Canon nFD50mm f/1.4 with strange aperture problem

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RvL001 New Member • Posts: 14
Canon nFD50mm f/1.4 with strange aperture problem

Hi all,

recently I purchased a Canon nFD 50mm f/1.4 in an optically very good state. No visible scratches, no fungus or haze and only a minimal amount of dust given its age. I'm using it with my Canon M50 regularly.

It only has one strange problem with the aperture. The blades look fine, no oil to be found. They close down just fine.

First thing I noticed is that the aperture blades close slightly when engaging the aperture lever on the back of the lens. This can only be seen through the front element. When looking straight through the back element there's no difference between engaged and not engaged so I think this works as designed. But when looking in Av mode I sometimes get 1/3 shutter speed difference between applied and not applied.

When closing down the blades using the aperture ring, things get stranger. Normally, each click of the ring is half a stop.

I first noticed that the aperture blades didn't change from f/11 as if they were already fully closed. So I went to a room (bath room) where the lighting doesn't change and conducted some tests in Av mode and high ISO (low artificial light).

At wider apertures, adding a full f stop (2 clicks of the ring) also adds a full shutter stop. In some cases 1 1/3. But from f/5.6 onwards things get strange and each click seems to be a full stop. So by the time I reach f/11, the blades are fully closed.

Some measurements I did and the shutter speed the camera calculates. Up to f/5.6 only the full stops. From f/5.6 I also report the half stops.

Apt lever on back not applied: 1/4000
f/1.4: 1/3200
f/2 : 1/2000
f/2.8: 1/800
f/4 : 1/320
f/5.6: 1/125
f/6.7: 1/60 (actually f/8?)
f/8 : 1/30 (actually f/11?)
f/9.5: 1/13 (actually f/16?)
f/11 : 1/5 (actually f/22?)

Closing the aperture further down from f/11 to 16 or 22 doesn't change the aperture blades nor the shutter speed in Av mode.

So it seems all specified aperture are available but the scale is off. Or at least the setting doesn't correspond with the scale.

Has anyone ever experienced a similar problem? I can't imagine Canon meant it to be this way. When knowing it, I can take it into account but a fix would be even better.

Canon EOS M50 (EOS Kiss M)
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