Aperture issue with Sigma 30 1.4

Not sure what's going on and whether it is the lens or the body but yesterday the lens seemed to be stuck at f1.4. Every shot was hugely overexposed despite the camera showing and appropriate aperture and the exposure seemingly correct. After doing a bit of on off, battery in out stuff and using my 70-200, which was fine, I worked out if I pressed the depth of field preview button you could see the blades close and open and then it would work fine. If, however I take the lens off and reattach the problem resurfaces until I press the DOF preview button.

Any thoughts, lens or body?

check your body first. what mode are you in? what's the exif data say?

The OP said the camera body works fine with the 70-200 so I doubt it is the camera. The camera also displays normal exposure values, so that is what will show up in exif data.

I would first clean the electrical contacts on the lens (and body just to be sure). It is strange that the aperture works fine after you use the DOF button just once. Try doing it different apertures to see if large or small aperture settings makes it behave differently.

It might mean sticky aperture blades which means the lens will need to be serviced. With lots of use it may get better(or worse) so I would use it like it is for awhile and see if it clears up.

Thanks for that I will keep playing and might drop in to my local shop this weekend. Would be interested if anyone else has encountered this.