Advice on aperture repair on camera lens

[lukashts]

I bought a used Sigma 20mm f/1.4 DG Canon EF-mount lens which had its aperture and autofocus broken.

I fixed the autofocus simply by cleaning the switch but the aperture remains stiff. The aperture works in video mode if it goes from f/1.4 to f/16, but doesn't work in photo mode above f/3.5 because it gets stuck because of too much resistance. I tried cleaning the aperture with isopropyl alcohol but that didn't fix the problem.

What could I try to fix the aperture? Should I disassemble the mechanism and clean it comprehensively? I don't want to send it away for repair since that would cost more than what I paid for the lens. The rubber also reeks of cigarette smoke but that'll probably go away with time.

[ApertureScience]

I have no experience with repairing modern lenses. With old manual lenses I sometimes cleaned the whole aperture assembly with an ultrasonic cleaner, using alcohol as a solvent. That usually removed all oil and dirt.

[lukashts]

That seems like a great idea, but I don't have an ultrasonic cleaner. Maybe I can justify getting one for other uses as well.

[Koray]

It is odd that the aperture behaves normally in video mode but not in photo mode. Could it be software related?

K.

[lukashts]

It isn't software related. The aperture motor doesn't have the oomph to close the aperture for a split second in photo mode but in video mode the aperture increases in small steps so there's less resistance.

[AndrewEp17]

If you are able the get the aperture diaphragm out i would recommend completely disassembling and cleaning the blades with isopropyl alcohol but be careful not to dent or bend the blades. and take it apart slowly and pay attention to how it is assembled so it will be easier to remember how to put it back together

[jchw4]

That seems like a great idea, but I don't have an ultrasonic cleaner. Maybe I can justify getting one for other uses as well.

Given that the smallest and simplest one costs $20 delivered https://www.amazon.com/Vpcok-Direct-SU-745-Ultrasonic-Jewelry/dp/B0BJJS9RPP I see no reason not to buy one.
These are sold under zillion "brands" and they are all the same.

I have one that looks approximately the same and it works well for home use.
In my case the timer is fixed at 1 minute and I have to run it several cycles to really clean stuff, hopefully now you can buy one with a flexible timer for 1..10 minutes.
Not very powerful but I use it once or twice per year and it just works.

[ApertureScience]

You could start by submerging the assembly and working it a few times. Reassembly can be quite tedious. If the material permits you can also try acetone, be very careful.

[lukashts]

I will try repairing it after a trip.