Sigma 20mm f1.4 broken aperture mechanism

lukash · Aug 23, 2024

I bought an used Sigma 20mm f1.4 DG art for cheap and it's definitely the sharpest lens I own. The main problem with it is that the lens doesn't really go beyond f2 or f2.8 when taking photos. With videos I can go all the way to f16. The aperture motor doesn't spin enough to go beyond f2. When returning from f16, the motor spins continuously until I shut my canon eos RP off. I suspect it's a problem with the electronics or the motor itself since this happens without any aperture blades. I don't know how to continue with fixing the problem. Obviously the most sure option is to send the lens to Sigma, but as usual, it will cost more than what I bought it for.
These videos show the problems:

https://drive.google.com/drive/folders/189HuRle9z-o9dCjoH2JDADqLvaZjNQ3X

Vasandra · Sep 30, 2025

lukash said:
I bought an used Sigma 20mm f1.4 DG art for cheap and it's definitely the sharpest lens I own. The main problem with it is that the lens doesn't really go beyond f2 or f2.8 when taking photos. With videos I can go all the way to f16. The aperture motor doesn't spin enough to go beyond f2. When returning from f16, the motor spins continuously until I shut my canon eos RP off. I suspect it's a problem with the electronics or the motor itself since this happens without any aperture blades. I don't know how to continue with fixing the problem. Obviously the most sure option is to send the lens to Sigma, but as usual, it will cost more than what I bought it for.
These videos show the problems:

https://drive.google.com/drive/folders/189HuRle9z-o9dCjoH2JDADqLvaZjNQ3X

Hello. Have you found the solution? I am going to buy broken Sigma 20mm f1.4 DG art for 100$, as seller says "with broken aperture, does not work at all and broken hud".
Hud is nothing, replacing takes about 15 mins and a bit of skill, but aperture. In the internet different youtubers said that you need to dissasemble almost whole lens, then just "unstick" uperture blades, and everything will be fine. I had some expirience with replacing broken ribon cables and resoldering them to different lenses, but they were ef or ef-s old.
Will test the lens on the post and reply with a results and next steps. Maybe this will help somebody.

Vasandra · Oct 8, 2025

Update! So when I connected the lens to the a6300, it worked completely fine (for some reason). The only thing I noticed was that the aperture blades were not really even, but this sub 0.01 mm difference only when closed to f/16 is not a big deal. MAYBE the lens was broken, but during the post procedure it repaired itself with vibration (stuck aperture blades or dislocated gear from falling).
I was testing the lens all day long and didn't see any problems at all. Will change the blend and paint scratches on the lens body.

loccdor · Oct 8, 2025

That's great. I have the Sigma ART 135mm f/1.8 and it's also the sharpest lens I own I don't have the skill to work on electronic lenses.

I did see this about the warranty for the OP's country, transferable between owners:

SIGMA LENSES

Our Sigma lenses automatically comes with a standard warranty of two(2) years warranty* when purchased in Europe , but we are extending the warranty of our world class lenses by three(3) years free of charge, to provide our Sigma Nordic and Baltic customers with a total of five(5) years warranty*.

Sigma 20mm f1.4 broken aperture mechanism

lukash

Vasandra

Vasandra

loccdor

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