as possible throughout the test.

Start by putting your camera in Aperture Priority mode and selecting the widest aperture.  Take a reading and observe the shutter speed the camera wants to use.  If you are using a digital camera, also observe the ISO value.  Switch to Manual mode and select the shutter speed and if necessary the ISO value that you obtained in Aperture Priority mode.  Also ensure that the aperture value is still at its widest.

Now let's stop and think for a minute.  If we go ahead and take our first picture with these settings, it will have correct exposure.  Subsequent photos will be darker and darker as we use smaller and smaller apertures.  If the lens has an aperture range of several stops, the last two or three photos could be completely dark.  This would hinder our ability to judge whether the smallest apertures are working.  For this reason, I propose that we alter our initial settings so that we begin in the overexposure range.  We can do this by slowing our shutter speed by two stops or so.  After making this change, take the first photo.  Then increase the f-stop by one stop and take another photo.  Repeat this process until you have taken a photo at the smallest aperture on the lens.

If you are using a digital camera, you may be able to compare the photos on the LCD to see if the photos become darker for every increase in the f-stop.  If you are unsure, you have the option of viewing the histogram for each photo or viewing them on a computer.  Remember, if the last two or three photos are completely dark, you can repeat the procedure again starting with a slower shutter speed.  If the last few photos are not completely dark, but you can't tell a difference in darkness then the blades might not be stopping down to the smaller values.

Attention film photographers:  If possible make your comparisons on the negatives or slides.  If you have these test photos scanned or printed, the lab may automatically adjust them so that the over- and under-exposure are far less pronounced.

 

Testing Method 3 - TTL Flash with Flash Meter

As a method of testing the aperture blades, this method is probably only of interest to film photographers, although it will work with digital cameras also.  It has the benefit of giving immediate results without having to wait for the film to be developed.  In fact, if your camera will perform this test without film loaded then good.  If not, then there are a couple of other options.  You can use a roll of expired film.  If you have a film leader puller and are testing multiple cameras, lenses, and flash units then you can use the same roll of film over and over again.

Note:  Both digital and film photographers can also use this method to test the function of TTL flash with their particular camera and flash unit.  It requires a TTL flash unit, a camera compatible with TTL flash, and a flash meter.

Perform this test indoors.  There is no need for a lamp as in method 2.  Have your camera on a tripod with a TTL flash unit mounted on the hot shoe.  Place your flash meter on a table a few feet in front of the camera.  Set the meter for incident metering.  Set the camera on Manual mode.  Set the shutter speed to any speed at or below the sync speed.  Set the ISO to a fairly high value.  I use 800.  Set the lens to its widest aperture.

Take a photo and measure the flash output with the flash meter.  Try and keep the flash meter in the same spot on the table in between each shot.  Take subsequent photos, increasing the f-stop each time and measuring the flash output each time.  Every time you increase the f-stop, the resulting flash should be more intense as measured by the flash meter than the previous flash burst.  If you see that the flash unit begins to fire at its maximum power at higher f-stops, then begin the test again with a higher ISO.  Film photographers:  if you change the ISO manually on your camera, be sure to change it back to match your film speed after the testing is complete.

If you are unable to verify that each flash burst is more intense with each increase in f-stop, try a different flash unit before assuming that the lens is at fault.  You may also need to try a different lens and/or camera to make sure that the results make sense before drawing any conclusions.

Have fun and good luck!

 

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