Summicron-S 100 mm – Aperture Stuck? Underexposing by 2–3 Stops

[Stef63]

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Of all my S lenses, the Summicron-S 100 mm is — together with the 120 mm — my most beloved lens.

Recently, however, I noticed a very strange behavior: the lens consistently underexposes by about 2 to 3 stops. Also, the diaphragm does not fully open at f/2.0. When I use the stop-down button, it’s clearly visible that at f/2.0 it’s already partially closed — and with each click it closes further, step by step, all the way down to f/22. In other words, for every aperture setting on the camera there is a corresponding physical opening, but it starts off wrong from the beginning.

I also see the same behavior when using the lens on an SL2 with adapter, so it’s definitely not camera-related.  No other S lens has this problem on my S3. 

Apart from the obvious advice to send it back to Leica for repair — has anyone experienced something similar, or does anyone have suggestions on what might be going on?

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[John McMaster]

On my just serviced 100-S, if I come up to it I can barely see the diaphragm.  However, if I stop the lens down with the stop-down to say f8.0 then open it up to f2:0 I can see more of the diaphragm....

john

[Milan_S]

Hi @Stef63 sorry to hear this. I am also Antwerp based, so I can have a look at your S3 and Summicron-S since I have the same combination. 

We can compare both cameras and lenses and swap it to see where is exactly the issue. 

[Pieter12]

Interesting. My 100 was just serviced. I tried stopping down, previewing and letting the diaphragm open back up again--I could see the edges of the blades. After doing this a number of times at smaller and smaller apertures, the blades opened back up completely. I don't think it will affect exposure in any significant way since the difference between wide open (no diaphragm visible) and the slight intrusion of the blades seems minimal. But it certainly is bothersome on a lens that just came back from Wetzlar.

[Stuart Richardson]

I had a Hasselblad 110 f2 that did this. I think in fast lenses with a lot of glass in the barrel there is not a lot of room for the iris and the designs require more complicated action, when compared to slower lenses with more space in the lens barrel. In my lens the blades would slightly bind, causing it to catch a bit at certain apertures, rather than fully opening and closing. Service fixed it up. Perhaps you have the same issue.