I've recently purchased a Canon AE-1 Program and a FD 50mm f1.8, as a entry way into the world of photography. As soon as i got the lenses, i got excited to check it out. I noticed something weird though, as things would be as much out of focus in 1.8 as they would be in f22. So, curious, i decided to put the camera in bulb mode, put the aperture to 1.8, open the film cover and shoot.

This is the result:

enter image description here

This looks normal, but when i tried the same with the aperture set at f22, i got this result:

enter image description here

This is not normal. I googled the problem but came across a lot of confusion. Some, with the same problem, said the problem is indeed a stuck diaphragm and that this is quite a well known problem for the FD series. Other instead said that this is a result of not inserting the lens properly.

Is there a magic trick to inserting the lens, or are their diaphragm just stuck?

Thank you very much!


2 Answers 2


You can easily check this.

First, you may not have a problem at all. From your post, I infer that you haven't taken any pictures yet and instead tried to judge depth of field through your viewfinder. The viewfinder of your camera always shows you the image with the aperture fully open, and therefore what you describe is expected. If you want to see the image with the actual depth of field, push the "DOF preview" or "stop-down" button on your camera. It should be located left of the camera-side lens mount. This procedure is described on page 18 of the camera's manual: AE-1 Manual. You will notice how your viewer image also becomes darker when stopped down.

If this does not work, something might actually be wrong. In this case, the next step is to find out if the camera or the lens is responsible.

To do that, remove the lens from the camera, close the diaphragm to f/22 and look at the lens bayonet.

Canon FC bayonet, image courtesy of Wikipedia

The lever on the top left in this picture is the one that controls the diaphragm. Try to gently move it counter-clockwise. There should be the kind of resistance you expect from a small spring, like in a ballpen, and the diaphram should open. When released, it should immediately snap back into its inital position.

The first 20 seconds of this video show how it should look like: https://www.youtube.com/watch?v=vlVoxAnvpSQ

If the lever is blocked, the diaphragm doesn't follow the lever's movement, or it feels like you have to overcome considerable friction to move it, then something is likely wrong with the lens. Otherwise, the problem is likely with the camera.

  • 1
    \$\begingroup\$ Hi! Thank you very much! I tried the DOF preview and it did absolutely nothing. I then tried to move the little lever you indicated, with no success whatsoever. The thing just wouldn't move. I emailed the person i bought the lens from. He apologized profusely and explained that he had two of the lenses, but only one worked. And he accidentally shipped the wrong lens. \$\endgroup\$ Commented Apr 13, 2017 at 22:04

The viewfinder view through your Canon AE-1 is nice and bright. This is because the Iris Diaphragm (aperture) is being held in the wide open position. The idea is to present to you, as you compose and focus, a max brightness image. Additionally, the max opening of the Iris delivers the shallowest depth-of-field. This affords you the best opportunity of obtain a tack sharp focus.

Consider that the image you are viewing had to contend with a gauntlet of obstacles. The image forming rays were diverted by a first surface mirror. Next this upside-down and inverted image encounters a penta-prism (roof-prism). This is a multi-faceted solid glass prism that resembles a gabled roof. The image forming rays reverberate inside multiple times. The result is a right-side-up and correct left-to-right image. This image is projected on a ground glass viewing screen. Because of the after effects of all this reverberation, the image tends to be vignetted (dim at the edges).

Atop the ground glass is a Fresnel lens. This is a flat plastic cover that is embossed with concentric circles. This shape fakes a condensing lens. It’s task is to diminish the vignette. You now view this image through a eyepiece magnifying glass that presents this view to your eye only millimeters away from the ground glass view screen.

Needless to say, the sum total of all this optics seriously dims the image. It’s so dim that we need all the light we can get to do the composing and focusing.

Pre-set Iris to the rescue: As you compose and focus the Iris is being held wide-open by a spring. The actual diameter setting for the exposure is called a pre-set. The built-in light meter or your manual aperture setting determines the pre-set f-number. You press the shutter release, reflex mirror flips out of the way of the film. As this happens the Iris closes down to a pre-set diameter, the shutter opens and the exposure is made. Now the reflex mirror repositions to send the image from the lens to the viewfinder. The Iris springs full open. All you see is a wink!

This is the mechanism we know and love as the SLR (single lens reflex). The idea is to present to us a view that mimics as close as possible, the view seen by the film (digital chip) during the actual exposure.

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    \$\begingroup\$ @AlanMarcus, you should seriously consider writing a book \$\endgroup\$
    – Olivier
    Commented Apr 13, 2017 at 19:29
  • \$\begingroup\$ @ Philip Kendall -- I think most gentle readers will conclude that the iris diaphragm (aperture) is not malfunctioning. It is being held open by a spring. When mounted on camera, the iris will remain fully open for composing and focusing. When the go button is depressed, the iris will close down to the pre-set f-number for the exposure occurrence. After the exposure is complete, the iris re-opens to it's max diameter again for viewing, composing, and focusing. \$\endgroup\$ Commented Apr 13, 2017 at 20:36
  • \$\begingroup\$ My thanks to Oliver – I have seriously considered writing a book but when they were passing out phonics I missed the boat. Plagued by poor spelling I barely squeaked by in my 55 + years in this industry. Now pushing 80 and out outside looking in, no fresh stuff from me. But thanks, I like the encouragement. \$\endgroup\$ Commented Apr 13, 2017 at 20:48

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